27 Mechanisms of reactions in solution† N. Winterton The Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool, UK L69 3BX 1 Introduction Full understanding of a chemical change requires the identity and nature of intermediates formed between reactants and products to be identified and knowledge of the processes of bond making and breaking and of electron transfer to be established.Inorganic reaction mechanisms are thus of intrinsic interest. Moreover, they underpin our understanding of key environmental, biological, medical and industrial processes. This review of the 1998 literature does not, except for studies of particular importance or interest, cover homogeneous catalysis or redox processes involving organic substrates, organic reactions of the heavier p-block elements, or fluxional, photochemical, electrochemical, solid-state or heterogeneous processes.A new journal, Inorg. React. Mech., and a special issue of Transition Met. Chem. are devoted to inorganic mechanisms. Activation volumes in solution,1 models for electron transfer,2 the use of density functional theory3 and electron-transfer reactions of polyoxymetalates4 have been reviewed and the application of genetic-algorithm5 and non-linear optimization methods6 to the treatment of kinetic data described.Stopped- flow EXAFS has been used to study7 short-lived intermediates. Studies of processes on the femtosecond timescale8–18 are noted. 2 Redox reactions A series of papers on electron transfer in proteins19–23 and a review by Marcus on theory and experiment in electron-transfer reactions24 have appeared in a special issue of J.Electroanal. Chem. Long range electron transfer Di§erent sites on cytochrome bc 1 are associated with oxidation and reduction,25 with the Fe–S protein component of cytochrome bc 1 acting as an electron shuttle between †Dedicated to the memory of Professor Bob Hay, an esteemed friend and colleague.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 535ubiquinol and cytochrome c. Proton-coupled electron transfer involving the [Fe 3 S 4 ] cluster in Azobacter vinelandii ferredoxin I has also been studied.26 Electron transfer from the reduced [Fe 2 S 2 ] centre in putidaredoxin to the camphor-oxidising cytochrome P450 occurs27 via Cys-39 and Asp-38 of the former, across the interprotein interface to Arg-112 of the latter, and finally to the haem-group via a propionate.Electron transfer between cytochrome b 5 and hemoglobin (Hb) involves prior complexation28 and is faster for the a-chains than for the b-chains. Binding and redox processes have also been studied29 in complexes between cytochrome c and a horseheart apomyoglobin reconstituted with artificial terminally-functionalised porphyrins.Spacer flexibility a§ects photoinduced electron transfer in semisynthetic myoglobins into which [Ru(bipy) 3 ] moieties have been incorporated, linked via XNHC(O)(CH 2 ) 2 , spacers [X\CH 2 , (CH 2 ) 3 O(CH 2 ) 2 or (CH 2 ) 3 O(CH 2 ) 2 O(CH 2 ) 2 ].30 The rate constant for electron transfer between an iron(II) sub-unit of a mixed-metal Hb hybrid and a zinc(II)- or magnesium(II)-substituted sub-unit varies little with temperature from 5 to ca. 200 K.31 Redox processes have been described involving pseudoazurin (from Achromobacter cycloclastes) and cytochrome c from two sources,19 cytochrome c and cytochrome c peroxidase,20 cytochrome c and dtpa-modified zinc myoglobin21 and galactose oxidase,32 various cytochrome c’s and an iron(II) complex,33 FeIII reduction in Coprinus macrorhizus peroxidase by dithionite,34 and oxidation of a plastocyanin from the green alga Ulva pertusa by [Fe(CN) 6 ]3~ or [Co(phen) 3 ]3`.35 A distance attenuation factor for electron transfer, b, of 1.1^0.4Å~1 is reported36 for mono-, di- and trivaline-bridged [RuII(bipy) 2 (cmbipy)]2`–[CoIII(NH 3 ) 5 ] donor –acceptor pairs.Polymer conformation a§ects37 the quenching kinetics by 1,1@- didodecyl-4,4@-pyridinium in the presence of L-tyrosine esters of RuII* bound to poly(1- vinylimidazole). The distance attenuation factors for electron transfer in Ru-modified b-sheet proteins are smaller than for those with helical structures.22 The increase in the rate on cooling from 220 and 170K of CuI to RuIII electron transfer in the blue copper protein, azurin, isolated from Pseudomonas aeruginosa, with the [Ru(im)(bipy) 2 ]- moiety bound at His-83, has been ascribed38 to reductions in the lengths of hydrogen bonds which mediate the coupling.A reduction in electron-transfer rate from CuI to native and the His-46-Asp mutant forms of azurin, RuIII-modified at His-83, reflects39 changes associated with the binding site for copper rather than driving-force or reorganisation-energy e§ects. Intramolecular (pH-dependent) electron transfer from CuI to [RuIIIL(terpy)]His-59-modified Scenedesmus obliquus plastocyanin (L\bipy, 4,4@-Me 2 bipy, 4,5,4@,5@-Me 4 bipy) occurs40 with a Cu–Ru separation of 15.6Å.Rates of intramolecular electron transfer in [RuIII(NH 3 ) 4 L]-modified manganese(II)-substituted cytochrome c increase along the series, L\NH 3 , py, isn, to an extent less than expected for a driving-force change of 0.3 eV, suggesting41 that the electron transfer step per se may not be rate-limiting.Volume profiles for reversible intramolecular electron-transfer in two cytochrome c’s modified, respectively, at His-33 (horse heart) and His-39 (Candida krusei) by trans-[Ru(NH 3 ) 4 L] (L\NH 3 , py, isn, 3,5-lut) are asymmetric,42 an e§ect associated with increased electronic coupling arising from pressure-dependent changes in non-bonding interactions.These and related processes are the subject of further theoretical and modelling studies.23,43–49 Protein folding associated with electron transfer has been reviewed.50 Distance-dependence51–58 and other aspects59–62 of long-range electron transfer mediated by DNA have been studied, including further work on non-covalently- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 536bound donors and acceptors.63–68 *-[Ru(phen) 2 (dppz)]2` and *-[Rh(phi) 2 (5- Xphen)]3` (X\amidoglutaryl) show low site-selectivity when individually intercalated with aDNAdecamer duplex, though together each binds specifically towards the end of the duplex, with a four base-pair separation.The cleavage of DNAand RNA by metal complexes has been reviewed69,70 and the role of complexes of iron,71 cobalt,72 nickel,73 platinum,74 copper,71,75–79 ruthenium68,80 (and its inhibition by Mn2`81) and rhodium82 described.Studies of electron- and energy-transfer across M(l-L)M@ bridges83–102 have included [Ru(terpy)(l-L)Ru(terpy)]5` (L\L1, L2, L3)84 [Ru(bipy) 2 (l-L4)Ru- (bipy) 2 ]5`,86 [Mmer-Ru(NH 3 ) 3 (bipy)N2 (l-L5)]3` (R\H, Me, Cl)85 [MRu(NH 3 ) 5N2Ml- (C 5 H 4 N-2)(C 2 H 2 )n(C 5 H 4 N-2)N]5`,92 [Ru(bipy) 2 (l-tppa)Os(bipy) 2 ]4`,93 [Ru(terpy)- (l-tpypyz)RhCl 3 ]2`,96 [MRu(terpy)N2 (l-L6)(MLn)]5` MMLn \[M@(bipy) 2 ] (M@\Cu, Os), [Ni(H 2 O) 4 ]N,97 [Ru(bipy) 2 (l-L7)Cu(phen)(H 2 O)]3`,87 [Ru(Bu5 2 bipy) 3~nM(l- L)Pt(Bu5 2 bipy)Nn]2` (H 2 L\1,10-phenanthroline-5,6-diol),94 [Ru(bipy) 2 (l- L8)Mn(H 2 O) 4 ]5`,88 [RuIII(NH 3 ) 5M(NC)MIII(N 4 )(CN)NRuII(NH 3 ) 5 ]6` (M\Cr, Co, Rh; N 4 \a tetraazamacrocycle),89 [Fe(CN) 4 (l-L)Ru(bipy)(l-L@)Rh(terpy)LA]4` (L\L@\2,3-bpp or bipym, LA\N-methyl-4,4@-bipyridinium),90 [MFe(CN) 4N2 (l-g4- bmtz)]3~,91 and zinc porphyrin units linked covalently by 2,6-disubstituted 4,4@- diphenylethylene bridges98 and other groups.100 Long-range electron transfer in other donor (D)–spacer (S)–acceptor (A) systems continues to be actively investigated.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 537Electron transfer in [RuII(bipy) 2Mphen-(S)(A)N]2` linked covalently to a 2,4-dinitrophenylamino moiety (A) via amino acid bridges (S)103 involves a through-bond, r-hole tunneling process without involvement of p-orbitals of aromatic substituents in the amino acid.Intramolecular and intervalence electron transfer Activated electron-transfer between two adjacent metal centres in the (crystalline) mixed-valence complex [M 3 O(O 2 CBu5) 6 (py) 3 ] (M\Fe) occurs at a rate lower than the infrared timescale at room temperature.104 For M\Mn, localised valence states are seen.Related studies of the [Fe 3 S 4 ]0 cluster in a 7Fe8S ferredoxin from Bacillus schlegelii have been reported.105 Changes in intramolecular electron-exchange coupling on protonation of l-O in [FeIII 2 O(O 2 CBu5) 2 Tp 2 ] I account106 for di§erences in Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 538bimolecular electron-transfer kinetics involving I or [IH]` and [Ru*(4,4@-Me 2 - bipy) 3 ]2`. Energy-transfer rates have been measured within complexes formed from [Ru(Bu5 2 bipy) 2 (X-bipy)]2` and [Os(Bu5 2 bipy) 2 (Y-bipy)]2`, linked by double (X/Y\adenine/thymine moieties) or triple (X/Y\cytosine/guanine moieties) hydrogen bonds.107 A related hydrogen-bonded complex between a zinc–porphyrin and 1,8: 4,5-naphthalenetetracarboxylic diimide has also been studied.108 Intervalence transfer in dinuclear cyano-bridged complexes of FeI–FeII, FeI–MnII 109,110 RuII–RuIII 111 and PtII–FeIII,112 and in pyz-bridged complexes of OsII–OsIII 113 and RuII–RuIII 114 have also been investigated.Dipole-moment changes arising from intervalence transitions in [MII(CN) 5 (l-CN)M@III(NH 3 ) 5 ]~ (M\Fe, Ru; M@\Ru, Os) lead to estimates of the charge-transfer distances which are only 50–65% of the geometric separation between the metal centres acting as donor and acceptor.115 Other workers have questioned these conclusions116 and reported additional data on [RuIII(NH 3 ) 5 (l-NC)MLn]m` [MLn \M@II(PPh 3 ) 2 Cp (M@\Ru, Os), n\3; Cr0(CO) 5 , n\2].Reviews117,118 and new studies are reported (some119–127 in special volumes of Coord. Chem. Rev.) on the dynamics (including on the fs timescale9,10) of photochemical and photophysical processes, including those devoted to carbonyl–a-diimine complexes of MnI,119,128–130 ReI,119–121,128,131–142 RuII 120,121,136 and OsII,134 other mononuclear M(a-diimine) complexes, M\Fe,143 RuII,122,123,126,144–161 OsII 162 and PtII,127,163 (some with electron-donor133,140,164 and acceptor103 moieties substituted in the a-diimine and electron-acceptors134–136 bound to the metal) and catecholate complexes of CoIII 165,166 and CrIII.167 The dpypyz-localised MLCT state in [Ru*(bipy) 2 (dpypyz)]2` increases the basicity of the non-bonded nitrogens su¶- ciently to complex to PtIV and RhIII.126,154 The delayed photoejection of an electron from [Cu(NH 3 ) 3 ]` involves168 a dinuclear exciplex, [Cu 2 (NH 3 ) 5 ]2`. Photoreduction of [PtII(N 3 ) 2 (dppp)] first gives [PtI(N 3 )(dppp)] and then [Pt0(dppp)] via intramolecular electron transfer.169 Outer-sphere electron-transfer and self-exchange reactions A two-term rate law has been established170 for the reaction, 2 [RuII(NH 3 ) 5 - (isn)]2`]I 2 ]2 [RuIII(NH 3 ) 5 (isn)]3`]2 I~, associated with outer-sphere electron transfer to form I 2 ~ or I 2 ~]I~, respectively.The self-exchange rate constant for I 2 /I 2 ~ was estimated to be 5]102M~1 s~1. Outer-sphere (and Br~-inhibited) bromine oxidation of [RuII(NH 3 ) 5 (pyzH)]3` is 500-fold slower than for [RuII(NH 3 ) 5 - (pyz)]2`.171 Self-exchange rate constants for the couples Br 2 /Br 2 ~,171 [Fe(CN) 5 (NO 2 )]3~@4~,172 [CuL 2 ]2`@` (L\2,9-Me 2 -4,7-Ph 2 phen173 and L\L9),174 and [W(CO) 5 L]0@~· 175 have been reported.Sowrey, MacDonald and Cannon176 have used NMR line-broadening to measure self-exchange kinetics for the couple [Fe 3 O(O 2 CBu5) 6 (py) 3 ]`@0 in dichloromethane.Ferrocene derivatives are included in a wider study177 of self-exchange kinetics involving organic substrates. Outer-sphere electron transfer is proposed for the two-step two-electron reduction of [NiIV(L10)]2`by nitrite and the single-step two-electron reduction of [NiIV(L11)]2`by nitrite at pHp6.0,178,179 for reduction of [IrCl 6 ]2~ in alkali,180 and for the oxidations of [Mo(CN) 8 ]4~ by [Mn(cdta)]~.181 Sykes and co-workers182,183 have studied the outer-sphere reactions of single- and double-cube complexes, e.g.[M 3 InS 4 (H 2 - Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 539O) 12 ]5` (M\Mo, W) and [Mo 6 ES 8 (H 2 O) 18 ]8` (E\As, In) by [Co(pydca) 2 ]~. Helicate complexes184 of copper(I), [CuI 2 (L12) 2 ]2`, may be reversibly oxidised to [CuII 2 (L12) 2 ]4` via a mixed-valence intermediate. Li and Ho§man185 have challenged the assignment186 of inverted Marcus behaviour for the bimolecular reductive quenching of [Ru*(a-diimine) 3 ]2` by phenolate. (See also refs. 187–190.) Rates of [e(H 2 O)n]~ and [CO 2 ]~· reduction of [Ru(bipy) 2 (g2-L)]2` (L\2,2@: 3@,2A: 6A,2@@@- quaterpyridine),191 have been measured, with rates of protonation for an unbound N in the product, [Ru(bipy) 2 (g2-L·)]2` also reported.The ligand-centred radical anion resulting from pulse-radiolytic reduction of [Ru(menbipy) 3 ]2` reduces [Co(acac) 3 ] with an enantioselectivity of 2.7, favouring * over ".192 In water, enantioselectivity in the quenching of the excited state of rac-[TbIII(pydca) 3 ]3~ by *-([)-[Ru(phen) 3 ]2` increases193 with applied pressure.Diastereomeric outer-sphere ion association between [Co(dien) 2 ]3` and [CoL(ox) 2 ]2~ (L\gly, b-ala),194 and between [Fe(4,4@- Me 2 bipy) 3 ]2` and [P(C 6 Cl 4 O 2 -O,O@) 3 ]~ 195 has been studied. Stereoselective electron transfer between rac-[CoIIILX]` (L\alamp, promp, X\H 2 O,196 py, Him, pyz197) and optically active complexes of iron(II) takes place via an inner-sphere mechanism with a carboxylato moiety on L acting as the bridge.Stereoselection in metal complex –protein binding,198 energy-199 and electron-200 transfer is noted. Inner-sphere electron and atom transfer [FeIII 2 (l-O)(phen) 4 (H 2 O) 2 ]4` II oxidises [S 2 O 3 ]2~ in the presence of [phenH]` by an inner-sphere process201 to give [S 2 O 3 ]~·, [Fe(phen) 3 ]2` (an inhibitor of the reaction) and [Fe(phen) 3 ]3` with [S 2 O 3 ]~· undergoing a self-reaction to give [S 4 O 6 ]2~.In the absence of phen, [Fe(S 2 O 3 ) 2 ]~ and [Fe(S 2 O 3 ) 3 ]3~ are involved.202 A similar mechan- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 540ism (via NHOH· to N 2 O) occurs with II and NH 2 OH.203 Inner-sphere processes are also suggested204 from the kinetics of N 2 H 4 oxidation of [MnIV 2 (l-O) 2 (l- O 2 CMe)(bipy) 2 (H 2 O) 2 ]3`.Reduction of trans-[Pt(CN) 4 Cl 2 ]2~ by [Pt(NH 3 ) 4 ]2` in chloride-ion solution follows two parallel pathways,205 both involving chloridebridged transition states, [(H 2 O)PtII(l-Cl)PtIVCl] and [ClPtII(l-Cl)PtIVCl]. Hindmarsh, House and van Eldik206 have reviewed redox processes involving platinum(II) and platinum(IV) complexes.The rate of PtIV reduction and cytotoxicity are qualitatively associated for a series of anticancer PtIV complexes.207 Atom-transfer is the dominant primary process127,208 in the quenching of the triplet state [3Pt 2 (l- H 2 pop) 4 ]4~ by [Co(CN) 5 X]3~ (X\Cl, Br, I) in aqueous acid, giving aquacobalt(II) complexes and [PtIII(l-H 2 pop) 4 X 2 ]4~. Inner-sphere mechanisms are proposed for N-bromosuccinimide oxidation of [CoII(nta)(H 2 O) 2 ]~ 209 [and its iron(II)-catalysis210] and of [CrIII(hedta)(H 2 O)].211 Oxidation of dithiophosphates, [P(OR) 2 S 2 ]~, L, by copper(II) proceeds212 via [CuIIL 2 ].Rate di§erences, M\Mo[W, for the oxygen-atom transfers, [MO 2 (mnt) 2 ]2~]P(OR)nR@3~n ][MO(mnt) 2 ]2~] P(O)(OR)nR@3~n, are associated213 with di§erences in the ease of reduction of MVI to MIV and the strength of the M––O bonds.Transfers of O from MoVI to P,214 MnV to P,215 FeIV to P,216 RuIV to P,217 and from NV or NIII to MoV 218 and ClI, IIII or NV to RuIII 219 are also reported. ReVII and SII engage in reversible oxygen-atom transfer.220 Substituent e§ects and rate correlations for RR@S to RR@SO oxygenation by oxochromium( V)221 point to sulfide to CrV outer-sphere electron transfer as being ratedetermining.The OsVI-to-P N transfer between [OsNCl 2 (terpy)]` and PPh 3 222 involves the intermediate [OsIV(NPPh 3 )Cl 2 (terpy)]`. Steric e§ects dominate rates of –– NRtransfers223 in imide–imine metathesis reactions involving [Mo(––NR) 2 Cl 2 (dme)].Miscellaneous redox reactions In basic solution, NO reduction of [CuL 2 (H 2 O)]2` to HNO 2 and [CuL 2 ]` is faster for L\Me 2 phen than for L\phen,224 and involves NO binding inhibited by OH~ or the bu§er conjugate base. Disproportionation of [Cu(NO)(TpM4,H)] III to [Cu(NO 2 )(TpM4,H)] and N 2 O involves225 rate-determining attack of NO on III.Dinitrosyl intermediates are proposed226 in similar reactions of a manganese tropocoronand, [Mn(NO)(L13)]. Kinetics have been studied for reactions involving NO, NH 3 and CuII,227 N 2 O and [Co(tpp)]2~,228 N 2 and NbIII,229 NH 2 OH, HNO 3 and TcVII 230 or PuIV,231 NH 3 and MnVII,232 and N 2 H 4 233 or NH 2 OH234 and RhIV. The couple [CuII]/[CuIII]` is responsible for the accelerating e§ect of trace quantities of copper on the processes, 3I~]CrVI]1.5 I 2 ]CrIII,235 and 2 I~]2VV]I 2 ]2VIV.236 Photolysis of [FeX(H 2 O) 5 ]2` (X\Cl, OH) at j\347nm leads to [Fe(H 2 O) 6 ]2` and Cl·, [Cl 2 ]~·, [ClOH]~· and OH·.237 Related studies238,239 on the photoreduction of [IrCl 6 ]2~ are reported.Chromium(II)- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 541catalyzed aquation of cis-[Cr(N 3 )(bipy) 2 (H 2 O)]2`,240 reduction in the rate of pyridine exchange on one-electron reduction of [Fe 3 O(O 2 CCBu5) 6 (py) 3 ],176 RuIII-catalyzed oxidation of [Fe(CN) 6 ]2~ by [IO 6 ]5~,241 reaction of OBr~242 or OCl~243 with NpIV and PuIV, of NCS~ and TcVII,244 H 3 PO 2 and TlIII,245 PuV disproportionation,246 RuIII-catalyzed oxidation of SeIV by MnIII,247 CrVI 248 and MnVII 249 oxidation of AsIII and FeIII 250 or CrVI 251 oxidation of PIII have all been described.Geochemically- and environmentally-relevant studies of the oxidation of iron(II),252–255 reduction of chromium(VI),255 iron(III),256 iron(IV),257 manganese(III)258 and mercury(II)259 and of the autoxidation of sulfur(IV)260–266 have appeared.Only in very acidic droplets is chromium concluded260 to have any role in atmospheric aqueous-phase SIV autoxidation. In a revised mechanism of manganese-catalyzed autoxidation of [HSO 3 ]~ 261 [MnIII(OH)(H 2 O)n(l-O)MnII(H 2 O)n]2` initiates a chain reaction by complex formation with [HSO 3 ]~ to produce chain-propagating [SO 3 ]~· several orders of magnitude faster than iron(III).The synergistic e§ect of iron(III) is associated with an increase in [MnIII] and of the catalytically-active dimer. [Fe(H 2 O) 4 (l-SO 3 )(l-OH)Fe(H 2 O) 4 ]3` forms in the reaction between sulfite and excess iron(III).262 Zinc(II) catalyzes the oxidation of RSH by CrVI 267 in acetate bu§er with [CrO 3 (SR)]~ formation from zinc–thiolate and acetatochromate species being ratedetermining.L-Cysteine, glutathione and DL-penicillamine (RSH) oxidations by [Cr(OO)(H 2 O) 5 ]2` proceed268 via a thiol complex, with [Cr(OOH)(H 2 O) 5 ]2` and [CrO(H 2 O) 5 ]2` as intermediates. O 2 participates in related processes involving RSH and [CrVO(ehba) 2 ]~.269 Propagation in the oxidation of AcrH 2 by H 2 CrO 4 involves270 reaction of AcrH 2 and a CrV species to give (by H· abstraction) chaincarrying AcrH· and CrO2` in competition with H~ transfer giving Cr3` and AcrH`.Other oxidants have also been studied.271 Other sulfur- (and selenium-272) centred oxidations have been described involving [FeO 4 ]2~,273,274 [Fe 2 (CN) 10 ]4~,275,276 tungstocobaltate(III),277,278 [Co(ox) 3 ]3~,279 [Co(OCrO 3 )(NH 3 ) 5 ]`,280 and complexes of NiIV,281 VV,282 FeIII,282,283 ReV 284 and CeIV.285 Ferrate(VI) oxidation of cyanide yields cyanate and then nitrite.286 Oxidation of H 2 ox by [CrVO(ehba) 2 ]~ involves287 mixed-valence CrV–CrVI complexes.Several studies relevant to water oxidation, both on photosystem II288–290 and model systems,291–294 have also appeared. Reactions of oxygen-containing oxidants and reductants Mechanistic studies continue to focus on a better understanding of enzymatic processes involving binding and activation of oxygen-containing oxidants with oxidases,20,32,295–298 oxygenases,299 superoxide dismutases,80,300 peroxidases301–304 and catalases and related model systems.305–307 [FeII(tpp)], generated in situ in MeOH, and O 2 react to gives308 [O 2 ]~· and [FeIII(tpp)]` via slowly-dissociating [FeII(O 2 )(tpp)] rather than by outer-sphere electron transfer.p-Nitroperbenzoic acid oxidizes [FeIII(tdcpp)] to either [FeV–– O] or [FeIV–O·] in the presence of methanol or [FeIV(––O)(tdcpp`·)] in its absence.309 The rate constant for formation of [MoVIO(O 2 )(tmp)] from [MoVIO(tmp)] and O 2 is ca. 105-fold smaller310 than those for porphyrins of iron(II). Peroxocomplexes are key intermediates311,312 in reactions of binuclear dicopper complexes, [CuI 2 (L14)(NCMe) 2 ]2`311 and [CuI 2 (L15)]2`312 in Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 542which L14 and L15 contain a benzene moiety capable of hydroxylation linking two chelating groups. An intramolecularly-bridged ‘‘side-on’’ l-g2:g2 peroxo intermediate was detected312 using resonance Raman spectroscopy.Aliphatic hydroxylation, observed for [CuI(L16)]` IV and O 2 , proceeds via rate-determining formation of a l-g2: g2 peroxodicopper(II) intermediate from IV and a monomeric superoxocopper( II) species.313 (See also refs. 314–316.) In the presence of excess oxygen, primary rather than tertiary C–Hof an N-isopropyl moiety of (mononucleating) Pr* 3 [9]aneN 3 (L) in [(CuIIL) 2 (l-g2: g2-O 2 )] is oxygenated, via317 a novel [MCuII(L[H)N2 (l-1,1- OO) 2 ] intermediate.The e§ect of L and R on the reactivity of a series of alkylperoxocobalt( III) complexes, [CoIII(OOR)L], with alkanes has been studied.318 (Reviews are noted for other metal-complex catalyzed oxygenations and hydroxylations of organic substrates299,319–323 not otherwise surveyed.) O 2 and the diiron complex, [FeII 2 (l-L17)(l-O 2 CR)(O 2 CR)(N) 2 ] (N\pyridine- or imidazole-derived ligand) react in non-polar solvents according to kinetics324 that are first order in each reagent. (See also ref. 325.) Monomeric and dimeric peroxoiron(III) complexes are proposed326 as intermediates in O 2 oxidation of [FeII([15]aneN 4 )(H 2 O)n]2`. Solvent dissociation is not rate-determining in the reversible binding of O 2 at the vacant site of some new cyclidene complexes of cobalt(II),327 with the rate of binding being a§ected primarily by the size of the lacuna andO 2 -dissociation rates by the axial base.The rate law328 for the oxidation of [CuI(fum)] by O 2 involves only reaction via [Cu(O 2 )]` formed from, and reacting with [Cu(H 2 O)n]` to give [CuII(H 2 O)n]2` and H 2 O 2 .Related studies329 on [NiIII(edta)]~show that ligand oxidation is first order inO 2 and second order in the complex. [P 2 O 8 ]4~ formation from [H 2 PO 2 ]~ and O 2 , in the presence of [CrO 4 ]~ 330 and [Ru*(bipy) 3 ]2`331 has been described. In acid, ozone oxidation of [Mn(H 2 O) 6 ]2` proceeds (without OH· formation) via oxygen-atom transfer to give [MnIVO(H 2 O)n]2`332 which itself reacts rapidly with excess manganese(II) to give manganese(III).The terminal label in O 3 isotopomers scrambles unimolecularly only at elevated temperatures.333 In the absence of added H 2 O 2 , surface-catalysed formation ofH 2 O 2 controls334 the initiation ofO 3 decompo- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 543sition in aqueous acid.The kinetics of the liquid-phase reaction between O 3 and H 2 O 2 335 and SnHR 3 336 have been reported. [O 3 ]~· is a proposed intermediate337 arising from the photolysis of strongly alkaline oxygenated [S 2 O 8 ]2~ solutions. The hitherto unexamined equilibrium O 2 ]HO 2 ~]OH~H2[O 2 ]~·]H 2 O, has been studied.338 Rate contants have been reported339 for the copper-catalyzed dismutation of [O 2 ]~·, a process believed to be an important environmental sink for superoxide formed on photooxidation of organic matter dissolved in surface waters.Superoxide dismutation by copper–zinc enzymes has been reviewed300 and studied for model systems involving complexes of iron,340,341 manganese341,342 and copper.343,344 In the presence of sulfate, [HO 2 ]· oxidizes [Ni([14]aneN 4 )]2` to [NiIII([14]- aneN 4 )(SO 4 ) 2 ]~.345 Reaction of H 2 O 2 with [CrO 4 ]2~ gives rise to the chromium(V) complexes [Cr(g2- O 2 ) 3 (OH)]2~, [CrO(g2-O 2 ) 2 (H 2 O)]~ and [CrVO(g2-O 2 )(H 2 O)n]` as well as the known [Cr(g2-O 2 ) 4 ]3~.346 Related studies on dioxovanadium complexes have also been reported.347 Complexes with singlet O 2 co-ordinated to vanadium(V) have been proposed348 in oxidations involving H 2 O 2 , vanadium(V) and acetic acid.Reaction of H 2 O 2 with MnIII,332 NpVI 349,350 and NpVII,349 [S 2 O 8 ]2~ oxidation of PuVI,351 NpVI,352 UIV 353 and mixed-valence dimeric complexes of ruthenium,354,355 and CoII to CoIII oxidation of [CoIIW 12 O 40 ]6~ by [HSO 5 ]~ 356 have been reported. Catalysis by sulfito-bound cobalt(III) of the self-decomposition of peroxynitrite, 2 [ONOO]~ ]O 2 ]2 [NO 2 ]~,357 a related process involving [FeIII(TMPyP)],358 [MnIII(TMpyP)],306 catalysis of perborate oxidations by [Fe(CN) 6 ]3~ 359,360 or [MO 4 ]2~ (M\Mo, W)361 peracetic acid decomposition catalysed by cobalt(II) and vanadium(V)362 and the disproportionation of H 2 O 2 by manganese,363–370 dinuclear manganese–copper371 and iron372 complexes, by [WO 4 ]2~ 373 and calcium hydroxide374 have been described. The [Fe(H 2 O) 6 ]2`-catalyzed chain reaction, [Rh(H)L]2` V]Bu5OOH]H`][RhL]3`]MeH]Me 2 CO (L\[14]aneN 4 , Me6 [14]- aneN 4 ), involves375,376 rapid hydrogen-atom abstraction from V by methyl radical.Espenson and coworkers and others377–379 report further studies of [ReMeO 3 ]- catalyzed oxidations of arenes,380 alcohols381 (and isomerisation of allylic alcohols382), alkenes,377–379,383,384 hydrazones385 and silyl enol ethers.386 Other non-metal redox reactions The redox chemistry of peroxynitrite, which plays an important role in host defence against invading pathogens, continues to receive intense study.306,357,358,387–393 In neutral solution containing [HCO 3 ]~ and [NO 2 ]~, [ONOO]~ and CO 2 form an adduct,[ONOOCO 2 ]~.This homolyses to generate [CO 3 ]~· which oxidises nitrite to Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 544[NO 2 ]· (which subsequently hydrolyses to [NO 3 ]~ and [NO 2 ]~).387 Carbonateradical production from the same adduct (and subsequent reaction with a series of substrates to give superoxide) has been proposed independently388 in the formation of [O 2 NOO]~ from [ONOO]~.Oxidation of [ONOO]~ by [CO 3 ]·~, [N3 ]·~ and [ClO 2 ]· has also been studied.389 In carbonate- and oxygen-free water, [ONOOH] gives390 ca. 10% [OH]· in addition to nitric acid (formed by collapse of the radical pair [OH·/NO 2 ·]). An oxygen-atom shift from O to N is the preferred394 mechanism from kinetic isotope e§ect studies of the HONO 2 to HNO 3 isomerization.While homolysis is considered unlikely,395 decomposition of [O 2 NOO]~ has been reported to involve, 396 in part, homolysis to give NO 2 · and [O 2 ]·~. Further studies of the oxidation of nitrous acid by H 2 O 2 397,398 and of nitrite by oxygen399 and detailed studies of the thermal decomposition of nitramide400–403 have also appeared.Stanbury404 has reviewed the oxidation chemistry of aqueous hydrazine. New studies of the oxidation by iodine of N 2 H 4 405 and of NH 2 OH406 over wide pH ranges have been reported. At pH\1, I 2 reacts directly with [N 2 H 5 ]`. A proposed I 2 N 2 H 4 adduct loses I~ and H` (general base-assisted) in the rate-determining step at higher pH. A series of pulseradiolysis studies of aminyl and other inorganic nitrogen radicals has appeared.407–409 [NH 2 ]· and O 2 react more rapidly than previously reported with equilibrium formation of [NH 2 O 2 ]· which protonates and then isomerizes, giving NO which then reacts to give [ONOO]~.407 The self-reaction of [NH 2 ]· and its reaction with NH 2 NH 2 have also been studied.408 Iodide is oxidised by N-chloro compounds (for a general review see ref. 410) via rate-determining transfer of Cl` to I~411 in a process subject to general-acid catalysis. Rates of oxidation of aqueous arsenic(III) to arsenic(V) are relevant to the persistence412 of arsenic(III) entering the environment from geothermal sources. [(H 2 N) 2 CS], [(H 2 N)(HN––)CSO 2 H] and [(H 2 N)(HN–– )CSO 3 H] all give [SO 4 ]2~ on oxidation with oxyhalogen species.413 Oxidation of hypotaurine (H 2 NCH 2 CH 2 SO 2 H) and taurine414 (and formaldehyde415) by [ClO 2 ]~ and of cysteine by [BrO 3 ]~ 416 has also been studied.N-Bromination without C–S cleavage occcurs in the reaction of acidic bromate and taurine.417 Thiosulfate reacts with chlorine dioxide with irreversible formation of [S 2 O 3 ClO 2 ]·2~, which then forms [S 4 O 6 ]2~ and [ClO 2 ]~ via [S 4 O 6 ]·3~.418 Ab initio calculations on [HSO 3 ]~ and SO 2 oxidation by H 2 O 2 ,419 [SO 3 ]2~ oxidation by [BrO 3 ]~,420 S-nitrosothiol ascorbate reactions,421 and formation of chlorine-atom adducts with R 2 S422 have been described.In studies relevant to the chemistry of the troposphere, rate constants for the equilibrium, Cl·]Cl~a[Cl 2 ]~·423 for the reactions of Cl· and [Cl 2 ]~· with water (in two independent studies423,424) and for [Cl 2 ]~· and [OH]· 424 have been reported. Oxidation of elemental mercury by aqueous [OCl]~/HOCl (using chloramine as a reservoir) may also have implications for atmospheric chemistry.425 Oxidation of [ClO 2 ]~ by HOBr to produce ClO 2 and [ClO 3 ]~ involves426 a steady-state intermediate ([HOBrOClO]~or [HOBrClO 2 ]~) which gives [BrOClO] or [BrClO 2 ] by a general-acid catalyzed reaction, the latter reacting with OH~ (or H 2 O) to give [ClO 3 ]~ and Br~ and with [ClO 2 ]~ to give 2ClO 2 and Br~.The disproportionation of chlorous acid in strong acid427 and the role of chloride ion in the reaction of chlorous and hypochlorous acids428 are important industrially. ClOO and Cl formation and OClO reformation in the photochemistry of chlorine dioxide has been studied Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 545in aqueous and acetonitrile solution.429–431 Bromate formation, of serious environmental concern in the ozonation of drinking water, arises from the oxidation of bromine atoms by O 3 .432 The kinetics have been reported433 of reaction of [e(H 2 O)n]~ and [BrO 3 ]· (generated from [SO 4 ]~· and [Br- O 3 ]~) and of [BrO 3 ]· dissociation to [BrO]·.Further studies have appeared dealing with geminate recombination following triiodide photodissociation434 and dissociation of iodine encapsulated in cyclodextrin.11 Oscillating reactions and chemical chaos The activation energy for the oscillation of a wholly inorganic Belousov–Zhabotinskii (B-Z) reaction based on435 [BrO 3 ]~–[H 2 PO 2 ]~–MnII–[Fe(phen) 3 ]2`–H 2 SO 4 has been estimated from the variation with temperature of the oscillating period.Related B-Z systems with amino acids or peptides,435,436 organic acid–ketone mixtures,437–440 cyclohexanedione441,442 (a related metal-free 1,4-dihydroxybenzene –acidic bromate system exhibits Landolt-type dynamics443), bromomalonic acid,444 vanillin,445 pyrogallol446 or gallic acid447,448 as organic substrates have also been described.Oxidation of bromomalonic acid by CeIV 449 proceeds via two pathways, one at a low [CeIV]: [bromomalonic acid] (leading to bromoethenetricarboxylic acid, CO 2 and Br~) and one when this ratio is high (leading to complete oxidation to CO 2 ).A ‘‘Radicalator’’ model simulates observations in a B-Z system, controlled by the concentration of radicals (such as the malonyl radical) rather than by bromide, in which Hacac introduces an induction period before oscillations start.450 Photoinduced bifurcations of the B-Z reaction in a continuous-flow stirred tank reactor (CSTR) above a critical level of irradiation,451 stochastic resonance on illumination of a B-Z medium unable to support sustained wave propagation,452,453 light- flux and flow-rate control in a model photosensitive system,454,455 transient response to pulsed visible light,456,457 modulation of behaviour under continuous illumination, 458 and periodic and chaotic chemiluminescence459,460 in ruthenium(II)-catalysed B-Z reactions and the e§ect of an external emf461,462 of oxygen,463–465 and of oxygen and surface-to-volume ratio on chemical oscillations in mm-sized droplets466 have all been described.Pulsed-visible light responses467 and other phenomena468 of the [Fe(CN) 6 ]4~–H 2 O 2 –[SO 3 ]2~ system have also been examined. Chemical waves have been studied in catalytic membranes,469 polymer gels,470,471 beads of ion-exchange resin,472,473 and capillaries in which surface tension e§ects are suppressed.474 Redox indicators475 and a platinum indicator electrode476 reveal the otherwise invisible spatial patterns of uncatalyzed bromate oscillators.Microelectrodes also reveal macroscopic concentration gradients (arising from a coupling of chemical reaction with micromixing) in the most turbulent zone of a B-Z reaction in a CSTR.477 Electrode geometry e§ects have been noted478 for the oscillatory behaviour observed in [IO 3 ]~ alkaline reduction.Chemically-coupled Marangoni instabilities induced by surface tension gradients have also been proposed.479 Mixing-e§ects on the kinetics of the ferrous-ion reduction of nitrite in a CSTR have been studied.480 Spatial and temporal inhomogeneities in the coupling of two identical B-Z oscillators481 and growth dynamics of Turing patterns in a ClO 2 –I 2 –malonic acid–polyvinyl alcohol reaction –di§usion system482 have been reported.The suppression of oscillations in the Bray–Liebhafsky (BL) reaction, the [IO 3 ]~-catalyzed disproportionation ofH 2 O 2 ,483 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 546by the removal of iodine vapour from the reaction solution, confirms the importance of iodine interphase transport.484,485 Iodine autoxidation in acidic solution486 is also relevant to an understanding of the BL system. The 1H NMR signal for solvent water in the BL reaction displays a periodic chemical shift and line splitting.487 The role of bulk water was also revealed in related studies using D 2 O.488 pH and potential oscillations have been studied for the H 2 O 2 –[SO 3 ]2~–[CO 3 ]~–H 2 SO 4 system.489 Studies of the Briggs–Rauschler reaction, [IO 3 ]~–H 2 O 2 –organic substrate–Mn@ (n`1)`,490–492 reveal photoinduced chaos linked to photoautocatalysis of HIO 2 ,491 and suppression of oscillations by Br~.492 The autocatalytic exothermic oxidation of thiourea by chlorite493 also displays a range of non-linear behaviour. 3 Substitution High pressure kinetic approaches to the study of the interactions of small molecules with transition metal centres494 and the chemistry of ruthenium–polyaminocarboxylate complexes495 have been reviewed. Six-co-ordination Wilkins has produced a short overview496 of substitution processes in labile octahedral metal complexes.The forward and back rate constants (25 °C; pH 3.5) for the equilibrium, [Cr(H 2 O) 6 ]2`]bipyma[Cr(bipym)(H 2 O) 4 ]2`, are497 1.6]108M~1s~1 and 4.3]104 s~1, respectively. Complexation of [M(H 2 O) 6 ]n` VI (M\Fe; n\3) by 2-acetylcyclohexanone,498 2,6-dimethylheptane-3,5-dione,499 acetoacetamide500 [via the amide-tautomer; copper(II) and dioxouranium(VI) also studied] and a 3-hydroxy-4-pyridinone,501 of VI (M\Al; n\3) by dihydroxychalcones, 502 amino acids,503 b-diketones,504 and of VI (M\Ga; n\3) by substituted 8- hydroxyquinoline ligands505 have all been reported. Formation and dissociation of [Mn(bipy)(H 2 O) 4 ]2` at pH[6 involves506 only the unprotonated ligand. 1: 1 complex- formation of [Mg(H 2 O) 6 ]2` with monoprotonated methyl thymol blue is more rapid than that with the neutral ligand, ascribed507 to ligand intramolecular hydrogen- bonding.Reversible complexation between [Co(H 2 O) 6 ]2` and 2-(2- aminoethyl)benzimidazole and related ligands has also been reported.508 Binuclear complexation between VI (M\Co; n\2) occurs with both [CoIII(tetren)(Hpyca)]3` and [CoIII(pyca)(tetren)]2` (via an I$ mechanism) whereas forM\Ni (n\2) reaction only proceeds with the deprotonated form.509 Dissociation of [MCoIII(pyca)(tetren) NM(H 2 O) 4 ]4` is acid catalysed for M\Co but not for M\Ni.Bimetallic intermediates are also formed510 in the transfer of a tetradentate ligand from [MIICl 2 L] [L\bis(2-picolyl)-1,3-dithiopropane] to CuII. Studies involving ternarycomplex formation between [CoIII(pyca)(NH 3 ) 5 ]2` and [NiL(H 2 O) 6~n]m,511 and between [CoIII(nsa)(en) 2 (NH 3 )]2` and [Ni(H 2 O) 6 ]2` or [NiL(H 2 O) 4 ]2`,512 have also been reported. The substitution, [Ru(H 2 O) 6 ]2`]L][Ru(H 2 O) 5 L]2`]H 2 O (L\CH 2 –– CH 2 ) is confirmed513 to be I$.The ligand exchange, [Ru(H 2 O) 5 L]2` ]L@][Ru(H 2 O) 5 L@]2`]L (L\L@\CH 2 –– CH 2 , dmso, CO) involves rate-determining loss of axialH 2 Oand the intermediacy of trans-[Ru(H 2 O) 4 LL@]2`.Slower loss of equatorial H 2 O leads to cis-[Ru(H 2 O) 4 LL@]2`. Substitution of adenosine, L, in Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 547Fig. 1 Heterotrinuclear dichromium monorhodium aqua ion.518 [Rh(OH)(H 2 O) 5 ]2` to give [Rh(OH)(H 2 O) 3 L]2` is514 an I! process in the pH range 3.0–4.3.An initially-formed ion-pair between [CrIII(OH)(H 2 O) 5 ]2` and [ox]2~ gives515 [CrIII(ox)(OH)(H 2 O) 3 ] via an I! process. Anation of [CrIII(ox)(H 2 O) 4 ]` by glycine516 has also been studied. Substitution by dma of terminal H 2 O in [CrIII 3 (l3 - O)(l-O 2 CR) 6 (H 2 O) 3 ]` is dissociatively activated517 with changes in rate reflecting the influence of the r-donor character of RCO 2 on the strength of the cis-Cr–OH 2 bond.Acid cleavage of [RhCr 2 (l-OH) 4 (H 2 O) 9 ]5` (Fig. 1) giving [M(H 2 O) 6 ]3` (M\Rh, Cr) in a 1: 2 ratio follows518 [H`]-dependent and [H`]-independent (I$) pathways. Bridge-cleavage studies have also been reported for [MCr(phen) 2N2 (l-OH)a(l-O) 2~a]n` (a\2, n\4; a\1, n\3; a\0, n\2),519 [MCo(Hbig) 2N2 (l-OH) 2 ]4`520 and [O(tmpa)VIV(l-O)VVO(tmpa)]3`.521 Anations of [Cr(acac) 2 (H 2 O) 2 ]` and [Cr(N 3 )(acac) 2 (H 2 O)] are522 substantially more facile for N 3 ~ than for SCN~.Acid-catalysed aquations of [Cr(N 3 )(acac) 2 (H 2 O)] and [Cr(N 3 ) 2 (acac) 2 ]~ and anations of [Co(salen)(H 2 O) 2 ]`523 and of [Co(Hbig) 2 (H 2 O) 2 ]3`524 have been reported. Forward and reverse rate constants, k& and k$, for aqua substitution in [Fe(CN) 5 (H 2 O)]3~ by S- and N-heterocycles, 525 4- and 3-pyOR526 (R\H, Me), 3-methyl and 3-phenylsydnone527 have been measured.For 4-pyOH k& is [103-fold slower than for L\4-pyOMe, ascribed to tautomerism in the former. The sydnones are also relatively unreactive, arising from their mesoionic character rather than from a change in mechanism.The aqua-substitutions [IrCl 5 (H 2 O)]2~]MeCN,528 [Ru(NH 3 ) 5 (H 2 O)]2`]2-pyX (X\Cl, F)529 and [Ru(edta)(H 2 O)]~]2-mercaptobenzoic acid,530 cysteine, HSCH 2 CH 2 OH, glutathione531 and 4-sulfanylpyridine,532 have all been studied. For 4-pySH, both Nand S-bound species are formed with second-order kinetics.532 The N-co-ordinated species isomerizes slowly to the thermodynamically-favoured S-bound isomer by a dissociative process. Chloride-loss from cobalt(III) complexes continues to be studied, particularly by House and co-workers.533–536 House has concluded533 that no correlation exists between ground-state d C0–C- and the solvolysis reaction rate for chloropentaaminecobalt( III) complexes.The loss of the first Cl~ from trans-(R,S)(S,R)- [MCl 2 (223-tet)]` VII (M\Co, Cr) gives trans-aquo, chloro-complexes.534 However, forM\Co further aquation leads to trans-to-cis-b isomerization and, forM\Cr to Cr–N cleavage.Hg2`-assisted chloride-loss from VII, M\Co, gives trans-[M(223- tet)(H 2 O) 2 ]3` followed by trans-to-cis-b isomerisation. The kinetics of chloride loss from cis-a- and cis-b-[MCl 2 (amp)]` (M\Co, Cr),535 trans-[CrCl 2 (Me 8 [14]ane- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 548N 4 )]`537, cis-[RuClL(dppe) 2 ]`538 and from cis,trans-[RuCl 2 (Me 2 NCH 2 CH 2 PPh 2 ) 2 ],539 aquation of [Co(diox) 2 (NO 2 )X]n,540 [Cr(NCS)L]~, [CrL(NCSHg)]` (L\R-pdtrp, edtrp),541 and [Co(ebb)L@]2` (L@\Gly, Ala),542 and cis- [Co(CO 3 )([15]aneN 4 )]`,543 anation of [Cr(ox) 2 (H 2 O) 2 ]~ by CO 2 ,544 [Co(cyclen)( NH 3 )(H 2 O)]3` by N 3 ~545 and of [RuI(NH 3 ) 5 ]2` by SCN~546 and the substitution of X in [PtIVMe 3 X(N–N)] (N–N\phen, bipy) by Y\SCN~, N 3 ~547 have also been described.The relative rates of Hg2`-assisted removal of Cl~ from [CoCl(N) 5 ]2` [(N) 5 \tetren, (tacn)(diamine), mer-(dien)(diamine) and related complexes] suggest536 that the trajectory of the incoming H 2 O is a major factor influencing the interchange mechanism.trans-OSO 2 -O (L) in [CrL(salen)L@]n has little labilising e§ect on the loss of L@\NCS, N 3 , py, Him.548 Similar rate constants for bipy dissociation from [Fe(bipy) 3 ]2` and from [Fe(bipy) 2 (dmf) 2 ]3` suggest that the latter is also a complex of low-spin t 2' 6 iron(II).549 Complexation with 1,4,7-tris(2,2@-bipyridin-5-ylmethyl)- 1,4,7-triazacyclononane was also studied.Rate constants for the processes, [NiX(bipy) 2 (dmf)]`]bipy][Ni(bipy) 3 ]2`]X~ and [NiX 2 (bipy) 2 ]]bipy] [Ni(bipy) 3 ]2`]2X~ (X\Cl, NCS) are significantly smaller than for [Ni(bipy) 2 (dmf) 2 ]2`]bipy][Ni(bipy) 3 ]2`.550 Ternary complex formation between [Cr(nta)(H 2 O) 2 ] and Mordant Orange 3551 or 3-phenylazo-5-sulfosalicylic acid,552 [Ni([12]aneN 4 )(H 2 O) 2 ]2` and phen, en or Gly,553 and between [FeL(NCMe) 2 ]2` ML\N,N@-dimethyl-2,11-diaza-[3.3](2,6)pyridinophaneN and dbsq554 and the formation of [Al(edta)F]2~ in an I! process555 have been studied.Axial dmso,556,557 CO,558 1-Meim,559H 2 O,560 NO,561 andONO561 substitutions in six-co-ordinate phthalocyanine556,558 and porphyrin557,559–561 complexes of iron(II),556–558 chromium(III),559,560 and ruthenium(II)561 have been analysed kinetically. The resolution of the sequential replacement of dmso of [Fe(pc)(dmso) 2 ] by pyor Him into two observable processes has been confirmed.556 Formation of [Fe(pc)(CN) 2 ]2~ from [Fe(pc)(CO)(dmso)] does not occur via formation of [Fe(pc)(CN)(CO)]~. The rate of cyanide binding558 is governed by the rate of CO loss, with dmso-lability reduced by the trans-CO.Base hydrolysis Syn(N),anti(O)-, syn(O),anti(N)- and syn(N),syn(O) forms of [CoM(S)-AlaON(cyclen)]2` equilibrate in base more rapidly than they hydrolyse to give [(S)-AlaO]~ and cis- [Co(OH) 2 (cyclen)]`562 via rate-determining loss of [AlaO]~ from ring-opened [CoMg1-(S)-AlaON(OH)(cyclen[H)]. OH~-catalyzed hydrolyses of [CoX(NH 3 )(cyclen)] n` have also been studied.545 The proportions of trans-[CoX(NH 3 ) 4 (NH 2 Me)]2` VIII (X\NO 2 , ONO) formed in competition between nitrite and water in the base hydrolysis of VIII (X\Cl, Br, NO 3 ) di§er563 from those observed in similar competition studies with [CoX(NH 2 R) 5 ]2` (R\H, Me).Isomerisation of the nitrito to the nitro forms of VIII in base is an intramolecular conjugate-base process564 occurring with full stereochemical retention.Base hydrolysis of [Co(OSO 2 -O)(tetren)]` is faster than for [Co(OSO 2 -O)(Meim)(en) 2 ]`, the latter only displaying isomerization (in excess sulfite) to [Co(SO 3 -S) 2 (en) 2 ]~.565 Isomerization in base566 of trans-meso- to trans-primary, rac-[CoMeL(H 2 O)]2` (L\5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca- 4,11-diene) involves inversion at a secondaryN–H.Base hydrolysis (by a D CB pathway) of ab(R)-[Co(tetren)(dmf)]3` is more rapid567 than that of the Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 549aa-isomer (and occurs without hydrolysis of dmf). Equilibrium constants have been measured568 for 1: 1 complex formation between OH~ and [Pt(N–N) 2 ]2` (N–N\bipy, bipyz, bipym, bipdz). Solvent and medium e§ects have been described for the base hydrolyses of [CoL(tetren)]2` ML\O 2 C(CH 2 )nNH 2 , n\1, 2569 or O 2 CC 6 H 4 OH-p570,571N and of [Fe(N–N) 3 ]2` (N–N\1,2-diazabutadiene or Schi§ base ligands).572,573 Four-co-ordination [Pd(H 2 O) 4 ]2` reacts reversibly with both [HSO 4 ]~ and [SO 4 ]2~ whereas [Pd(OH)(H 2 O) 3 ]` reacts predominantly with [HSO 4 ]~.574 Biphasic kinetics575 are observed for parallel formation of [Pd(O-O 2 CCH––CHCO 2 H)(H 2 O) 3 ]` IX from [Pd(H 2 O) 4 ]2` and H 2 mal and [Hmal]~.Ring closure gives a product also formed by irreversible attack of free ligand via the olefin moiety on IX. Complex formation between [Pd(H 2 O) 4 ]2` and (S)-carboxymethyl-L-cysteine576 and thioglycolic acid577 and between tu and [PtL(H 2 O) 2 ]2` (L\en, phen)578 has also been described.Bicarbonate- inhibited and bicarbonate-promoted reactions account for the complex kinetics observed579 for reactions of [PdX(dien)]` with [HCO 3 ]~. Hydrogen-bonding e§ects have been noted580 on rates of substitution of bipy by en in [Pd(bipy)(R 4 en)]2` (R\H, Me, Et, Ph).In chloroform, the rate of dmso exchange in [PtMe(dpa)(dmso)]` is little a§ected581 by either ion-pair formation or ligand deprotonation. In methanol, the complex shows little nucleophilic discrimination in reactions with a series of charged nucleophiles. The use of PtII (and more recently PtIV 582,583) complexes in anti-tumour therapy584 continues to be a major motivation for mechanistic and related structural work, including studies of interactions with DNA and synthetic oligonucleotides.585–590 Chloride loss from cis-[PtCl 2 (NH 3 )(CyNH 2 )]591 Ma metabolite of the orally-active PtIV anti-cancer drug, cis,trans,cis-[PtIVCl 2 (OAc) 2 (NH 3 )(CyNH 2 )]N, from cis- [PtCl 2 (NH 3 )(n-Mepy)] (n\2, 3),592 and displacements of Cl~ in [PtII 2 Cl 2 (hdta)]2~ by inosine593 have been described.The rates of substitution of Cl~ by H 2 O, and of H 2 O by 5@-GMP cis to 2-Mepy in cis-[PtCl 2 (NH 3 )(2-Mepy)] are ca. 2–12-fold slower than for the analogous processes with the corresponding 3-Mepy complex.594 The kinetics of the reaction of cis-[PtCl 2 (NH 3 )(n-Mepy)] and glutathione,594 of L-cysteine595 and cysteine-derived ligands596 with [Pt(en)(H 2 O) 2 ]2`,595,596 and [Pd(en)(H 2 O) 2 ]2`,596 of L-MetH with cisplatin597 and with [PtCl(15N-dien)]`598 have also been measured.[Pt(dien)(L-MetH-S]2` X, dominant at neutral pH, forms [Pt(L-Met-N)(dien)]` at pH[8. In acid, [Pt(dienH-N,N@)(L-Met-S,N)]2` is produced (half-life of days at pH 4.0) which slowly forms X. Hydrolysis for both Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 550[PtCl(dien)]` and [PtCl(NH 3 ) 3 ]` (and fast reverse Cl~ anation for the aqua complex formed) has been characterised kinetically.599 Rate constants for the equilibrium, [PtCl(terpy)]` XI]L a[PtL(terpy)]`]Cl~ (L\NH 3 , pyridine derivatives)600 reveal the e§ect of basicity, p-acceptor character and steric e§ects of L.When L\thiol substituted with OH, CO 2 H, NH 3 `,601 the e§ect of hydrogen-bonding with the departing Cl~ is seen. Compound XI reacts with cysteine and thioglycolic acid according to second-order kinetics,602 whereas methionine is unreactive under identical conditions. Competitive binding603 between XI and the sulfur-containing peptide, Ac-Gly-Met, and 5@-GMP has also been studied. Pt–S cleavage in [Pt(terpy)MSCH 2 CH 2 N(CH 2 CH 2 NH 2 ) 2N]` at pH 7–8, leading to XI, is accelerated by MCl 2 in the order M\Zn[Cu[Ni.604 Rapid intramolecular disproportionation for dinuclear [MPt(en)ClN2M(l-L-S) 2N]2` [L\C 2 H 4 (NMeCSNMe 2 ) 2 ] yields [PtCl 2 (en)] and [Pt(en)L]2`.605 Bridge-cleavage of [PtII 2 Me 4 (l-SMe 2 ) 2 ] byMe 2 S in CH 2 Cl 2 gives [PtIIMe 2 (SMe 2 ) 2 ] without intermediate formation.606 Bulky tmtu in [PtCl(tmtu)L]` (L\en, dach) decreases both the rate of Cl~ hydrolysis and binding of 5@-GMP and the nucleopeptide r(GpG)605 compared with related reactions for cisplatin.The rate ratio of the two steps (ring opening and ligand loss) in cbcda removal in the acid hydrolysis of carboplatin, cis-[Pt(cbdca)(NH 3 ) 2 ], is greater at lower acidity.607 Displacement of cbdca from the carboplatin analogues, [Pd(cbdca)L] (L\en, bipy), by tu, tmtu, I~ or 5@-IMP occurs consecutively by associative processes.608 Equilibria relevant to the role of platinum complexes as anti-cancer therapeutic agents involving acid–base609 and outer-sphere complexation (‘‘supermacrochelate’’ formation) of platinum(II)–nucleotide complexes, such as cis- [Pt(dGMP) 2 (NH 3 ) 2 ]2~,610 cis-[Pt(dGuo)(dGMP)(NH 3 ) 2 ],611 cis- [Pt(dCMP)(NH 3 ) 2 ],612 or [Pt(5@-GTP) 2 (en)]n~ (formally n\8)613 have been described. At pH 5 and room temperature, [MPt(en)(5@-GTP) 2NLa(H 2 O)n]5~, in which La3` is bound by the two triphosphate residues, is more stable than [La(edta)- (H 2 O)n]~.613 Temperature dependent line-broadening of the 1H NMR spectrum (at pH 7) is ascribed to atropisomerization of the * and " head-to-tail forms, trapped by La3` complexation, and not to La3` exchange.Chelate-ring conformational e§ects on atropisomerization rates in [Pt(G) 2 L] (L\S,R,S,R-, S,R,R,R- and R,R,R,R-bmap, G\9-substituted guanine) have been reported.614 Mixtures of the A-frame complexes [Ni 2 (C––CH 2 )X 2 (dppm) 2 ] (X\NCS, Cl) redistribute X according to an I! process.615 The uncatalyzed cis–trans isomerization of [Pd(C 6 F 5 ) 2 (tht) 2 ] involves a non-rate-determining dissociation of tht and rate-limiting topomerization of the three-co-ordinate intermediate.616 cis-[PdRI(PPh 3 ) 2 ] (R\C 6 Cl 2 F 3 ) converts617 to the trans-isomer by four pathways, two of which are initial direct and solvent-assisted replacement of PPh 3 giving cis-[MPdRI(PPh 3 )N(l- I)MPdR(PPh 3 ) 2N] followed by rearrangement to the trans-isomer and cleavage by PPh 3 .Isomerization also occurs via pseudo-rotations in [PdRI(thf)(PPh 3 ) 2 ]. Ringclosure for cis-[PtPh 2 (CO)MPh 2 P(CH 2 )nPPh 2N], to give [PtPh 2MPh 2 P(CH 2 )nPPh 2N] is rapid for n\2, 3 but observable for n\1, 4.618 The doubling of the observed rate of reaction of an excess of (R)-1,1@-bi-2-naphthol (H 2 binol) with [Pt(O 2 CO)M(S,S)- chiraphosN] to give619 [Pt(binol)M(S,S)-chiraphosN] compared with that for (S)- H 2 binol is ascribed to di§erences in stereochemical interactions between ligands across the square plane of co-ordination.Replacement of chloride in trans- [AuIIIX 2 Cl 2 ]`@~ (X\CN~ XII, NH 3 XIII), by the thione moiety in 4-thiouridine Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 5515@-monophosphate or 4-thio-2@-deoxyuridine are associative interchange processes [with reactivities of the gold complexes some 106-fold greater than the analogous platinum(II) complexes]. For XIII, the rates are similar for the two ligands, indicating620 that the outer-sphere electrostatic association with the phosphate group adjacent to the preferred binding site does not further accelerate an already rapid process.For XII, the nucleotide reacts significantly more slowly than the thiodeoxyuridine. In related reactions with thione-containing single-strand 16-omer oligonucleotides, XIII reacts more quickly than with the corresponding nucleoside whereas XII reacts more slowly,621 the e§ect being primarily reflected in di§erences in *St.The acceleration of the substitutions, [Fe 4 S 4 X 4 ]2~]EtSH][Fe 4 S 4 X 3 - (HSEt)]~]X~ (X\Cl, PhS) with increasing [lutH`] arises from protonation of the cluster core, probably at l3 -S. Core diprotonation further labilises the complex to thiolate exchange whereas, for [Fe 4 S 4 Cl 4 ]2~, chloride substitution is inhibited.622 At very high [lutH`], a kinetically characterized623 thiol dissociation from triprotonated [FeS 2 (SH) 2 (SPh) 2 (HSPh)] to give [FeS 2 (SH) 2 (SPh) 2 ] may lead to reduction of H` to H 2 or of added C 2 H 2 to C 2 H 4 .Intermediate formation and rates of assembly of the [Fe 4 S 4 ]-centre have been studied in the reaction of the apo form of the high potential iron protein from Chromatium vinosum with iron(II) and thiol.624 Five-, seven- and higher-co-ordination Rapid formation of an isolable 2: 1 *R-thepc12 (L18):EuIII intermediate, in which L18 is bound only by the OH moieties of eight pendant CH 2 CH(Ph)OH groups, is followed by a much slower first-order process to give the eight-co-ordinate 1: 1 complex.625 Rate constants for the "a* [Eu(pydca) 3 ]3~ interconversion in a racemic mixture have been obtained626 using time-resolved chiroptical luminescence.The transition state for formation of [FeII(CO)(PhBzXy)]2` from [FeII(PhBzXy)]2`, Fig. 2, involves almost complete association of CO into the cyclidene cavity and is followed by a change in spin state at the six-co-ordinate iron(II).627 CO loss involves a late transition state, with a change of spin state, movement of FeII out of the ligand plane and partial CO dissociation.CO photodissociation from a fully reduced cytochrome bo 3 628 involves CO transfer from Fe to Cu B . Rates of O 2 and CO binding to a series of capped iron porphyrin complexes629 and of H 2 O rebinding to ferrous sperm whale myoglobin630 have also been reported. Pyridine for water substitution in trigonal bipyramidal [Cu(Me 3 tren)(H 2 O)]2` follows an I! mechanism whereas the analogous process for [Cu(Me 6 tren)(H 2 O)]2` is more complex, involving two parallel competing paths, one a dechelation reaction.631 The log of the second-order rate constant for the I! axial substitution of SbPh 3 in square pyramidal [CoIIIL 2 (SbPh 3 )] (L\4,5-dichloro-1,2-benzosemiquinonediiminato),632 is a linear function of the nucleophilic reactivity of the entering ligand.The kinetics have been reported for axial co-ordination of im, Meim to [Fe(tmtp)Cl] in acetone,633 concerted coordination of two py to the axial sites of photoexcited [Ni(oep)],634 for the reaction ofNO 2 , NO,O 2 and py with [MnII(tpp)] following photodissociation of [MnIII(tpp)(ONO)] in toluene, 635 for NO][MIIMmeso-tetra(p-X-phenyl)porphyrinN] (M\Co, Mn; X\H, NO 2 , OMe),636 NO][CoPc]637 and for NO dissociation from [MII(tpp)(NO)] (M\Fe, Co).638 [ReIII(CN) 7 ]4~ is the ultimate product, formed via [ReVO 2 (CN) 4 ]3~, Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 552Fig. 2 Iron(II) cyclidene complex [FeII(PhBxXy)]2`.627 from [ReV 2 O 3 (CN) 8 ]4~ and CN~ after a fast pre-equilibration.639 Thermal reactions of photogenerated [M(CN) 7 (OH)]4~ with 5-NO 2 phen (M\W)640 and triethanolamine (M\Mo, W)641 have also been studied.Mechanisms of reactions of other high co-ordination number lanthanide and actinide complexes are discussed in the following two sections. Ligand exchange The rate constant for water exchange on [Al(OH)(H 2 O) 5 ]2`642 is 104-fold greater than that for [Al(H 2 O) 6 ]3`, a process also studied computationally.643 Water lability for [AlL(H 2 O) 4 ]`, where L are geochemically important ligands,644 is greater for methylmalonate than for [ox]2~. Limiting dissociative exchange for water cis and trans to the inert bridgingOHin [Rh(H 2 O) 4 (l-OH) 2 Rh(H 2 O) 4 ]4` occurs with similar rates.645 Exchange646 of H 2 O in p- and t-[CoXL(H 2 17O)]3` (L\tren, cyclen, NMecyclen; X\NH 3 , H 2 O) and for OH~ in [Co(OH)(tren)X]n` (X\NH 3 , n\2; X\OH~, n\1) is much slower than for [Co(p-OH)(tren)(t-H 2 O)]2` and [Co(OH)(cyclen)(H 2 O)]2`. Ligand–OH~ exchange proceeds via an aqua–conjugatebase complex, [Co(NR 2 )(H 2 O)]2` in equilibrium with [Co(NHR 2 )(OH)]2`.Associative water exchange in [PtMC 6 H 3 (CH 2 NMe 2 ) 2 -2,6N(H 2 O)]` is 107-fold faster647 than for [Pd(H 2 O) 4 ]2`. The faster exchange for H 2 O approximately trans to l-oxo in [Mo 3 (l3 -S)(l3 -O) 3 (H 2 O) 9 ]4` compared with that forH 2 Oapproximately trans to l-S arises648 from a pathway involving [Mo 3 (l3 -O) 3 (l3 -S)(OH)(H 2 O) 8 ]4`.Ab initio calculations have been made for water exchange,643,649–651 density functional theory applied to intramolecular oxygen exchange in [UO 2 (OH) 4 ]2~ 652 and molecular dynamics simulations for water exchange on Li` and Na`.653 Calculation649 replicates the experimentally observed D process for water exchange in [M(H 2 O) 6 ]3` (M\Al, Ga) whereas forM\In, an A or I! process is preferred.(New experimentally estimated lower limits for water exchange for [In(H 2 O)n]3`, [Lu(H 2 O)n]3` and [Zn(H 2 O)n]2` of 1]107 s~1, 1]107 s~1 and 5]107 s~1 respectively, were also reported.649) The structure of [MII(H 2 O) 7 ]2` (M\first-row transition metal), a putative intermediate or transition state for water exchange, is computed650,651 to be a pentagonal bipyramid distorted in the equatorial plane.Associative processes are feasible for water exchange for MII with less than seven d electrons, with cis-attack Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 553Fig. 3 Two diastereoisomeric representations (H 2 O molecules omitted) of [Eu(dotem)(H 2 O)]3`.660 predicted for VII (d3), FeII (d6) and CoII (d7) and trans-attack for CrII (d4).Solvent exchange for [Mn(NCR) 6 ]2` becomes less associative with increasing bulkiness of R.654 Reactivity towards dissociatively activated cyanide exchange in [MOX(CN) 4 ]n~ (M\WIV,MoIV, ReV, TcV, OsVI, X\O2~, OH~, H 2 O,CN~, HCN) follows the order MIV[MV[MVI.655 Oxygen exchange in [H 5 IO 6 ],656 ligand exchange in [VO(His) 2 ]657 and [GdIII(ATP) 2 (H 2 O) 2 ]5~ 658 have also been studied.The search for improved contrast agents for magnetic resonance imaging659 is dependent on understanding and control of water exchange rates.660–664 In a signifi- cant advance, rate constants for the dissociative exchange of water in the square antiprismatic M, Fig 3(a), and twisted antiprismatic m, Fig. 3(b), diastereoisomers660 of [Eu(Ln)(H 2 O)]3` (Ln\dotam) have been found to di§er by a factor of ca. 200. Water exchange has also been studied for [Gd(ttaha)(H 2 O) 2 ]n`,661 neutral [Gd(dtpabmea)( H 2 O)]662 and [Gd 3 (taci[3H) 2 (H 2 O) 6 ]3`.663 Related studies of proton exchange and water proton relaxivity have also been reported.663–675 Inter- and intra-molecular exchange of L and of F~ in [UVIO 2 LF 3 ]n~ (L\[ox]2~, [CO 3 ]2~, [pyca]~) has been studied further.676 Reactions of co-ordinated ligands and linkage isomerism [CoL(NH 3 ) 5 ]3` (L\carboxylate O-bonded phthalamic acid) slowly solvolyses in aqueous acid, whereas the amido N-bonded linkage isomer undergoes amido-N to amido-O as well as amido-N to carboxylato rearrangement in addition to solvolysis. 677 In excess sulfite, cis-[CoL(OSO 2 -O)(en) 2 ]` (L\Hbim, Meim) isomerizes565 by loss of L and formation of trans-[Co(SO 3 -S) 2 (en) 2 ]~.cis-Labilisation by O-bound sulfite and steric acceleration were noted. The keto-form of [CoMO 2 CC(O)MeN(NH 3 ) 5 ]2` gives [CoMO 2 CC(SO 3 )(OH)MeN(NH 3 ) 5 ]` on reaction with [SO 3 ]2~ and [HSO 3 ]~ rather than ligand substitution.678 Chen and Shepherd show that complexes formed between [RuII(hedta)(H 2 O)] and L\pyz,679 2,3- Me 2 pyz,680 pym,681 pydz,682 6-azauridine683 are fluxional, with the Ru centre migrating between various N-bonded and j2-bonded sites.[ReBr(CO) 3 L] (L\bbip, bmbip, bdmbip, btmbip,684 or mcpt, mcpmt, mmtt, bmtt685), [MReBr(CO) 3N2 L] [L\bis(terpy) ligands],686 fac-[PtXMe 3 L] (L\bip687 or mcpt685), [M(C 6 F 5 ) 2 (mcpt)] (M\Pd, Pt),685 [Ru(edta)L]~ (L\benztriazole)688 and trans- [Pt(o-tol)(PEt 3 ) 2 (N 3 Ar 2 )]`,689 [PdMeX(bppy)]690 and tris(pyrazolyl)triazine pallad- Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 535–591 554ium complexes691 Mbut not [W(CO) 5 (pyz)]692N display similar metallotropic processes. Syn-anti isomerization of four-co-ordinate [Pd(g3-C 5 H 9 )(j2N,S-bmtpy)]` proceeds via an associative process involving attack by the non-co-ordinated sulfur at Pd.693 The linkage isomerization [W(CO) 5MjP-PPh 2 CH 2 CH(PPh 2 ) 2N] a[W(CO) 5MjP-PPh 2 CH(PPh 2 )CH 2 PPh 2N] is also associative, being 104-fold faster694 than [W(CO) 5MjP-PPh 2 CH 2 CH 2 P(p-tol) 2N]][W(CO) 5MjP-(ptol) 2 CH 2 CH 2 PPh 2N].[Co(O 2 CO)L]n` (L\tripodal tetradentate ligands, 5-dptma, i-dtma, aeida) hydrolyse in aqueous acid via [H`]-dependent and [H`]-independent processes, one involving the inert cis-[Co(O 2 CO)(H 2 O)L]n` formed from the intermediate cis-[Co(O 2 COH)(H 2 O)L](n`1)`.695 [Co(O 2 CO)(Meim) 4 ]` hydrolysis has also been studied.696 Rhodium(III)-amine-catalyzed methylglyoxal–lactate transformation involves697 a 1,2-hydride shift in a chelated substrate.Mechanistic reports have appeared related to RNA/DNA cleavage,698–707 nerve gas disposal,708–711 and prodrug and drug behaviour.712,713 More specifically, reports have appeared exploring metal-complex-promoted hydrolysis,689,700,701,703, 704,708–712,714–744 dephosphorylation730,745 or transesterification746–749 of carboxylic esters,712,715,718,725,726,729,733,740,746 peptides,709, 719,720,722,724,727,732,743 phosphate mono-,705,709,715,730,736,744 di-700,701,703,704,716,717,731,734,737,738,747, 748 or tri-esters,708,710,711,714,723, 726,727,729,735,741,742,744,749 phosphorothiotes,728 phosphonate mono-708 or di-esters710,721 by chiral palladacycles,715 mono-709 or di-nuclear738 cobalt(III), nickel(II)–terpy–poly(ethylenimine),748 copper(II)705,708,710, 714,717,721–723,726,728,729,745 complexes (including a dinuclear cis-diaqua-calix[4]- arene compound734 and a trinuclear ATP-AMP complex730), alkaline earth complexes of thiol-pendant crown ethers,746 magnesium(II),735 zinc(II),728 cadmium(II),728 palladium(II)-aqua,719,724,732,743 -thiolato,720 platinum(II),712 or zinc(II)-azamacrocycle714,717,726,733,740 complexes, mixed zinc(II)–lead(II)-compartmental ligand,741,742 cerium(IV),737 europium(III),747,749 gadolinium,728 and other lanthanide( III)718,736,739 ions, complexes of lanthanum(III) Mwith the bu§er Bis-Tris [2,2- bis(hydroxymethyl)-2,2@,2A-nitrilotriethanol]N,716 and of zirconium(IV).731 Catalytic activities718 for lanthanide ions in the hydrolysis of methyl and ethyl esters decrease along the series: CeIII, NdIII[SmIII[EuIII[GdIII[CeIV[PrIII[DyIII[TbIII[ ErIII[HoIII, TmIII[LaIII, LuIII, YbIII.Hydrolysis of phenyl acetates in the presence of [Zn([9]aneN 3 )(H 2 O)n]2` does not involve nucleophilic attack by a co-ordinated water or OH~, but water or OH~ attack on a co-ordinated ester.740 The catalysis by [Zn(OH)(tren)]` and [Cd(OH)(Me 6 tren)]` but not [Zn(OH)(Me 6 tren)]` of the hydration of acetaldehyde is ascribed to steric inhibition of the displacement of MeCH(OH) 2 by water.750 A nonadecapeptide (being the C-terminal segment of the protein myohemerythrin) which binds the cis-[Pd(en)(H 2 O)]2` moiety to His-5 and His-9 is regioselectively cleaved via the His-5 residue at the Val-3-Pro-4 amide bond.719 While K M values from the saturation kinetics seen for the hydrolysis of [P(O)(OAr) 2 O]~ (Ar\C 6 H 4 NO 2 -4), in the presence of Ln3` are similar,700 k#!5 increases 60–70-fold for La3` to Er3` and decreases for Yb3` and Lu3` (correlated with Ln ion diameter). Copper(II)-promoted hydrolysis of nucleoside 5@-triphosphates to nucleoside 5@-diphosphates and inorganic phosphate is further facilitated730 by the antiviral 9-[2-(phosphonomethoxy)ethyl]adenine.The role of weak aromatic-ring stacking interactions745 in influencing such processes has been discussed.Lanthanides more e¶ciently cleave 3@-AMP and 3@-GMP (to adenosine and guanosine respectively) Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 555compared with the 5@-mononucleotides.736 Zinc(II) has been implicated751 as a cofactor in the 1017-fold enzymatic acceleration of the decarboxylation of orotidine 5@- monophosphate (t 1@2 for the spontaneous process\7.8]107 years).Heterodinuclear iron(III)–zinc(II) complexes are responsible for the synergy701 between iron and zinc in the hydrolysis of the RNA-model phosphate diester, adenylyl(3@-5@)adenosine. Hydrolysis of the latter, accelerated 105-fold by [Co(trien)(H 2 O) 2 ]3`, displays703 a maximum at pH 7 and a D 2 O solvent isotope e§ect of 2.0.Adenylyl(3@-5@)adenosine hydrolysis, catalysed by lanthanum(III) or neodymium(III), may be further significantly accelerated by the addition of hydrogen peroxide.704 However, the accelerated phosphate mono- and di-ester hydrolysis observed706 in the presence of H 2 O 2 and [Cr 3 O(OAc) 6 (H 2 O) 3 ]` is not the mechanism by which this combination cleavesDNA (which is believed to involve hydroxyl radicals).Cleavage of a plasmid DNA can be accelerated by dinuclear macrocyclic complexes of lanthanides.699 Hydrolytic scission of phosphodiester linkages in a DNA 22-mer by cerium(IV) has been demonstrated.702 Methanol-loss from the phosphonate monoester, [PhP(O)(OMe)O]~, bonded as a bridging ligand in a dinuclear cobalt(III) complex (along with bridging peroxide), is accelerated ca. 1011–1012-fold compared with the non-co-ordinated substrate.752 Peroxide was shown not to be involved in intramolecular nucleophilic catalysis; rather, intramolecular catalysis by co-ordinatedOH~ is observed. The half-life for the hydrolysis of [P(O)(MeO) 2 (OH)] (calculated at pH 7) is reduced by CeIV from 8454 years to 22 min.737 While [Fe(S 2 MoS 2 )(bipy) 2 ] is a more active catalyst than [Ni(S 2 MoS 2 )(bipy) 2 ] in the hydrolysis of MeCN to MeC(O)NH 2 and MeC(O)OH,753 the Ni complex is the more selective towards acetamide formation.Base hydrolysis of O-bonded dmf in [Co(NH 3 ) 5 (dmf)]3` gives the formato ligand 104-fold faster than the free ligand.722 Aminoguanidinium hydrolysis,754 and urea755,756 and acetonitrile755 alcoholysis have been studied. Kaminskaia and Kostic� show756 that cis-[Pd(en)(H 2 O) 2 ]2` XIV accelerates 105-fold the conversion of urea and MeOH into methyl carbamate and ammonia, by initial urea binding to PdII, direct methanolysis of O- and N-bound urea, formation of N-bound carbamic acid, the methanolysis of the latter and the fast dissociation of NH 2 C(O)OMe.Intramolecular alcoholysis in cis-[PdL(H 2 O) 2 ]2` (L\2,6-dithiaoctane-1,8-diol) is 100-fold faster than for intermolecular alcoholysis by XIV. Amides of a-amino acids718 are more readily hydrolysed by cerium(IV) than by lanthanide(III) ions. Hydrolysis of azomethine moieties in trinuclear cobalt(II) complexes757 and of benzidine-Schi§ bases by MII, M\Fe, Co, Ni, Cu,758 has also been studied.The rate constant for reaction of OH~ with co-ordinated NO in [RuII(NH 3 ) 5 (l-NC)Os(CN) 4 (NO)] increases759 105-fold on one-electron oxidation of RuII. Metal-ion complexation with macrocycles The predominantly dissociative process for Li` exchange in [Li(C221)]` in methanol between 5 and 25 °C changes to predominantly associative between [30 and [60 °C.760 Tl` and hexacyclen behave similarly,761 whereas for hexamethylhexacyclen the process is dissociative over the entire temperature range.In MeCN–MeNO 2 mixtures, Li` exchange in [Li(C222)]` is bimolecular over the entire temperature range studied.762 Intra- and inter-molecular exchange kinetics for *[M(L18)]` Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 556(M`\Li`, Na`, K`, Cs`) in dmf763 and the solvent dependence of ligand exchange in [Ba(18-crown-6)]2` have also been described.764 Formation of a 1: 1 complex between Cs` and 5,11,17,23-tetra-p-tert-butyl-25,26,27,28-tetramethoxycalix[4]arene proceeds via initial complexation of its partial cone (pc) conformer followed by a change to the 1,3-alternate (alt,3) conformer, from which Cs` cannot directly escape.Exchange between the two Cs` binding sites in the latter involves interconversion to the pc complex.765 An intramolecular racemisation process has been characterised for the binuclear complexes [Lu 2 L(dmf) 5 ] (L\p-nitro- or p-tert-butyl-calix[8]- arenes).766 Acid dissociation kinetics for [CuL]2` (L\L19 767 and L20 768) have been described. In studies on ultra-rigid ligands,769 a lower limit for the half-life for the dissociation of the cross-bridged L21 from [Cu(L21)]2` in 1MHClO 4 at 40 °C is[6 years, the complex being ca. 100-fold less labile than even the square-planar red isomer of [Cu(tet-A)]2`. Substitution of CuII by NiII in [Cu(amben)] in dmf–[NR 4 ]X follows complex kinetics, with the overall reactivity trend order X\[ClO 4 ]~ \Br~\SCN~@Cl~ linked770 to the tendency to form nickel complexes with reduced co-ordination number. [FeIII(salmH)(H 2 O) 2 ]2` forms from the ligand and [Fe(H 2 O) 6 ]3` or [Fe(OH)(H 2 O) 5 ]2` by I! and I$ mechanisms, respectively.771 Iron(III) transfer from ferrioxamine B ([Fe(Hdfb)]`) to edta, catalyzed by [ox]2~,772 proceeds by four parallel paths involving [Fe(ox)(H 2 dfb)], [Fe(ox) 2 (H 3 dfb)]~ and [Fe(ox) 3 ]3~ which react with [H 2 edta]2~, with the observed dependency on [H`] ascribed to fast protonation of both ferrioxamine B and [ox]2~.Albrecht-Gary and Crumbliss773 have reviewed mechanisms of iron(III)–siderophore complexation. Removal of Zn2` from Zn 7 –metallothionein by nta follows biphasic kinetics in contrast to a hybrid partially silver-substituted Zn 4 Ag 6 –metallothionein.774 This di§erence arises from di§erences in reactivity of the metal bound to the C-terminal a- and the N-terminal b-domains.Step-by-step ring closure accounts for the three stages revealed for the complexation of [Ln(H 2 O)n]3` (Ln\Eu, Gd, Tb) with arsenazo III775 and par or pan.776 Di§erences in pressure dependence of the first stage for the three metals reflect changes in co-ordination number along the lanthanide series.775,776 The reduction by a-, b- or c-cyclodextrins of the second-order complex formation rate between M2` (M\Co, Ni, Cu, Zn) and 5-Br-paps is ascribed777 to the consequences of binding between ligand and cyclodextrin.The kinetics of formation of [Ce(dota)]~ Annu. Rep. Prog. Chem., Sect.A, 1999, 95, 535–591 557studied at higher pH than hitherto,778 suggest the intermediacy of [Ce(H 2 dota)]` and [Ce(Hdota)] and rate-determining general-base-catalyzed deprotonation of [Ce(Hdota)] (H 2 O-assisted at pH\7.5 and OH~-assisted at higher pH) followed by rearrangement. Related studies on the formation and dissociation of cerium(III) complexes of tetraazadioxa macrocyclic tetracarboxylates779 and of europium(III) complexes of hepta- and octa-dentate tetraaza macrocycles with a range of aryl, hydroxyethyl, carbamoylmethyl and carboxamidoarylalkyl pendant groups749 have also been described.In contrast to the acid-promoted dissociation of the [Eu- (H 2 O)n]3`–edta complex, the dissociation of edtp from its complex with europium(III) is inhibited by protonation.780 Stopped-flow EXAFS7 has been used to establish the structure of the heterobinuclear intermediate formed during CoII for HgII exchange in the homobinuclear [Hg 2 (tpps)]2~.[Cu(H 2 tpp)]2`, a ‘‘sitting-atop’’ complex formed from copper(II) and H 2 tpp in MeCN, deprotonates to [Cu(tpp)] with second-order kinetics on subsequent addition of py.781 [Cu(tpp)] formation in the presence of py was also characterised kinetically.The formation of [Zn(tpp)] from the reaction between H 2 tpp and a mixture of ZnII, CdII and HgII has been described782 in terms of similar intermediates. PbII 783 and CdII,784 respectively, catalyze the reaction of 3,8,13,18-tetramethyl- 21H,23H-porphine-2,7,12,17-tetrapropionic acid with CoII 783 and MnII.784 Kinetics for the reactions of HgII with the water soluble [H 2 tppsBr 8 ]4~,785 of ZnII, CdII and HgII with H 2 tmtp786 and of ZnII with TMPyP2~ 787 have also been described. Kinetics and mechanisms involving metal-containing complex assemblages are now receiving more attention.In benzene, an initially formed 1: 1 complex788 between 18-crown-6 and [Zn(tol) 2 ] more slowly transforms to a threaded rotaxane,789 with formation and dethreading both requiring the presence of non-complexed [Zn(tol) 2 ].Reaction of L22 and FeCl 2 in ethylene glycol at 170 °C yids two helicate complexes, 790 a triple helix XV and a circular double helix XVI, with the latter accumulating at longer reaction times. The rates of formation of both species increases with reagent concentration, with XV disappearing790 via a double exponential decay, suggesting that there may be a number of pathways leading to XVI.Racemisation Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 558kinetics791 for the bi- and tri-nuclear helicates, [Ti 2 L23 3 ]4~ and [Ti 3 L24 3 ]6~ point to a stepwise non-dissociative mechanism. Fast equilibration between two enantiomeric forms of helical dicopper(I) complexes of p-cyclophane-linked bis(N 2 S 2 ) chelate ligands involve both solvent-dependent and -independent processes.792 Main-group reactions The lower activation barriers for B–N bond-breaking in boranes and boronates containing C 6 H 3 (CH 2 NMe 2 ) 2 -2,6 compared with those containing C 6 H 4 (CH 2 NMe 2 )-2 supports793 an S N 2 process for B–N dissociation.Hydrolysis kinetics for NH 2 CN (and its conversion to dicyanodiamide), (NH 2 ) 2 CNCN,794,795 [NH 4 ]SCN and [NH 4 ]OCN796 have been studied at high temperature (400–575 K) and pressure (275 bar). Rate-determining decomposition of [NH 4 ]SCN proceeds via initial formation of [NH 2 CSO]~ and OCS as undetected intermediates, with the latter rapidly giving CO 2 and H 2 S.[NH 4 ]OCN reacts ca. 3]103 faster at 543K than [NH 4 ]SCN. Isomerization of [NH 4 ]SCN to thiourea in solution797 and the thermal decomposition of solid [NH 4 ][CO 2 NH 2 ]798 have also been described. Liebig and Wohler’s classic conversion of [NH 4 ]OCN to urea (in the solid state) has been reinvestigated.799 The kinetics of formation of a silicate cubic octamer in aqueous tetramethylammonium silicate (pH 13.2–13.6) show800 that the cation participates directly in the formation and stabilization of cage polysilicates rather than in the templating role suggested for molecular-sieve formation.The e§ects of ammonium ions on [SiF 6 ]2~ hydrolysis have been examined801 and substitution processes in fluorocomplexes of the Group III–V elements reviewed.802 Rate constants for Si exchange between [SiO 4 ]4~ and [Si 2 O 7 ]6~ in highly alkaline KOH solution have been reported. 803 [Si 3 O 10 ]8~ and [Si 3 O 8 ]4~ show comparable reactivity in forming or breaking the Si–O–Si moiety. The condensation reactions of [SiH 3 OH] and [SiH(OH) 3 ] have been treated theoretically.804 The [1.1.0]bicyclobutane analogue, [Si 4 (SiBu5Me 2 ) 6 ], rearranges805 to the corresponding cyclobutene analogue via 1,2- silyl migration and not by skeletal isomerization.Study of the addition reactions of silenes806–808 suggests that Me 2 Si–– CHRmoieties are formed as reactive intermediates on irradiation808 of [Me 3 SiSiMe 2 (CH–– CH 2 )] and related compounds in hexane solution. Steric e§ects are responsible809 for the 109–1012-fold slower rate of addition of phenol to [mes 2 Si––Simes 2 ] than that of alcohol addition to [MePhSi––SiMePh].[HN(NO 2 ) 2 ] decomposition in aqueous HNO 3 proceeds401,402 via the mixed anhydride, N 4 O 6 . de Ja� ger and Heyns810 propose that hydrolysis of sodium polyphosphate in water, pH\0, involves nucleophilic attack of water on protonated terminal phosphate.The hydrolysis of [O 3 PSe]3~ has also been reported811 and that of N 2 O 5 812 and SO 3 813 studied theoretically.Rate-determining proton loss from the nitrosated intermediate, [RC(O)NH 2 ·NO]`, is confirmed814 for the reaction of primary organic amides with nitrous acid, with the e§ect of variations in R suggesting that reactivity is controlled by the basicity of the amide favouring the formation of the intermediate rather than loss of a proton.S-Nitrosothiols, RSNO, react with [20- fold excess of RSH to give RSSR with clean first-order kinetics which are una§ected by Cu2` or edta.815 Nitrosation of 2-mercaptopyridine has also been studied.816 Kinetics of [(H 2 N)(HN––)CSSC(NH 2 ) 2 ]` decomposition, catalyzed by [SCN]~, involves817 nucleophilic displacement of thiourea and formation of [(H 2 N)(HN––)CSSCN].In Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 559studies stimulated by its immunomodulatory properties, the reaction of ammonium trichloro(dioxoethylene-O,O@)tellurate(IV) with cysteine is shown818 to be first order in [Te] and second order in [RSH]. Exchange between [MBu5Ga(l3 -Te)N4 ] and the element, E\S, Se, is first order in complex, to give [MBu5Ga(l3 -E)N4 ], and is faster for S than for Se.819 All mixed Te–E cubanes were observed.Understanding the aqueous chemistry of ClO 2 is important in understanding its role in the atmosphere. Under photolysis at 400 nm, aqueous ClO 2 su§ers 93^3% dissociation to ClO and O (which may recombine in the solvent cage) and 7^3% to Cl and O 2 . As photon energy is raised, cage escape of O increasingly occurs.820 Further studies of the femtosecond dissociation of [I 3 ]~ in solution have been reported. 12,13 Iodine–magnesium exchange in the system PhCH 2 CHI 2 and Pr*MgBr occurs via kinetically characterized821 [PhCH 2 C(I)HIPr*]MgBr. 4 Organometallics r-Bonded organotransition-metal compounds Further studies have addressed cobalt–carbon bond cleavage in coenzyme B 12 (5@- deoxy-5@-adenosylcobalamin, AdoCbl),822–826 modified cobinamides,827–829 methylcobalamin (MeCbl)824,830 and model831–833 systems as well as processes834–836 mediated by B 12 and related systems.Co–C homolysis for AdoCbl, weakly bound to ribonucleotide triphosphate reductase,823 is accelerated 1.6]109-fold at 37 °C (associated with a much reduced *Ht and very similar *St) compared with the nonenzymatic thermal homolysis. 27% of the initially photoexcited MeCbl homolyses830 on the sub-ps timescale, with the remainder giving a metastable cob(III)alamin photoproduct which deactivates within 1.2 ns. On Co–C photohomolysis826 in AdoCbl (also studied by time-resolved photoacoustic calorimetry824) 76^4% geminate radical pairs recombine (biphasically) and 24^4% give solvent-separated radicals.The much reduced rates831 of Co–C thermal homolysis (in the presence of a spin trap) for [CoIIIR(salen)] [R\(CH 2 )n, n\3, 4, bridging cobalt and ligand intramolecularly] compared with non-bridged analogues are linked to in-cage radical recombination.Cage e§ects in these systems have been reviewed.827 Photolytic metal–alkyl bond homolysis121,128 of [ReR(CO) 3 (N–N)] (R\alkyl, benzyl; N–N\4,4@-Me 2 bipy, Pr* 2 dab) occurs131,837 via a triplet-state precursor.CO loss and isomerization occurs838 on irradiation of trans,cis-[RuRI(CO) 2 (4,4@-Me 2 bipy)] (R\Me) whereas Ru–C homolysis occurs for R\CH 2 Ph. The kinetics of Co–C homolysis in [CoRL] (L\tpp, tmp, R\CMe 2 CN; L\tap, R\CMe 2 CO 2 Me, CHMePh) have been investigated832 by 1H NMR line-broadening.Oxidation of [PtMe 2 (N–N)] XVII (N–N\R 2 dab) in MeCN to a mixture of [PtIIMe(NCMe)(N–N)]` and fac- [PtIVMe 3 (NCMe)(N–N)]` occurs839 via methyl transfer involving [PtIIIMe 2 (N–N)]`·. Reduction of XVII has also been reported.840 [RhII(por)] react with alkyl halides via S N 2 reactions of [RhI(por)]~ formed by disproportionation rather than via radical processes involving the rhodium(II) species.841 Structure-activity e§ects in the alkylation of [Co(dmgH) 2 (PBu 3 )] by alkyl chlorides have been reported.842 [SnBu 3 ]~ reacts with primary butyl halides as a nucleophile843 and sec- and tert-butyl halides by electron transfer.844 New data at pH 7–10 reveal845 that [ReMeO 3 (OH)n]n~ (n\1, 2) are involved in the base hydrolysis of [ReMeO 3 ] to CH 4 and [ReO 4 ]~.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 560Photolytic ring-opening of [FeR(CO) 2 Cp] (R\2,2-dimethylcyclopropylmethyl) by a concerted four-centre rearrangement is preferred846 to a radical process. Thermolysis of [MIV(CH 2 R)(N 3 N F )] MM\Mo, W; 3 N F ]3~\ [(C 6 F 5 NCH 2 CH 2 ) 3 N]3~; R\Me, Pr/, SiMe 3 , CMe 3N is a first-order process847 leading to [MVI(N 3 N F )(–– – CR)] and H 2 .a-Hydrogen exchange has been directly observed848 between [W(CH 2 Bu5)(SiBu5Ph 2 )(––CHBu5) 2 ] and [W(CH 2 Bu5) 2 (SiBu5Ph 2 )(–– – CHBu5)]. [Ta 2 (CH 2 R) 4 Cl 2 (––CHR) 2 ] (R\SiMe 3 ) eliminates RMe with first-order kinetics to give849 the unstable intermediate [Ta 2 (CH 2 R) 3 Cl 2 (–– CHR)(–– – CR)]. The absence of deuterium incorporation into CHPh of [RuDX(PBu5 2 Me) 2 (––C––CHPh)] (X\Cl, Br) suggests850 that [RuX(PBu5 2 Me) 2 (–– – CCHDPh)] is not readily accessible.Calculations suggest that while [RuHCl(PH 3 ) 2 (––C–– CH 2 )] and [RuHCl(PH 3 ) 2 (–– – CCH 3 )] are close in energy, a very high activation barrier exists between them. (See also refs. 851, 852. Migratory insertion processes of these and related compounds are described in the next section.) Ph 2 S is 4–5 times more reactive than cyclooctene towards [Fe(CH 2 )(CO) 2 Cp]`, formed reversibly by loss of Ph 2 S from [Fe(CH 2 SPh 2 )(CO) 2 Cp]`.853 Isomerisation of [MHCl(CH 2 )(CO)(PBu5 2 Me) 2 ] (M\Ru, Os) Mfrom [MHCl(CO)(PBu5 2 Me) 2 ] and CH 2 N 2N to [M(CH 3 )Cl(CO)(PBu5 2 Me) 2 ] requires854 phosphine dissociation.The rate of the reaction [Ti(g2-N 2 CHR)Cp* 2 ]]PhCH––CH 2 ] [TiMCH(R)CH(Ph)CH 2NCp* 2 ] (R\C 6 H 4 X-4) via a complexed carbene intermediate855,856 is insensitive to X. The role of ruthenium–carbene complexes in olefin metathesis,857 reactions of carbene complexes of Cr,858–863 W,860,864 and of allenylidene complexes of Mo and W865 have been studied. The e§ect has been described866 of phosphine, L, on the rates of equilibrium approach between the two isomers of [Os 3 (l-g2-CH––CH 2 )(l-H)(CO) 9 L].A l3 -CH 2 -bound intermediate is proposed867 in CH 2 exchange between the two RhCo(l-CH 2 ) sites in [(CoCp) 2 (RhCp*)(l-CH 2 )(CO) 2 ]. Related studies868 have been described for [Ru 2 (l- CH 2 )(CO) 2 (NCMe)Cp 2 ]. Theoretical studies rule out [WVI(HC–– – CH)] and [WVI(C––CH 2 )]869 as being of importance in acetylene polymerisation processes.The isomerization [Ti(g3-C 3 H 5 )(fv)Cp*] to [TiM(E)-CH––CHMeN(fv)Cp*] proceeds870 via [Ti(g2-CH 2 ––C––CH 2 )Cp* 2 ], [Ti(g1-CMe––CH 2 )(fv)Cp*] and [Ti(g2-CH–– – CMe)Cp* 2 ]. (See also ref. 871.) The [AuR(tht)]-catalyzed isomerization of trans- to cis-[PdR 2 (tht) 2 ] involves872 associative aryl-exchange between palladium(II) and gold(I) and ratedetermining tht loss from palladium.The interchange of agostic and non-agostic methyls and exchange of hydrogens within the agostic methyl groups in the b-agostic complex [PdMCH(CH 2 -l-H)(CH 3 )N(RN––CR@CR@––NR)]`873 have similar activation barriers. Carbonyl-insertion and alkyl-migration reactions [FeMe(CO) 2 Cp]874 and [FeMC(O)MeN(CO) 2 L] (L\Cp, ind)875 lose CO to give the corresponding monosolvento complex on photolysis.Using time-resolved infrared spectroscopy,876 the *Vt for trapping of the intermediates with CO (or other ligands) to give [FeMe(CO) 2 L] is more negative than that for the corresponding methylmigration reaction.875 When L\ind, ring-slippage is not favoured.Added salts, NaI, NaPF 6 or CaI 2 , increase877 the rate of the migratory-insertion reaction, [FePr*(CO) 2 L]]L@][FeMC(O)Pr*N(CO)L@L] for L@\PPh 2 CH 2 (aza-15-crown-5), Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 561L\ind, whereas there is no e§ect for L\Cp. The methyl group in [CrMe(CO) 3 (g5- ind)] migrates first to C-1 and then irreversibly, via a trimethylenemethane-type transition state,878 to C-3 of the ind moiety in [Cr(CO) 3 (g6-Meind)].Iridiumcatalyzed methanol carbonylation879 involves CO migratory insertion that is faster for [IrMeI 2 (CO) 3 ] than for [IrMeI 3 (CO) 2 ]~. Phosphite-induced CO insertion of the latter is inhibited by iodide ion and involves880 initial [IrMeI 2 (CO) 2MP(OR) 3N] formation. Rhodium-catalyzed carbonylation881,882 of CH 2 I 2 to malonate esters by [RhI(CO)(PEt 3 ) 2 ] involves loss of I~ from [RhMC(O)CH 2 INI 2 (CO)(PEt 3 ) 2 ], ROH addition to the resulting ketene complex [Rh(CH 2 ––C–– O)I 2 (CO)(PEt 3 ) 2 ]` then CO insertion to give [RhMC(O)CH 2 CO 2 RNI 2 (CO)(PEt 3 ) 2 ]. Insertion of norbornadiene into the Pd–acyl bond of [PdMC(O)MeN(L 3 )]` (L 3 \a tridentate nitrogen ligand883 or Cl with a bidentate nitrogen ligand884) involves partial dissociation of the chelated ligand.Migratory insertion in [PdMe(CO)(dippe)]` to form [PdMC(O)MeN(CO)- (dippe)]`885 and in [CuIIMe(CO)(H 2 O)n]2` to [CuIIMC(O)MeN(H 2 O)n]2`886 have been studied. Insertions of CH 2 ––CHCO 2 Me into PtII–Me,887 alkenes into Zr–C,888 alkynes into RhI–C,889 PtII–Si,890 ArNC into NiII–C,891 CO 2 into Ni–O,892 Ni–C,893 and into O–O in [RhCl(O 2 )(PEt 2 Ph) 2 ],894 and O 2 into CoIII–C895 have all been described.Theoretical studies have been made of CO 2 insertion into RhIII–H,896 of SnCl 2 into PtII–Cl,897 of CH 2 –– CH 2 into [M(C 2 H 5 )L] (M\ScIII, YIII, LaIII, LuIII, CeIV, ThIV, VV,898), TiIV–C,898–900 Zr–C,898,900 Hf–C,898,900 Pd–C,901–903 PtII–H904–906 or PtII–Si906 and of related metal-catalyzed olefin polymerization900,907–910 processes.Experimental studies of iron(II)-911 or palladium(II)-complex912,913 catalyzed ethylene oligomerization, palladium(II)-complex catalyzed copolymerization of ethylene with CO914,915 or methyl acrylate,916 norbornene with CO,917 styrene918 or p-methylstyrene919 with CO have been reported. Shaw has proposed920 a new mechanism for Heck reactions.Rates of Co to N migration of R in [CoR(tpp)]`, formed on oneelectron oxidation of [CoR(tpp)] by [Fe(phen) 3 ]3`, di§er921 106-fold for R\Ph and Bu. (See also ref. 922.) Migration of R from Fe to Nin [FeIVR(oep)]` is faster the more electron-donating is R.923 Values of the activation parameters for the thermolysis of [Rh(13CH 2 CH 2 Ph)(bocp)] to give [RhMCH(13CH 3 )PhN(bocp)] (and other data) support924 a stepwise cis b-hydride elimination/olefin Rh–H insertion mechanism.An elimination/insertion mechanism is also proposed925 for the formation of [RhIIIEt(acac)] from protonation of [RhI(CH 2 ––CH 2 ) 2 (acac)]. [RuHX(H 2 )(PBu5 2 Me) 2 ] (X\Cl, I) reacts with PhC–– – CD to give [RuDX(––C––CHPh)(PBu5 2 Me) 2 ] via alkyne insertion into Ru–H and a-H migration in [RuC(D)–– C(H)Ph] to give [RuD(––C––CHPh)].926 [RuHClL 2 ] (L\PPr* 3 ) reversibly binds EtOCD–– CH 2 , with D incorporation into RuH and P–Me.Slower formation of [RuHClM–– CMe(OMe)NL 2 ] follows.927 [IrEt(CH–– CD 2 )(PMe 3 )Tp@] reacts with D` to give two rotameric forms of [IrEt(––CHCD 3 )(PMe 3 )Tp@]` XVIII which then gives [IrH(CHMe––CHCD 3 )(PMe 3 )Tp@] (without D incorporation at vinylic sites).In rate comparisons with [IrH(––CHMe)(PMe 3 )Tp@]` XIX it is concluded928 that migratory insertion of ––CHMe into Ir–H of XIX is faster than into Ir–Et of XVIII. Related protonation processes of [Rh(Me 3 [9]aneN 3 )(CH––CH 2 ) 3 ] have been described.929 Ligand-displacement reactions of metal carbonyl and other low-valent compounds At high [L] (L\Pr 2 dab, 4,4@-Me 2 bipy), when a competing reaction to give [Cr(CO) 6 ] Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 562is unimportant, the thermal ring-closure reaction of [Cr(CO) 5 L] is dissociatively activated930 with positive values of *Vt. Rates of ring-opening have been measured for [W(CO) 4 L] (L\Bu5SCH 2 CHRSBu5, R\H, Me).931 [M(CO) 4 (phen)] (M\Cr, Mo, W) gives [M(CO) 3 (PR 3 )(phen)] by a dissociative process932 on ligand-field (LF) photolysis whereas charge-transfer photolysis may lead to associative substitution, depending on M and R.Photochemical substitution of [W(CO) 4 (en)] to [W(CO) 4 (NCMe)(en)] is more e¶cient via the LF singlet state than the lowest lying LF triplet state.933 Wavelength and solvent dependence for the photoreaction [Ru(g6- arene) 2 ]2`]3 solv][Ru(g6-arene)(solv) 3 ]2`]arene934 has also been reported.Photochemistry of [Cr(CO) 6 ]14,15 and [Fe(CO) 5 ]16–18 has been studied on the fs timescale. The role of LF and CT excited states in M–CO photodissociation935,936 (also discussed in previous sections) and other aspects of organometallic photochemistry937 –939 have been reviewed.*St for displacement by CO of solvent in [Cr(CO) 5 (RH)] becomes less negative as the alkane chain length increases,940 whereas *Ht remains unchanged. cis- And trans-[Cr(CO) 4MP(OPr*) 3NL] XX (L\ClPh) formed transiently in the presence of P(OPr*) 3 , each react to cis- and trans-XX [L\P(OPr*) 3 ]. The cis isomer very rapidly converts to the trans isomer non-dissociatively.941 Related studies have been reported on [Nb(CO) 3 (g2-C 2 H 4 )Cp] (CO-for-ethylene substitution in supercritical ethylene as solvent),942 [Cr(CO) 5 L] (displacement by alkenes of X-bonded L\PhX, X\Cl, Br,943,944 and of arenes, L\C 6 H 6~nMen 945), [Mo(CO) 2 (alkane)(g6-arene)],940 [W(CO)L(PhC–– – CMe)Tp@]` (exchange of L\Et 2 O by MeCN;946 E/Z isomerisation for L\g1-Me 2 CO947), [Fe(CH 3 )(CO)(solv)Cp],874,875 [M(CO) 4 (g2-CF 3 C–– – CCF 3 )] (M\Fe, Ru, Os) (substitution of CO by PR 3 948), [Mn(CO) 4 (cod)]` (replacement of cod by MeCN949), and [Mn(CO) 3 (g5-C 5 H 4 CH 2 CH 2 X)] (displacement of CO by X\Br, Cl950).Intermolecular CO exchange rates have been measured for [W(CO) 4 L]2~ [L\orotate (L25), dihydroorotate].951 Calculations suggest863 that alkynes react associatively with chromium carbene complexes, [Cr(CO) 5M–– C(OH)(C 2 H 3 )N], in the Do� tz reaction, rather than by an initial CO dissociation.Broadening and coalescence of v(CO) IR bands in [Fe(CO) 3 (g4-cod)] Mand low-temperature studies on stereoselectively labelled [Fe(CO) 2 (13CO)(g4-cod)]N provide estimates952 for a rate constant for CO site exchange at 293K of 1.54]1012 s~1.Isomerization of cis-[Rh(CO) 2 I 4 ]~ to the favoured trans complex involves CO dissociation. 953 Thermal mer to fac isomerization of [MnBr(CO) 3 (N–N)] (N–N\4,4@- Me 2 bipy) in thf involves954 partial dissociation of Br~ whereas for N–N\Rdab, Mn–N cleavage is preferred. Magnetization transfer has been employed955 in a study of the stereodynamics of fac-[ReX(CO) 3 (L26)].Intramolecular CO exchange956 in the metal–metal bonded [RuI 2 (CO) 6 (dppe) 2 ]2` involves pairwise bridging carbonyls. Cleavage of the Mn–Mn bond in Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 563[Cp@(CO) 2 MnMn(CO) 5 ]~ by L\PR 3 gives957 [Mn(CO) 2 LCp@] and [Mn(CO) 5 ]~ with [N(PPh 3 ) 2 ]` as counter-ion and [Mn(CO) 3 Cp@] and [Mn(CO) 4 L]~ with Na`.Calculations and experiment958 suggest that photochemically generated [MFe(CO)Cp*N2 ] is a triplet species with terminal CO which relaxes to a singlet CO-bridged ground-state. [(CO) 5 MnMn(CO) 3 ] has been proposed959 as a possible structure for the intermediate, [Mn 2 (CO) 8 ], formed on photolysis of [Mn 2 (CO) 10 ]. Intramolecular scrambling of CO (with negative *Vt)960 is faster for [Rh 4 (CO) 12 ] than for [IrRh 3 (CO) 12 ] which has apical Ir.Limiting rate constants for the dissociative displacement of L from [Rh 6 (CO) 15 L] by P(OPh) 3 961 span a range of ca. 105 along the series L\dmso, MeCN, thf, EtOH, with evidence that the [Rh 6 (CO) 15 ] intermediate may be stabilised by co-ordination of the solvent CHCl 3 or additional bridging by CO.Isokinetic relationships in reactions of [Ru 5 C(CO) 14MP(OPh) 3N] with P-donors have been studied.962 Phosphines with Tolman cone angles, h, p143° form monosubstituted products from [Os 3 (CO) 9 (l-C 4 Ph 4 )] via associative adducts,963 whereas for hq145° a single bimolecular step leads to both mono- and di-substitution. Joerg, Drago and Sales964 have expanded their database of phosphine basicity parameters, though their approach has been criticised by Giering, Prock and co-workers.965 Arene exchange in triple-decker [(CoCp*) 2Mg4: g4-areneN] proceeds via an equilibrium involving [CoCp*(g6-arene)] and the 14-electron [Co(solv)Cp*].966,967 Similarity of rate parameters suggests968,969 that [RuL(g4-C 5 H 4 O)Cp]` (L\MeCN, py, tu) react with PR 3 to give 1,2-disubstituted ruthenocenes Mvia [Ru(PR 3 )(g3-C 5 H 5 )(g4-C 5 H 4 O)]` involved in a pre-equilibrium between g5- and g3- forms or via [RuL(g3-C 5 H 4 O-2-PR 3 )Cp]` depending on Rand LN.Hapticity changes are also observed in reactions of [Mo(CO) 2 L 2 (g5-ind)]0,` with MeCN (g5 to g3)970 and [Ru(g4-cod)(g6-C 10 H 8 )] with phosphines or phosphites (g6 to g4).971 A dissociative mechanism dominates972 in the endo–exo isomerization of [ML 3Mg4- C 6 Me 4 (CH 2 ) 2 -oN] (L\PMe 3 ) in the presence of L when M\Ru, whereas an addition process, giving r-bonded [ML 4Mg2-C 6 Me 4 (CH 2 ) 2 -oN] dominates forM\Os.Redox reactions Zavarine and Kubiak have reviewed the redox chemistry of 19-electron [W(CO) 5 - L]·~.175 Rate constants for the cis–trans isomerization of 16-electron [ReIII(NCR) 2 (dppe) 2 ]3` (R\aryl) formed by two one-electron oxidations of the rhenium(I) precursor, increase973 with the electron-withdrawing character of the nitrile. The rate constant for axial–equatorial phosphine-related stereo-isomerization in [Ru 3 H 3 (l3 -COMe)(CO) 6 (PPh 3 ) 3 ]n` is ca. 104-fold faster974 for the 47-electron n\1 species than the 48-electron n\0 species.Electron-transfer catalysis accelerates975 the substitution, [FeI(CO) 2 Cp]]PPh 3 ][Fe(CO) 2 (PPh 3 )Cp]`]I~ via more labile 17- and 19-electron intermediates even though the redox process which propagates the catalytic chain is endergonic. The Fe–X relative bond dissociation energy increases as a consequence of oxidation of [FeX(dppe)Cp*]976 along the series F@Cl\Br\I.Facile formation of aldehyde, RCHO, and [Fe(SnR@3 )(CO) 2 Cp] from R@3 SnH and [FeMC(O)RN(CO) 2 Cp] (R\Me, CH 2 CH 2 Ph) is initiated977 by an electron-transfer step. The 17-electron monocation formed from [MoHL 2 (PMe 3 )Cp] XXI (L\PMe 3 ) and Ag` (or Fc`) disproportionates following a second-order rate law,978 a process accelerated by a competing deprotonation by excess XXI.XXI Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 564(L\CO) gives [Mo(CO) 2 (NCMe)(PMe 3 )Cp]`, H 2 and Ag0 via a hydride-bridged adduct, [MoM(l-H)Ag(NCMe)nN(CO) 2 (PMe 3 )Cp]`.979 Unusually stable [RhIIH(CO)(PPh 3 ) 3 ]` displays980 very rapid electron self-exchange with its rhodium( I) precursor. Further oxidation to [RhIIIH(CO)(PPh 3 ) 3 ]` leads to rapid proton loss and reformation of the rhodium(I) parent.[PdIICl 2MPPh 2 C 5 H 4 ) 2 FeIIN] is formed photooxidatively981 in CCl 4 from [Pd0MPPh 2 C 5 H 4 ) 2 FeIIN] 2 via initial oxidation of iron(II) to iron(III) in the redox-active ligand,982 intramolecular electron transfer and disproportionation of a palladium(I) intermediate. Arylpalladium(I) transients are proposed983 in the reduction of [PdAr(PPh 3 ) 2 (solv)]` present in equilibrium with [PdArX(PPh 3 ) 2 ].Redox processes have been reported for cis- and trans- [Re(CO) 2 (dppe) 2 ]`,984 [Re(CO) 3 L(bipy)]` [L\PR 3 , P(OR) 3 ],985 (relevant to photochemical reduction of CO 2 985,986) [MoF(CO) 2 (dppe) 2 ]`,987 [CoI(g3- C 4 H 7 )Cp],988 [Rh(1,n-cod)Cpt] (n\3, 5)989 cis-[Mo(SAr) 2 (CNBu5) 4 ],990 [WX(CO) 2 LCp] (X\Cl, Br, I, L\CO; X\I, L\PCy 3 )991 [UCl(g-C 5 H 4 R) 3 ],992 and (the previously discussed) [FeIIIR(oep)]923 and [CoIIIR(tpp)] (R\CH 2 Ph, Bu).921,922 Homolysis of the Rh–In bond in [Rh(tpp)In(oep)] has beennvestigated. 993 Di§erences in cage-recombination e¶ciencies for the radicals [M(CO) 3 Cp@]· (M\Mo, W) formed from photolysis of [MM(CO) 3 Cp@N2 ] arise994 from di§erences in metal–metal bond energy or from spin–orbit coupling rather than from di§erences in radical mass.Reactions of the spin-triplet [MoCl(PMe 3 ) 2 Cp*] with CO and N 2 have been reviewed.995 Photolysis of [Os 3 (CO) 10 (a-diimine)] leads to zwitterion-forming heterolysis in co-ordinating solvents or to biradical-forming homolysis in non-co-ordinating solvents.996 Successive one-electron oxidations of [MRu(l-AsPh 2 )(CO)CpN2 ] give Ru–Ru bonded species.997 Oxidative addition and reductive elimination [PtIIMg1:g1-(CH 2 ) 4N(bipy)] oxidatively adds EtI ca. 2.5-fold more rapidly than [PtII(CH 3 ) 2 (bipy)].998 Variation with X of overall rates of oxidative addition of PhI in mixtures of [Pd(dba) 2 ] and P(C 6 H 4 X-4) 3 is associated999 with e§ects on the intrinsic reactivity and equilibrium concentration of Pd0 complexes.Oxidative addition of the disulfide in [W(CO) 3 (phen)(MeSSMe)] is slow and kinetically first-order at high [Me 2 S 2 ] and second-order at low [Me 2 S 2 ], consistent1000 with formation of [MW(CO) 3 (phen)N2 (MeSSMe)] prior to S–S cleavage. Aubart and Bergman1001 suggest that diaryl disulfides react with [Cp 2 Ta(l-CH 2 ) 2 CoCp] giving [Cp 2 Ta(l- CH 2 ) 2 Co(SR)Cp] with modest charge separation in the transition state.Stereoselectivity in the oxidative addition of RX to [Rh(CO)Mg1:g5-R 2 P(CH 2 )n(ind)N] (n\2–4) is governed by the spacer length, n.1002 Di§erent stereoselectivies for Ph–H and Cy–H have been observed1003 in photochemical C–H addition at Ir in the pro-chiral ligandcontaining [IrH 2Mg1:g5-Me 2 PCH 2 CMe 2 (C 5 H 3 Bu5)N].[IrMe(C 2 H 4 ) 2 Tp] reacts with benzene to give [MIrMePh 2 TpNnN 2 ] (n\1, 2).1004 The 10–20-fold greater rate of the exchange,1005 [PtR(O 2 CCF 3 )- (dmpe)]`]C 6 D 6H[Pt(C 6 D 5 )(O 2 CCF 3 )(dmpe)]`]RD, for R\C 6 H 5 compared with R\CH 3 supports rate-determining aryl C–D addition to the PtII centre. Spin saturation transfer experiments point1006 to scrambling of the four hydrogens of the OsHMe moiety in [OsIVHMe(dmpm)Cp*]` between [100 and [120 °C [via Os(CH 4 ), a species favoured over OsH 2 (–– CH 2 ) or OsHCH 2 ~]H`] via a di§erent Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 565transition state from that involved in reductive elimination seen above [95 °C. Diastereomeric and isotopic scrambling in [IrRH(PMe 3 )Cp*] [R\(RS),(SR)- and (RR),(SS)-2,2-dimethylcyclopropyl] involves1007 Ir–(r-alkane) intermediates. Cyclohexane activation by [Rh(CO) 2 Cp] proceeds via a single intermediate,1008 the g5 species [Rh(CO)LCp] XXII (L\c-C 6 H 12 ).The complex XXII (L\c-C 5 H 10 ) has been directly observed byNMRspectroscopy.1009 [M(CO) 2 (solv)Cp] (solv\SiHEt 3 ) bonded either via Si–H or Et,1010 formed on photolysis of [M(CO) 3 Cp] (M\Mn, Re),1011 reacts either via Si–H cleavage (on a timescale of 5 ps) or C–H cleavage (230 ns).Successive Si–H and C–H reductive-elimination oxidative-addition1012 leads to deuterium scrambling involving RhH, SiH, RhCH 2 and o-Me groups in fac- [RhHMSi(H)mes(C 6 H 2 Me 2 CH 2 )N(PMe 3 ) 3 ] formed from [RhMe(PMe 3 ) 4 ] and SiD 2 mes 2 .The p CO -dependence of product ratios and kinetic isotope e§ects for CyH dehydrogenation in the presence of [RhCl(CO)(PMe 3 ) 2 ] point to1013 C–H activation via [RhCl(PMe 3 ) 2 ]. C–C rather than C–F cleavage is preferred in the reaction of L27 (R\CF 3 ) with [MRhCl(C 2 H 4 )N2 ].1014 For R\C 2 H 5 ,1015 Ar–CH 2 is cleaved in preference to CH 2 –CH 3 . For R\OMe, Rh inserts into aryl–O at room temperature. 1016 When R\Me, an g2-RhH–CH 2 interaction results in a highly acidic proton in a complex1017 of rhodium(I). Related cleavages1018,1019 have been rendered catalytic. An unusual C–N cleavage in [Co(CNCH 2 Ph)(PMe 3 )Cp] to give [Co(CH 2 Ph)(CN)(PMe 3 )Cp] has also been studied.1020 The unusual thermolysis of [WH 3 (OCH 2 Ph)(PMe 3 ) 4 ] to [WH 2 (CO)(PMe 3 ) 4 ] and PhH occurs1021 viaH 2 elimination. Niu and Hall,1022 being unable to find a r-bond metathesis pathway using DFT methods, conclude that C–H activation in [IrMe(PMe 3 )Cp]` proceeds only via oxidative addition, in contrast to earlier suggestions.Other computational and theoretical studies have addressed activation of C–H,1023–1036 C–F,1035 C–C,1027 Si–H,1027,1037 H–H,1030,1036,1038–1042 O–H1043 and B–B1044,1045 activation by W0,1040 RhI,1023,1031,1035 IrI,1031,1038,1041 MnI and ReI,1010 Fe0,1038 Ru0,1024,1038 Os0,1024,1035 Pd0,1025,1026,1036,1043,1044 Pt0,1024,1027,1036,1042–1044 NiII,1041 PdII,1028,1031 PtII,1031 OsII,1029,1030,1034 and HgII.1032,1033 RH addition in [PtMe(RH)L 2 ]` (R\Me, Et) and RH elimination from [PtHMe 2 L 2 ]` or [PtMe 3 L 2 ]` (L\NH 3 , PH3 ),1045 CH 4 elimination from [WHMeCp 2 ] and [WHMeM(g5-C 5 H 4 ) 2 CH 2N],1046H–C 2 H 5 addition to [IrMe(PH 3 )Cp]`,1047 the rearrangement [MCl(PH 3 ) 2 (HC–– – CPh)] to [MCl(PH 3 ) 2 (––C––CHPh)] (M\Rh, Ir),1048 intermediate formation in the reaction of RH with [Rh(CO) 2 Tp*],1049 and C–S vs.C–H activation in thiophene by [Rh(PMe 3 )Cp*]1050 have been studied theoretically.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 566Fig. 4 Cyclometallated platinum(II) alkyl complex involving a2-(diisopropylphosphino) isodurene.1059 Rates of intramolecular concerted RSR@ loss from [PdR(SR@)(P–P)] (P–P\a chelating diphosphine)1051 depend on the state of hybridisation at C bound to Pd (sp2[spAsp3), are faster for electron-donating R@ and -withdrawing R and increase with diphosphine bite angle.Reductive elimination of RCN from [PdR(CN)(P–P)] (R\CH 2 SiMe 3 ) is 104-fold faster for P–P\diop (bite angle ca. 100°) compared with dppe (bite angle ca. 85°).1052 Multiple H–D exchange prior to methane loss accounts for CH 4 isotopomer formation from [PtMe(OEt 2 -d 10 )(tmen)] decomposition.1053 Reductive elimination of RH from [IrIIIHRMg2-Si(H)(mes)CH 2 CH 2 PPh 2N(PMe 3 ) 2 ] (R\Me, Et) in methanol proceeds1054 via the silylene-bound [IrHMg2- –– Si(mes)CH 2 CH 2 PPh 2N(PMe 3 ) 2 ] and subsequent cyclometallation.Si–H addition of SiPhH 3 to Ta–– N in [TaMeM––N(C 6 H 3 Me) 2 N(SiMe 3 )NCp*] and elimination of SiMe 3 H from [TaHMeM(PhSiH 2 )N(C 6 H 3 Me) 2 N(SiMe 3 )NCp*]1055 and SiMe 4 loss from [W(NO)(CH 2 SiMe 3 )(CPh––CH 2 )Cp*] to give [W(NO)(CPh–– – CH)Cp*]1056 have been studied kinetically.Pt–H and Si–H exchange in cis-[PtH(SiHmes 2 )(PCy 3 ) 2 ] by reductive elimination of SiH 2 mes 2 1057 rather than by PtH 2 (–– Simes 2 ) formation. Hydrogen migration from Pt to Si in the process [PtHMSi(SBu5) 2 (OTf)N(PEt 3 ) 2 ] ][PtH(NCMe)MSiH(SBu5) 2N(PEt 3 ) 2 ]` 1058 involves [PtHM––SiH(SBu5) 2N(PEt 3 ) 2 ]` as an intermediate.The cyclometallated complex cis-[PtR(g1-P)(g2-P[H)] (Fig. 4, R\Me) reacts with SiR 3 H to give SiR 3 Me and (Fig. 4, R\H).1059 Mechanisms of loss of PhH from [IrHClPh(CO)(PPr* 3 ) 2 ],1060 of SnHPh 3 from [RhH(SnPh 3 )(NCBPh 3 )(dmap)(PPh 3 )],1061 of EMePh 3 (E\Si, Ge) from cis- [PtMe(EPh 3 )(PMe 2 Ph) 2 ],1062 and of RONp from [PdR(ONp)(P–P)],1063 have all been described.New proposals for the Heck920,1064 and Stille1065 reactions have appeared. Negative Hammett o values for cyclopalladation in [Pd(L28)(NCMe)]2` point to1066 electrophilic attack of PdII on ortho-benzyl C. Rate constants for cyclopalladation of [Pd(L28)(NCMe)]2`follow the order dmf\dmso@py, which, with other data, establish1067 the importance for C–H cleavage of solvent assistance at the ortho proton.Hydrogen and hydrido complexes No evidence for a r-H 2 adduct was seen in a kinetic study of the equilibrium [RhI(bipy) 2 ]`]H 2a[RhIIIH 2 (bipy) 2 ]`.1068 Self exchange between [RhIIIH 2 (bipy) 2 ]` and [RhI(bipy) 2 ]` proceeds through a symmetric bis(hydridobridged) intermediate. [Ru(H 2 O) 5 (H 2 )]2` has been characterised.1069 Highly siteselective H–D scrambling and diastereomer interconversion for [IrH 2 (Me- DuPhos)(cod)]` formed by H 2 oxidative addition does not involve H 2 reductive Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 535–591 567elimination.1070 Para-hydrogen induced polarisation has been used to study H 2 addition to [TaH(L)Cp* 2 ] (L\benzyne)1071 or [RhI(CO)(PMe 3 ) 2 ],1072 H 2 exchange with [IrH 3 (CO) 3~x(PPh 3 )x] (x\1–3)1073 or [PtH 2 (P–P)] (P–P\unsymmetrical diphosphine)1074 and hydrogenations.1075–1077 Ammonia is formed1078,1079 on treatment of cis-[W(N 2 ) 2 (PMe 2 Ph) 4 ] and [RuCl(H 2 )(dppp) 2 ]` with H 2 at 1 atmosphere. Second-order rate constants, k 2 , for 20 single-step hydride transfers, MH][CPh 3 ][BF 4 ]][M(FBF 3 )]]CHPh 3 , span a range of 107,1080 M\[Mn(CO) 5 ]\cis-[Mn(CO) 4 (PCy 3 )]\[Re(CO) 5 ]; [W(CO) 3 Cp] \[Mo(CO) 3 Cp]\[W(NO) 2 Cp]\trans-[Mo(CO) 2 (PCy 3 )Cp] [trans-XXIII].k 2 is 103-fold smaller for cis-XXIII than for trans-XXIII (at [55 °C). The substitution, trans-[FeH(H 2 )(dppe)]`]L]trans-[FeHL(dppe)]`]H 2 (L\MeCN, dmso) involves1081 rate-determining attack of L on an intermediate with monodentate dppe followed byH 2 displacement on chelate ring closure.Inverse kinetic isotope e§ects for the common first-order reaction of HX and DX (X\BF 4 , Br, Cl, CF 3 CO 2 ) with cis-[FeH 2 L 2 ] [L\dppe,1082 (L 2 )\pp31083] to give trans-1083 or cis-[FeH(H 2 )- L 2 ]` 1082 suggest a late transition state. Related studies on trans- [W(N 2 )(NCR)(dppe) 2 ]1084 and [Ni(dppe) 2 ]1085 have been described.cis- [Re(H 2 )(CO) 4 (PR 3 )]` is su¶ciently acidic to protonate Pr* 2 O.1086 Exchange of thiolate in cis,cis,trans-[RuH(SR)(CO) 2 (PPh 3 ) 2 ] with R@SH involves phosphine dissociation, R@SH co-ordination, intramolecular proton-transfer to SR and loss of RSH.1087 Protonation of trans-[MH(SPh)(dppe) 2 ] (M\Ru, Os) gives1088 trans- [MH(HSPh)(dppe) 2 ]` with one equivalent of H` Mwith no evidence for the formation of [M(H 2 )(SPh)(dppe) 2 ]`N and trans-[Os(H 2 )(HSPh)(dppe) 2 ]2` in excess acid. Protonation at Re1089 of [Re(CO) 2 (MeC–– – CMe)Cp*] precedes H`-migration to alkyne, the resulting 1-metallacyclopropene rearranging to [Re(CO) 3 (g3-exo,anti- MeHCCHCH 2 )Cp*]`.Corresponding processes for [Re(CO) 2 (g2-MeC–– – CMe)Cp] (giving both g3-allyl and g2-allene products)1090 and for isomers of [Mo(g3- C 3 H 5 )(g4-C 4 H 6 )Cp]1091 have also been reported.Theoretical and computational studies have examinedH 2 addition toW0,1039 IrI,1036,1040 Fe0,1038 Ru0,1038 OsII,1030 NiII,1041 Pd0,1036 and Pt0,1036 as well as aspects of stability,1092 reactivity and dynamics1093–1100 or bonding.1101–1104 Relevant experimental studies of reactivity and dynamics1086,1098,1105–1119 and bonding1108,1120 are noted.Cooper and Caulton report1121 that variation in L causes *Ht and *St for exchange between chemically inequivalent hydride sites in [IrH 2 LL@2 ]n` to be varied over a wide range. Solvent and other medium e§ects Studies of solvent,10,111,118,149,150,524,528,562,570,571,573,761,762,807,1067 electrolyte and counter-ion,181,572,801,802,877,957 encapsulation114 and surfactant726,729 e§ects and reactions in supercritical media794–796,942 have been discussed in earlier sections.E§ects of mixed solvents on redox reactions between [Co(en) 2 L]n` (LH\HSCH 2 CH 2 NH 2 , n\2;1122,1123 LH 2 \HOSCH 2 CO 2 H, n\11124) and [S 2 O 8 ]2~ 1122,1124 or [IO 4 ]~,1123,1124 between [Fe(CN) 2 (bipy) 2 ]1125 or [Fe(CN) 3 (terpy)]~1126 and [S 2 O 8 ]2~, [Fe(solv)(dppe)Cp]` and HSR,1127 on solvolytic reactions of [CoBr(Hedta)]~,1128 [CoCl(CN) 5 ]3~,1129 cis- and trans- [CoCl(NO 2 (en) 2 ]` 1130 and on anation of cis-[Co(H 2 O) 2 (big) 2 ]` by glutamic acid1131 have been reported.Ionic strength e§ects have been treated using the mean Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 535–591 568spherical approximation1132 and studied experimentally for [Fe(CN) 6 ]3~ oxidation of ascorbic acid,1133 photoaquation of [Co(CN) 5 (SO 3 )]4~ in the presence of polyammonium macrocycles, e.g. M[32]aneN 8 H 8N8`,1134 ester interchange in non- and weakly- polar solvents1135 and oxidative quenching of [Ru*(bipy) 3 ]2`.1136 Copper(II) dodecyl sulfate produces1137 a 106-fold acceleration in a Diels–Alder reaction by a combination of Lewis acid and micellar catalysis. Surfactants, micelles and microemulsions a§ect nickel(II)1138 and palladium(II)1139,1140 complexation, [Fe(Ph 2 phen) 3 ]2` aquation,1141 electron-transfer quenching of [RuII*(phen) 2M4,7-(~SO 3 C 6 H 4 ) 2 - phenN],1142–1144 and oxidation of ferrocene derivatives.1145,1146 Complexation between 5-alkoxymethyl-8-quinolinoland NiII or ZnII at liquid–liquid interfaces has been studied kinetically.1147 Studies relevant to catalytic processes in novel media include investigations of the reactions [Nb(CO) 3 (g2-C 2 H 4 )Cp]]CO][Nb(CO) 4 Cp] ]C 2 H 4 in supercritical (sc) ethylene942 and [Mn(CO) 2 (g2-HSiEt 3 )Cp*] ]H 2 ][Mn(CO) 2 (g2-H 2 )Cp*]]SiHEt 3 in scCO 2 .1148 Palladium catalysts, solubilised into water using sulfonated dppp, rapidly copolymerise ethylene and COin the presence of excess TsOH.1149 Hydroformylations in fluorous media1150 and in sc CO 2 1151 have also been studied.Dec-1-ene reacts with CO and H 2 in toluene–C 6 F 13 CF 3 in the presence of [RhH(CO)L 3 ] (L\P[CH 2 CH 2 (CF 2 ) 5 CF 3 ] 3 ) though the catalyst is an order of magnitude less active than for L\PPh 3 .Palladiumcatalyzed C–C bond formation processes,1152 molybdenum-catalyzed alkene oxidation1153 –1155 and Friedel–Crafts alkylations1156 have also been studied in sc CO 2 . Photolysis of [M(CO) 6 ] (M\Mo, W) in a polyethylene matrix has permitted1157 the study of the formation and reactions of [M(CO) 5 (g2-H 2 )] and cis-[M(CO) 4 (g2-H 2 ) 2 ].Cage-e§ects on ion-pair recombinations in viscous media have been studied.994,1158 Magnetic field e§ects on redox processes have been the subject of several reports. 1159–1162 Di§erent isotope e§ects, k H‘O /k D‘O , have been reported for solutionand surface-catalyzed proton-coupled electron transfer, [RuIII(OH)(bipy)(terpy)]2` ][RuII(bipy)(terpy)(H 2 O)]2`.1163 The need to understand critical heterogeneous processes in the atmosphere has stimulated further studies of the dynamics of reactions of HOBr,1164 HNO 3 ,1165,1166 and XNO 2 (X\Cl, Br)1167 with solid alkali metal halides, of ClONO 2 , HCl and HOCl1168–1171 or N 2 O 5 , ClNO 2 and HNO 3 1172,1173 on water ice, of CH 3 SO 3 H on water droplets,1174 ofN 2 O 5 and XNO 2 on salt solution droplets1175,1176 and of ClNO 2 , NO 2 and HONOon a sulfuric acid aerosol.1177–1180 References 1 A.Drljaca, C. 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