2 Alkali and alkaline-earth metals I. B. Gorrell Chemical and Forensic Sciences, University of Bradford, Bradford, UK BD7 1DP 1 Introduction This report follows the pattern set in previous years with the emphasis on the organometallic and co-ordination chemistry of Groups 1 and 2 published in 1998. In general, compounds with polydentate or macrocyclic ligands or those of more interest to solid-state chemists, which are covered elsewhere in this volume, have been omitted.As usual, lithium–nitrogen compounds dominate the field but interest in the heavier elements of both groups is increasing. A timely review of recent developments in the organometallic chemistry of the heavier alkali metals has appeared;1 there have also been reviews on the chemistry of cyclopentadienyl alkali metal derivatives2 and the chemistry of alkaline-earth metal bis[bis(trialkylsilyl)amides].3 A summary of work concerning well defined lithium –sodium and lithium–sodium–potassium compounds which can exhibit reactivity superior to that of conventional organolithium reagents has been published, and a new class of lithium–magnesium amide, based around an oxo or peroxo core, presented.4 The chemistry of a-heteroatom-substituted 1-alkenyllithium reagents, initially postulated to be short-lived intermediates in a-eliminations, has also been reviewed.Such species can act as nucleophilic carbanions or as electrophilic carbenoids, depending on the heteroatom substituent.5 The first thermally stable and structurally characterized neutral adducts of isonitriles with some Group 1 and 2 metal complexes have been described including the preparation and crystal structures of [LiMN(R)C(Ph)NC(Ph)–– CR 2N(CNPh)] 2 , [LiNR 2 (CNPh)] 2 , [Mg(CHR 2 ) 2 (CNC 6 H 3 Me 2 -2,6) 2 ] and [Mg(CHR 2 )(l- Br)(CNBu5)] 2 (R\SiMe 3 ).The magnesium compounds are especially noteworthy since they do not isomerise to give the insertion products.6 An in-depth X-ray di§raction study of a series of hydrated 2- and 4-nitrophenoxides of the Group 1 and 2 metals, together with their complexes with the parent phenols, has been published. In general, the lithium and magnesium compounds are discrete molecules whereas the heavier metals form polymeric species.Stacking of the aromatic rings is a persistent theme.7 Chiral lithium, potassium and barium metallocene complexes with a dialkylphosphonium bridge have been prepared.Solution NMR studies and X-ray crystal- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 3lography of [Ba(C 5 H 2 Me-2-Bu5-4) 2 PMe 2 (thf) 3 ][BPh 4 ] and [K(C 5 H 2 Me-2-Bu5- 4) 2 PMe 2 ] show that these compounds form as racemic diastereomers.8 The D([)- mandelate salts, [M(C 8 H 7 O 3 )] (M\Li, Na, K, NH 4 ) and [M(C 8 H 7 O 3 ) 2 (H 2 O)n] (n\0,M\Sr; n\1,M\Ca, Ba; n\2,M\Mg) have been prepared from D([)- mandelic acid and the appropriate hydroxide in water.The X-ray structure of the magnesium complex revealed distorted octahedral geometry at the metal centre and cis-water molecules.9 Several radical anion and dianion alkali metal salts of perylene have been described.All of the radical anion species, [C 20 H 12 ]·~[M` 40-7] and [C 20 H 12 ·~· · ·C 20 H 12 ][M` 40-7] crystallise as solvent-separated ion pairs whereas the dianionic salts, [C 20 H 12 2~(M` 40-7) 2 ] and [C 20 H 12 2~][M` 40-7] 2 strongly depend on optimum solvation of the counter ion for their stability. Calculations were also presented.10 Reaction of trimethylsilyl-substituted phosphanes with alkali metal butoxides has led to the isolation of a series of phosphanide compounds.The crystal structures of [M(thf)P(SiMe 3 ) 2 ] = revealed ladder frameworks with five-co-ordinate phosphorus and three-co-ordinate metal centres.11 A theoretical study of the interaction of alkali metal cations with glycine in the gas phase has been reported.12 A series of new Me 3 SiCH 2 –, (Me 3 Si) 2 CH–, Me2 Bu5Si– and Me 3 Si-substituted cyclopentadienes and their complexes with alkali metals (Li, Na, K) has been presented.13 The crystal structures of a series of adducts of 1,3,4,5-tetramethylimidazol-2-ylidene with MCp 2 * (M\Mg, Ca, Sr, Ba) have been published; the shortest M–C bond is to the former carbene centre but the di§erence between this distance and theM–Cp distance decreases as the atomic number of the metal increases.14 Reaction of Li[C(SiMe 3 )(PMe 2 ) 2 ] with MCl 2 in thf yields M[C(SiMe 3 )(PMe 2 ) 2 ] 2 ·n(thf) (M\Be, n\0; M\Mg, n\2; M\Ca, n\3); with an additional equivalent of the lithium compound the magnesium species yielded the MMg[C(SiMe 3 )(PMe 2 ) 2 ] 3N~ anion.15 2 Lithium Carbon-donor ligands Highly coloured paramagnetic lithium 5,5@- and sodium 6,6@-naphthalenide complexes have been prepared on polymer or oxide supports and characterized by CP MAS, PES, EPR and FTIR.Solutions of organoalkali reagents were obtained, free from arene by-products, by reaction of these supported complexes with organic halides, nitriles and phosphates.16 Electronic structure calculations have provided strong evidence for ate complexes being the key intermediates in lithium (magnesium halide) –halogen (metalloid) exchange reactions.17 Reaction of [Al(NHBu5) 3 ] 2 with LiBu/ generates a complex in which a dimeric LiBu/ fragment is trapped by [Li 3 Al 2 (NHBu5) 3 (NBu5) 3 ].18 Treatment of Bu 3 SnCH 2 PPh 2 with LiBu/ yields [Li(CH 2 PPh 2 )(thf)] = ; X-ray di§raction revealed chair Li 2 C 2 P 2 rings and planar Li 2 C 2 rings arranged alternately.19 Recrystallisation of [LiCH 2 NMe 2 ] from hexane– thf a§orded [LiCH 2 NMe 2 (thf)] 4 the crystal structure of which revealed an open Li 4 tetrahedron with two tetrahedral (CNO 2 ) and two pentaco-ordinate (LiNC 3 ) lithium centres.20 The doubly lithiated aminals [LiCH 2 NR]CH 2 (R\Me, Pr*) have been prepared as insoluble powders which dissolved in organic solvents in the presence of pmdien–thf or tmen–thf so that NMR spectra could be recorded.Ligand transfer Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 4reactions to zirconium were also reported.21 NMR studies indicate that triple ions form in thf–hmpa solutions of a variety of localized lithiated carbanions such as aryllithiums and sulfur- and silicon-substituted alkyllithiums.22 Within the context of the 1,2-addition of LiR to ArCOCF 3 mediated by LiOR occurring with a 50: 1 enantioselectivity, low temperature 6Li and 13C NMR spectroscopies have revealed lithium cyclopropylacetylide (LiR1) to be a dimer–tetramer mixture in thf and 1(R),2(S)-C 4 H 8 NCHMeCH(Ph)OLi (LiOR2) to be a complex mixture of oligomers.In thf, LiR1 and LiOR2 a§orded stoichiometry dependent mixtures of 3: 1, 2: 2 and 1: 3 mixed tetramers.23 A family of unsolvated lithiosilanes have been prepared and characterized as oligomers in non-polar solvents but monomeric in diethyl ether or thf; Li NMR provided a useful structural probe.24 The structures of MCCH (M\Li, Na, K) have been determined using millimetre/submillimetre spectroscopy.25 The transition- metal-like behaviour of lithium has been examined by means of ab initio calculations on the interactions of LiR (R\H, Me, Ph) with benzene and CO.26 1,1-Dilithio-2,2-diphenylethene undergoes rearrangement into (E)-1-lithio-2-(2- lithiophenyl)-2-phenylethene at temperatures above [100 °C; the (Z)-compound forms from the (E)-compound above [25 °C.27 1-Iodo-1-lithioethene has been prepared from 1-iodoethene and LiNPr* 2 in thf and can be trapped by electrophiles.Ab initio calculations suggest that the species is monomeric in thf and that hydride migration and a-elimination of LiI occur in a concerted process.28 Lithiation of 1,2-diphenyl-1,2-bis(trimethylsilyl)ethene using the metal in thf gives the dilithioethane derivative;NMRstudies show high quinoid character in the phenyl rings and relatively mobile cations.29 In the solid state the lithium salt of the octasilyl[4]radialene dianion is a contact ion pair with the metal ions situated above and below the four-membered ring.This structure is maintained in toluene but in thf one lithium dissociates and there is evidence for a ‘lithium walk’ on the eight-centred ten-electron system.30 A mechanism for the reduction of diphenylacetylene by metallic lithium has been proposed.31 The syntheses and structures of [Li 2 (tmen)(g5-C 5 H 4 SiMe 2 CHSiMe 3 )] 2 , a centrosymmetric dimer, [Li 4 (tmen)(g5-C 5 H 4 SiMe 2 )MNC(Bu5)CHSiMe 3N2 ], which contains four di§erent lithium environments32 and [Li(dme)] 2 [C 8 H 6 (SiMe 3 ) 2 -1,4] in which the two lithiums are coordinated to the dianion in a g3-allyl-like fashion33 have been reported.The crystal structure of fluorenyllithium revealed dimers in which the two metal centres are sandwiched between two six-membered rings;34 in the ether complex the metal interacts with the anion in a g2-fashion and this is the optimum structure from quantum chemical methods.35 The preparation and crystal structure of the dilithium salt of hexasilylfulvene have been reported; one of the lithium cations is located above the centre of the five-membered ring whereas the other is bonded to the two carbon atoms which are common to the six-membered rings.This structure persists in solution.36 The preparation and X-ray structure of g3-N-Boc-N-(pmethoxyphenyl)- 3-phenylallyllithium·([)-sparteine have been described together with its use for enantioenrichment.37 The solid state structure of unsolvated, base-free phenyllithium has been determined using synchrotron powder di§raction.The molecules form dimers which are linked to adjacent Li 2 Ph 2 units forming a polymeric ladder structure; the Li 2 C 2 rings are planar and the p-electrons of the phenyl rings interact with the metal centres of neighbouring units.38 The compound forms a mixture of dimers and tetramers in diethyl ether; Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 5complete conversion to dimeric solvates is achieved by addition of thf, dioxalane, dme or tmen but with pmdien in diethyl ether, a monomer is obtained. In thf, LiPh is a mixture of monomers and dimers; addition of tmen forms a series of complexes but the monomer/dimer ratio is una§ected.The e§ect of other donors such as hexamethyltriethylenetetramine, dimethylpropylene urea, 12-crown-4 ether and hmpa were also studied; all increase the reactivity of LiPh in thf.39 Ab initio calculations have been carried out on the lithium and sodium salts of doubly deprotonated benzene and the bridged ortho dianions were found to be remarkably stable (LiPh was found to be more acidic than benzene) and, indeed, ions corresponding to these species have been observed in electrospray mass spectrometry. 40 Nitrogen- and phosphorus-donor ligands The preparation and X-ray structure of [LiMgMN(SiMe 3 ) 2N3 ] have been reported; a near-planar NMgNLi ring is present with trigonal planar (N 3 ) magnesium and tetrahedral (N 2 C 2 ) lithium due to two additional C–Li interactions with the bridging N(SiMe 3 ) 2 groups. In the presence of trace amounts of oxygen [Li 2 Mg 2MN(SiMe 3 ) 2N4 (O 2 )x(O)y] crystallises from solution and X-ray crystallography showed that the compound with x\0.715(7) and y\0.285(7) is the major component.The four indistinguishable metal centres are in a square planar arrangement with side-on co-ordination to the peroxide and bonded to two nitrogens in an eightmembered ring. With PhCN, [MPhC(NSiMe 3 ) 2N4 Li 4 Mg(O)] is obtained; its structure is based on a trigonal bipyramidal Li 4 MgOcore at the centre of which lies a l5 -oxygen atom.41 The compounds [Li 2 Mg 2 (O)MNCMe 2 CH 2 CH 2 CH 2 CMe 2N4 ] and [Na 2 Mg 2 (O 2 )x(O)yMN(SiMe 3 )N4 ] (x[32%, y[68%) have been prepared either by bubbling oxygen through the amine before reaction or placing a calcium chloride drying tube on the vessel containing the oxygen-free compounds.Their structures are similar to the LiMg peroxide compound described above.42 Rare examples of crystallographically characterized infinite one-dimensional ladders in lithium amide chemistry have been provided by [Li(NHCH 2 CH 2 NH 2 )], which contains double edgesharing LiN 4 tetrahedra,43 and the hemi-benzylamine complex of lithium benzylamide which possesses an infinite, twisted ladder of fused Li 2 N 2 rings; only one edge of the ladder is solvated with benzylamine molecules.44 Deprotonation of N,N-dimethyl-N@- Me2N N N Me2N Li Li(OEt2) SiMe3 SiMe3 1 trimethylsilylethane-1,2-diamine (HL) with LiBu/ yielded LiL in hexane but [LiL 2 Li(OEt 2 )] 1 in diethyl ether.The complex [LiLLiClLiL(thf)] 2 was formed as a side-product of the reaction of [LiL(thf)n] with [LiCl 3 (thf) 2 ] in thf. In the solid state a LiCl unit is bonded between LiL and LiL(thf) units generating a three-rung ladder which is further connected, by Li–Cl bonds, to a second LiLLiCl(thf) unit; the structure contains two rare Li 4 Cl moieties.45 The preparation and structure of [(tmen)Li(l- NPh 2 ) 2 LiClMl-Li(tmen)N2 NPh 2 ] have been published.46 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 6Lithiation of 1,8-bis(trimethylsilylamino)naphthalene in thf yielded [C 10 H 6MN[Li(thf) 2 ]SiMe 3N2 ] which, on stirring for ca. 15 h in dioxane or toluene, lost three molecules of thf to give [C 10 H 6MN[Li(thf)]SiMe 3NMN(Li)SiMe 3N] 2 in which the unsolvated cation is g3-bonded to one of the arene rings.47 The crystal structure of [Li(NPh)(NHPh)SiMe 2 (OEt 2 )] 2 , prepared from Me 2 Si(NHPh) 2 and LiBu/ in diethyl ether, revealed a centrosymmetric molecule based on a Li 2 N 2 ring.48 Treatment of the hydridosilylamines, NHR@SiR 2 H (R@\SiMe 3 ; R\Pr*, Bu5, Ph; R@\R\Bu5) with LiBu/ a§ords the hydridosilylamides, R 2 HSiN(Li)R@.These are dimeric in the solid state and possess a planar Li 2 N 2 ring with significant SiH · · ·Li and CH 3 · · ·Li interactions.49 New derivatives of general formula Li 2 [XMe 2 SiNSiMe 2 X] 2 (X\Ph, NMe 2 , NEt 2 , NHPr*, OPh, OSiMe 3 , C 4 H 3 O, C 4 H 3 S) have been synthesised and characterized. All possess a polycyclic arrangement with a central Li 2 N 2 ring to which four similar LiNSiY rings are fused along a common LiN edge (Y can be either a C atom of a p-system, N or O).These compounds appear to be dimeric in benzene but there is evidence of dynamic behaviour.50 X-Ray di§raction shows that Bu5N(Li)CH––CH(Li)NBu5 is an unsymmetrical dimer.51 The preparation of [Li 3 (thf) 3 N(CH 2 CH 2 NSiBu5Me 2 ) 3 ] has been published; distillation gave the base-free compound while cooling a§orded the bis-thf compound which was shown to be dimeric in the solid state with two Li 2 N 3 units each linked via Li–N bonds to a central Li 2 N 2 moiety.52 The thf adduct of lithiated 2,5-di(tert-butyl)pyrrolide is monomeric in the solid state and in solution.The three-co-ordinate metal is involved in agostic interactions with a methyl group from each Bu5 group.53 The competition between solvent thf and polyamine ligands for co-ordination sites around lithium has been studied by probing the solution structure of [6Li]-a-(phenylthio) benzyllithium using 1H–6Li HOESY.In most cases polyamine complexes, in the form of contact- and separated-ion pairs, are obtained and the amines bind more strongly when their nitrogen content is high.54 NMR (6Li and 15N) spectroscopic studies show that hexane solutions of LiNPr* 2 containing \1 equivalent of pmdien per lithium contain a mixture of unsolvated LiNPr* 2 oligomers, monosolvated open dimers and monosolvated monomers. With[1 equivalent of pmdien, monomers are the dominant species.Addition of pmdien to LiNPr* 2 in toluene gives open dimers at low pmdien concentrations and a mixture of LiNPr* 2 monomer and LiCH 2 Ph (via toluene deprotonation) at high pmdien concentrations. The results were compared to previous investigations of LiNPr* 2 with tmen and trans-N,N,N@,N@-tetramethylcyclohexanediamine. The reactivity of the LiNPr* 2 solutions was probed by studying the dehydrohalogenation of (^)-exo-2-bromonorbornane; all exhibited similar reactivity. 55 The binding of diamines to n-butyllithium dimers has been investigated using NMR (6Li and 13C) spectroscopies. Highly ligand-dependent solvation energies and correlated solvation e§ects were observed.56 The lithium salt of carbazole is a contact ion pair in thf whereas the caesium salt is a mixture of monomers and dimers; the monomers form a 1: 1 complex CsCb·CbH.57 Lithiation of 4H-imidazoles, using LiH, a§ords the stable delocalized anions.Surprisingly, X-ray di§raction studies revealed a coordinated water molecule and PM3 calculations predicted an increase in stability as diethyl ether ligands are substituted by water.58 Computational studies on lithium (2-methoxy-(R)-1-phenylethyl-(S)-1-phenylethyl)amide, using a solid state structure as reference showed that there are only small di§erences between X-ray, ab initio and PM3 structures.The distances from these structures can be used for the calculation of Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 7Li–H distances using Li,H NOE data. The solution structure of the thf adduct was found to be similar to the calculated and X-ray structures.59 The first example of a lithiated diazomethane, M[Me 3 SiC(Li)N 2 ] 2 ·3thfN= , has been reported.The repeat unit is tetrameric with two mono- and two bis-thf-complexed molecules. The former associate to form a central Li 2 C 2 ring; the latter which are terminally attached to the metal centres of this ring by N· · · Li contacts are also each lithiated at carbon. Further N· · · Li contacts cause polymerisation of these units.The structure is in marked contrast to that of the compound obtained when the lithiation is carried out in ether (see Ann. Rep. Prog. Chem., Sect. A, 1994, 91, 7). These results can explain the lower reactivity of lithiated diazomethanes in diethyl ether.60 Lithiation of 4-methyl-1,5-diphenyl-1,2-diazapenta-2,4-diene in diethyl ether yielded [(PhNLiNCHCMeCHPh) ·Et 2 O] 3 in which the cations are g1-co-ordinated to the nitrogen atoms of two W-shaped diazapentadienyl units as well as one diethyl ether molecule.61 The heteroatom assisted lithiation of 1,3-bis[1-(dimethylamino)ethyl]benzene with LiBu/ a§orded 2,6-bis[1-(dimethylamino)ethyl]phenyllithium. X-Ray di§raction revealed a dimeric structure in which four benzylic chiral centres are identical, pointing to stereochemical crystallisation. In contrast, 1,3-bis[1-(dimethylamino)propyl]benzene reacted to give a dimeric aggregate of the lithiated compound and LiBu/ in which two ArLi 2 units are bridged by two Bu/ groups in a ladder framework.62 The X-ray structures of [(Bu5NSiMe 2 C 6 H 4 X-2)Li] 2 (X\OMe, NMe 2 , CH2 NMe 2 , CF3 ) Ph3P O NC5H5 Li(thf) + + – NC5H5 2 revealed Li–X, as well as Li · · ·CH 3 (Bu5) contacts with almost planar Li 2 N 2 rings; [(Bu5NSiMe 2 C 6 H 4 OMe-2) 2 Mg] and [(Bu5NSiMe 2 C 6 H 4 CH 2 NMe 2 -2)(OMe)Mg] 2 were also reported.63 A stable betaine lithium salt, 2, stabilised by complexation with pyridyl ligands, has been observed during the course of a Wittig reaction.64 Enthalpies of Lewis base induced tetramer]dimer and dimer]monomer conversion, intramolecular Li–NMe 2 R complexation and stabilisation by a-SiMe 2 R have been reported for a group of primary alkyllithium compounds.65 The reaction of lithium morpholide with N,N-dibutylformamide and 1-formylpiperidine results in complex equilibria in which the mixed diamino lithium alkoxides derived from morpholine and the corresponding amide are formed first.These intermediates then collapse to the lithium morpholide carbamoyl anion which reacts with morpholine to give the lithium dimorpholinemethoxide.66 The p-pyrrole complexation of lithium by zirconium mesooctaalkylporphyrinogens to give encapsulated Li 4 H 4 and Li 2 Oin sandwich structures has been reported.67 The reactions between Li[CH(SiMe 3 ) 2 ] and RNC can yield an enamide, a b- diketiminate, a 1,3-diazaallyl or a 1: 1 adduct depending on reaction conditions and the nature of R (Bu5, Ph or C 6 H 3 Me 2 -2,6).68 Reaction of [Li(ER 3 )(thf) 3 ] with ArCN (Ar\Ph, C 6 H 3 Me 2 -2,6) yielded the 3-sila- and 3-germa-b-diketiminates [LiMN(R)C(Ar)E(R)C(Ar)N(R)N(D)n] (R\SiMe 3 , E\Si or Ge, D\thf or ArCN and n\0–2).The exact nature of the product was determined by the choice of starting Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 8material and work-up procedure. X-Ray structures of the compounds with E\Si, D\ArCN, Ar\C 6 H 3 Me 2 -2,6, n\2 (monomeric) and E\Ge, Ar\Ph, n\0 (dimeric) were determined and shown to have, in contrast to the carbon analogues, the anionic charge localised at Si or Ge.69 The alkali metal-1-azaallyl complexes [CH 3 CH 2 CH 2 C(H)C(Bu5)N(H)Li(hmpa)] 2 , [CH 2 C(Bu5)N(H)Li(hmpa)] 2 and [CH 3 CH 2 CH 2 C(H)C(Bu5)N(H)Na(hmpa) 2 ] 2 have been prepared by treating the appropriate metal alkyl with Bu5CNin the presence of hmpa.Each structure is centred on a M 2 N 2 rhombohedral ring but the nature of the azaallyl–metal bonding di§ers with the first two displaying a terminal g1-N arrangement and the third showing a chelating g3-NCC moiety. Ab initio calculations which examined the energetics of the ketimide–azaallyl isomerism involved in the formation of these compounds were also reported.70 The preparation and crystal structure of the lithium 1-azaallyl compound, [LiMg3-N(SiMe 3 )C(Bu5)CH 2N2 ] 3 have been published together with ligand transfer reactions to niobium.71 Detailed information concerning the structure of lithiated a-amino nitriles in the solid state and in solution has been presented along with a mechanism for their stereoselective reactions, especially Michael additions.72 X-Ray N NH LiL N LLi NH L = Me2N(CH2)2NHMe 3 crystallography showed the product of the reaction between ortho-methylbenzonitrile and [LiN(Me)(CH 2 ) 2 NMe 2 ] to contain a new type of (NCNLi) 2 core in the dimer 3; there are also very weak amido(N)–Li cross-ring contacts.In solution, co-ordination of the diamine varies with temperature.73 The 1: 1: 1 reaction of 3-methylpyridine with LiNPr* 2 and benzonitrile in the presence of pmdien a§orded the first monomeric iminolithium compound 4.This result together with the isolation of the cyclised N Ph HN Ph NLi(pmdien) 4 N NLi(thf)2 Ph 5 product, 5, using a 3: 10: 3 reactant ratio, suggested a mechanism for previously published cyclisation reactions of b-methylazines.74 An EPR study of argon matrices containing lithium atoms and HCN or MeCN has revealed a weak van der Waals, end-on complex, RCN· · · Li and a strong charge transfer, side-on complex, Li`(RCN)~ in agreement with MNDO predictions.Irradiation converts the former into the latter.75 Reaction of Bu5NCNBu5 with LiNHBu5 (1 equivalent) in thf yields Annu. Rep. Prog. Chem., Sect.A, 1999, 95, 3–22 9MLi[C(NBu5) 2 (HNBu5)]N2 ·(thf) which contains an Li 2 C 2 N 4 ring. Deprotonation of this species with LiBu/ (2 equivalents) gives MLi 2 [C(NBu5) 3 ]N2 after recrystallisation from pentane. The structure is based on a highly distorted Li 2 N 6 C 2 hexagonal prism containing planar C(NBu5) 3 2~ anions linked by four Li cations.76 Hydrolysis of MLi[Bu/C(NBu5) 2 ]N2 , prepared from LiBu/ and Bu5NCNBu5 in hexanes, produced the nineteen atom cluster MLi[Bu/C(NBu5) 2 ]N4 ·Li 2 O.X-Ray di§raction revealed a linear Li 2 O unit at the centre of a l6 -OLi 6 core and encapsulated by two Li 2 N 4 C 2 rings.77 The preparation and crystal structures of the triazenide compounds [Li(OEt 2 )(4- MeC 6 H 4 NNNC 6 H 4 Me-4)] 2 and [K(dme)(4-MeC 6 H 4 NNNC 6 H 4 Me-4)] = have been reported.78 Dilithiated hydrazine reacts with SiClMe 2 Ph to give the isomers (Me 2 PhSiNH) 2 and (Me 2 PhSi) 2 NNH 2 which, in turn, react with 1 and 2 equivalents of LiBu/ to a§ord [H(Li)NN(SiMe 2 Ph) 2 ] 2 and [(Me 2 PhSi)LiNNLi(SiMe 2 Ph)] 3 , respectively.Both were characterized by X-ray crystallography; the former contains two-co-ordinate lithium stabilised by additional g2-contacts to each phenyl ring and the latter contains an Li 6 N 6 polyhedron with three-co-ordinate lithium additionally involved in links to the ipso-carbon of each phenyl ring.79 The crystal structure of [LiNPPh 3 ] 6 reveals an Li 6 N 6 polyhedron which is peripherally shielded by the phenyl groups.80 The preparation and crystal structures of the heterocubane cis- [(Bu5NP) 2MBu5NLi(thf)N2 ] and the seco-heterocubane (cube with one corner missing) [Mg(thf) 2M(Bu5NP) 2 (Bu5N) 2N] have been reported81 as have those of the heterocubane, cis-[MMeSi(Bu5)NN2MBu5NLi(thf)N2 ].82 Metallation of PMCH(SiMe 3 ) 2N(C 6 H 4 CH 2 NMe 2 -2) 2 with LiBu/ a§ords Li[C(SiMe 3 ) 2MP(C 6 H 4 CH 2 NMe 2 -2) 2N] in which the ligand adopts an unprecedented tridentate PN 2 co-ordination mode with no short contacts between lithium and the planar carbanion centre.83 The syntheses and structures of the lithium derivatives of a variety of tridentate and macrocyclic ligands containing amides, amines and phosphines in a chelating array have been presented.84 The butane-1,4-diide salt, [Li(thf) 2 (Ph 2 P) 2 CCH 2 ] 2 has been prepared and structurally characterized; the lithium atoms are in distorted tetrahedral environments (2P]2O) at either end of the molecule.85 The crystal structure of [LiPBu5 2 (thf) 2 LiCl(thf)] 2 revealed a four-rung LiPLiClLiClLiP ladder; the structure of [Li(l-Cl)(thf) 2 ] 2 has been redetermined.86 Oxygen- and sulfur-donor ligands The 1: 1: 1 or 2: 1: 2 reactions of LiNHBu5, KOBu5 and tmen in hexane yields [Li 8 K 2 (O)(OBu5) 8 (tmen) 2 ] which consists of a discrete cage with a central O2~ anion co-ordinated to the eight cations of a surrounding Li 8 O 8 polyhedron, two opposing sides of which are capped by a tmen-chelated potassium cation.87 The corresponding 1: 1: 1 reaction involving RbOBu5, rather than KOBu5, a§orded [M(Bu5OLi) 5 (Bu5ORb) 4 (Li 2 O 2 )(tmen) 2N= ], the crystal stucture of which revealed (Bu5O) 9 (O 2 )Li 7 Rb 4 cages linked together via Rb–tmen–Rb bridges to give a sheet structure.88 The preparation and crystal structure of [Li(l2 -OAr)] 3 (Ar\2,6- diphenyl-3,5-di-tert-butylphenolato) have been reported; an Li 3 O 3 ring forms the central core.89 An Li 4 O 4 cube with one unsolvated lithium centre forms the central core of [MLiOC 6 H 2 Me 3 -2,4,6N4 (thf) 3 ].90 X-Ray structure analysis of crystals grown from a photolysed LiOBu5–LiBu5 mixture revealed the presence of [Li 33 H 17 (OBu5) 16 ] with an inner hydride-rich core and a butoxide-rich periphery.Such species have Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 10potential as models of a soluble form of LiH.91 Reaction of LiN(Me)(CH 2 ) 2 NMe 2 with 2-CF 3 C 6 H 4 CHO a§ords a tetranuclear mixed aggregate containing three chiral a- amino lithium alkoxide units and one benzyl alkoxide, formed via LiH transfer.The structure is based on an Li 4 O 4 cube with only d-N-mono-co-ordination of the metal centres to give seven-membered chelates showing anti-isomerism about the a-N–C bond.92 Lithiation of dibenzylhydroxylamine, (PhCH 2 ) 2 NOH, yields an unsolvated, hexameric compound with a core of two stacked Li 3 O 3 rings.The metal centres are further co-ordinated via intra-monomeric chelation by hydroxylamide N-centres to give three-membered NOLi rings. Ab initio calculations were used to suggest reasons for the preference for a hexameric structure.93 Metallation of the disiloxane, OMSiMe 2 CH(SiMe 3 ) 2N2 , using LiMe in thf, yielded [SiMe 2 C(SiMe 3 ) 2 LiC(SiMe 3 ) 2 SiMe 2 OLi(thf) 2 ] the crystal structure of which was reported.94 Reaction of lithium or LiPh with a series of p-quinones gives the lithium semiquinonates; EPR spectroscopy identifies both monomeric and dimeric species. Adducts with 4-methylpyridine were also reported.95 Quantitative measurements of the aggregation equilibria in Claisen reactions of lithium enolates with phenyl esters show that the monomers are more reactive than the dimers and tetramers also present.96 The lithium enolate of 1-biphenylyl-2-methylpropanone exists as a mixture of monomer and tetramer ion pairs in thf.The most reactive species are the monomers, even though the tetramers are present in higher concentration.97 Rate studies of the ortholithiation of anisole by LiBu/–tmen have demonstrated that the reaction proceeds via the transition state [(LiBu/) 2 (tmen) 2 (PhOMe)]t.98 The structure, stability and electronic state of [Li(H 2 O)n]m` (n\1–6, 8; m\0 or 1) clusters have been investigated using ab initio methods which included electron correlation.The structure with four water molecules in the first co-ordination sphere and more in the second sphere was found to be the most stable for nq4.Li–O interactions play an essential role in dictating the structures in which 1pnp4 and the balance between Li–O bonding and hydrogen bonding becomes important in larger clusters.99 The structure of [Pt(C–– – CBu5) 2 (PPh 2 O) 2 Li 2 (l-H 2 O)(Me 2 CO) 2 ] 2 contains two coplanar Li 2 O 2 rings connected by two water molecules.100 The crystal structure of LiI·3thf, prepared from LiH and iodine, revealed a distorted tetrahedral geometry around the metal.101 The preparations and crystal structures of [LiX(dme) 2 ] (X\Br, Cl, AsH 2 , PH 2 ) have been reported.All possess trigonal bipyramidal metal centres with X in equatorial sites.102 The e§ect of polyether ligands on the solution structure of thf solutions of [6Li]-a-(phenylthio)benzyllithium has been reported (see ref. 54). Only the crown ethers were strongly bound.103 The preparation and crystal structures of [Li(E)CNBu5Bu/] (E\O, S) have been reported; the compounds are hexameric with cores of Li 6 E 6 hexagonal prisms.104 Halogen-donor ligands Reactions of NH 4 PF 6 with LiBu/ in toluene in the presence of hmpa or pmdien a§ord the ion-separated species [Li 2 (hmpa) 5 ][PF 6 ] 2 which contains three bridging hmpa ligands, or [LiPF 6 (pmdien)] 2 in which PF 6 ~ anions bridge Li(pmdien) units via Li · · ·F interactions. Ab initio calculations were used to help explain these di§erent structures.105 The 2: 1: 1 reaction of [TiCp*F 3 ] 2 , Me 3 SnF and LiCl in thf has resulted in the trapping of LiF formed in situ to give [(TiCp*F 3 ) 4 (LiF)].The structure consists Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 11of two [Ti 2 F 6 Cp* 2 ] units bridged by a lithium atom and a fluorine atom.106 The synthesis and crystal structure of [(C 6 H 3 mes 2 -2,6)AlBr 3 Li] 2 have been described. The X-ray structure revealed an Li 2 Br 4 octahedron with g6-interactions between the lithium centres and one of the mesityl rings on each phenyl group.107 3 Sodium The preparation and crystal structure of [Na(tmen) 3 (L)] (L\9,9-bianthryl), the salt of a radical trianion with three Na`–p(aryl) contacts in only one half of the bianthryl system, have been reported.108 The preparations of [MN(SiHMe 2 ) 2 ] (M\Li, Na, K) have been published; the structure of the sodium derivative revealed an infinite all-planar ladder structure with three-co-ordinate sodium also involved in SiH · · ·Na interactions.109 The crystal structure of [(NaNHBu5) 3 (NHBu5)] revealed an infinite wavelike polymer whereas [LiNa(NHBu5) 2 (tmen)] 2 adopted a four-rung ladder array with central Li–N rungs and outer Na(tmen)–N rungs.110 Metallation of NH(PPh 2 ) 2 with NaH in the presence of pmdien a§ords [NaN(PPh 2 ) 2 (pmdien)] with five-coordinate sodium (4N]P) and weaker g1-C contacts to the amine and a phenyl group. With 12-crown-4 ether rather than pmdien, M[NaN(PPh 2 ) 2 ] 2 [12-crown-4] 5N is formed; the structure is non-ionic according to mass spectrometry and conductivity measurements.111 The preparation and crystal structure of Na 4 [PMSiF(C 6 H 2 Pr* 3 - 2,4,6) 2N(SiPr* 3 )] 2 , in which a rhomboidally distorted planar Na 4 ring is stabilised between two silyl(fluorosilyl)phosphanide anions, has been reported.Each P atom is l-bonded to an Na 2 edge and each F atom coordinates to three Na centres.112 The preparation of [Na 2 P 5 (SiBu5 3 ) 3 (thf) 4 ] by (i) reaction of [Na 2 P 4 (SiBu5 3 ) 2 (thf)n] with CF 3 CO 2 H in thf, (ii) dissolution of the P 4 compound in toluene or (iii) reaction of [Na(SiBu5 3 )(thf) 2 ] with P 4 in benzene, has been reported; X-ray crystallography shows a P 3 ring with two cis-PNa(SiBu5 3 ) substituents and one trans SiBu5 3 group.113 The formation and X-ray structure of [MO(Ph 2 SiONa) 2N4 (NaOH)(H 2 O)(py) 7 ], incorporating a unique assembly of three fused Na 4 O 4 cubane units, which share a l6 -OH site, and a l3 -water molecule at one corner, have been presented.114 Reaction of fluorenone with Na (2 equivalents) in thf gave the green Na–fluorenone dianion.An X-ray structure analysis revealed both sodiums to be bonded to the carbonyl oxygen with one additionally coordinated by two thf molecules and the other also bonded to the carbonyl carbon and the carbon adjacent; further intermolecular interactions gave rise to a polymeric structure.115 The structures of hydrated Na` in concentrated aqueous solutions have been studied using X-ray di§raction and Raman spectroscopic methods.116 The synthesis and characterization of a series of sodium and potassium thiolates, [ML(SC 6 H 2 Pr* 3 -2,4,6)]n, (L\OEt 2 , thf, tmen, pmdien, dibenzo-18-crown- 6; n\1, 2, 6,O) have been described.Several were studied by X-ray di§raction and the choice of donor was found to influence structure.117 4 Potassium, rubidium and caesium Reaction of potassium with (Me 3 Sn) 2 in refluxing toluene–thf yields red [KCH 2 Ph] 2 - (thf) owing to metallation of toluene by the K[SnMe 3 ] intermediate.The dimeric units Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 12form a polymer through bridging benzyl groups.118 The synthesis and structure of the paramagnetic tweezer complex [(g5-C 5 HMe 4 ) 2 Zr(g1-C–– – CSiMe 3 ) 2 ]K has been reported. 119 The trimethylsilysilanyl compounds (Me 3 Si) 3 SiR [R\H, Et, SiMe 3 , Si(SiMe 3 ) 3 , SiMe 2 Si(SiMe 3 ) 3 ] and Me 3 SiSiR@ [R@\H 3 , Me 3 , MePh(SiMe 3 )] react with KOBu5 in dme or thf to give the corresponding polysilanylpotassium compounds and Me 3 SiOBu5; the usual route to such species involves the use of poisonous mercury compounds.120 The synthesis and X-ray structure of [KNPr* 2 (tmen)] 2 have appeared; the dimers are based on an asymmetric K 2 N 2 ring with secondary K· · ·CH 3 (Pr*) contacts.121 The diazapentadienyl complex, [(C 6 H 3 Pr* 2 - 2,6)NC(Me)C(H)C(Me)N(C 6 H 3 Pr* 2 -2,6)K ·PhCH 3 ] = has been prepared and structurally characterized. The compound is polymeric in the solid state via g5-aryl · · ·K interactions but monomeric in toluene.122 The 1,3-diazaallyl [LiMN(R)C(Ph)NC(Ph)––CR 2N], has been prepared from [LiC(SiMe 3 ) 3 (thf) 2 ] and 2 equivalents of PhCN; the tin derivative reacts with potassium–graphite to give the potassium analogue which was structurally characterized as a centrosymmetric dimer.The two metal centres are bridged by l-g2:g2-ligands; the nitrogens are coplanar and this plane forms the common base for the two distorted pyramids with an axial potassium.123 Several potassium pyrazolato complexes, [K(3,5-R 2 pz)(thf)/] (R\Bu5, n\0; R\Ph, n\1), have been prepared; a crystallographic study of a pyridine complex with R\Ph revealed a hexameric structure with the pz ligands adopting a slipped g2-bonding mode and also bridging with g1-interactions to adjacent metal centres.124 The crystal structure of [KPH(C 6 H 3 mes 2 -2,6)] 4 revealed a four-rung ladder stabilised by K· · · p(aryl) interactions and steric e§ects.125 The synthesis and solid state structure of [K 3 (PHmes) 3 (thf) 2 ] = have been described; the latter revealed a chain of KP 5 tetragonal pyramids which share common edges, and two additional K atoms located over two adjacent edges of one trigonal face of the pyramid.126 Reduction of [PBu5] 4 with potassium in the presence of pmdien followed by stoichiometric hydrolysis has provided the first fully characterized example of an organodiphosphide anion, [KM(PBu5) 2 HN(pmdien)] 2 , in which the anionic P centres of the two ligands form the central K 2 P 2 ring with secondary interactions with the neutral P centres and pmdien-solvation of the cations.127 Ab initio structure solutions using high resolution powder di§raction have been reported for KOPh·2PhOH and KOPh·3PhOH; both exhibited polymeric zigzag chains with the metal at the centre of distorted octahedra (5 O]ring centre).128 The molecular structures of the rubidium and caesium derivatives of supermesitylphosphane, PH 2 (C 6 H 2 Bu5 3 -2,4,6), as well as several base adducts of these have been presented.X-Ray di§raction studies revealed infinite ladder structures with M–P rungs and the bases (py, en, Meim, thf) bridging the metals.Structures depend on the size of the cation as well as the nature of the base. Sodium and potassium compounds were also prepared.129 The preparation and molecular sructures of [MP(H)R] (M\Rb, Cs; R\C 6 H 3 mes 2 -2,6) have been described. The rubidium compound contains an Rb 4 P 4 cube with Rb · · · p(aryl) contacts whereas the caesium compound consists of a two-dimensional polymeric framework of Cs`MCs 2 [P(H)R] 3N~ contact ion-pairs.130 A layer structure has been obtained for [CsOC 6 H 3 Pr* 2 -2,6] in which Cs–O and Cs–arene chains are held together by interactions between para-carbons in one chain and metals in an adjacent chain; interestingly this compound is solvent-free even though it was crystallized from thf.131 Investigation of the UV–VIS spectra of the Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 13caesium enolate of 1-biphenylyl-2-methylpropanone in thf shows that a mixture of monomers, dimers and tetramers is present. The caesium ion pair is more highly aggregated and much more basic than the analogous lithium ion pair.The ion pair monomer is the dominant reactant in alkylation displacement reactions.132 5 Beryllium A matrix-isolation and density-functional study of the reactions of laser ablated Be, Mg and Ca atoms with methane has been reported. Only CH 3 MH was observed for M\Mg and Ca but when M\Be, CH 3 BeH, CH 3 BeCH 3 , CH 3 Be, H 2 CBeH and HCBeH were found.133 The predominant species in alkaline hydroxyberyllate solutions have been identified as [(BeOH) 4 (OH) 6 ]2~; X-ray crystallography revealed a Be 4 tetrahedron with six bridging OH groups along the edges and one terminal OH on each metal atom in an adamantane-type structure.134 The preparation (in basic solution) and crystal structure of Ca[Be(OH) 4 ](H 2 O) 3@2 , containing the [Be 2 (OH) 7 ]3~ anion, have been reported.This species is probably an intermediate in the formation of the [Be 4 (OH) 10 ]2~ ion described above.135 The results of a thermodynamic and multinuclear NMRstudy of beryllium(II) hydrolysis and complex formation with oxalate, malonate and succinate ions in aqueous solution are consistent with the formation of [Be 2 OH]3`, [Be 3 (OH) 3 ]3`, [Be 5 (OH) 6 ]4`, [Be 6 (OH) 8 ]4` and Be(OH) 2 , as proposed previously.Complex formation is promoted by entropic contributions.136 The synthesis, structure and 9Be NMRspectrum of the picrate salt of [Be 3 (l-OH) 3 (H 2 O) 6 ]3` have been reported; the cation adopts a cyclic structure with a skew-boat six-membered Be 3 O 3 ring with two terminal water molecules also on beryllium.The H 2 O and OHgroups are hydrogen-bonded to the picrate anions and to waters of crystallisation to give a network which seems to be the stabilizing factor for the trimeric species.137 The succinato beryllates M 2 [Be(C 4 H 4 O 4 ) 2 ] (M\Na, K, NH 4 ) are prepared by reaction of BeSO 4 , succinic acid and Ba(OH) 2 (1: 2: 1) in aqueous solution; equimolar quantities yield [Be(C 4 H 4 O 4 )(H 2 O) 2 ].Potassium bis(maleato)beryllate is made similarly. The compounds undergo slow hydrolysis in water and in neutral solution the maleato complexes are in equilibrium withM 3M[Be(C 4 H 2 O 4 )OH] 3N.138 Equilibria in aqueous fluoroberyllate solutions have been studied as a function of beryllium and fluoride concentrations and pH and involve [Be(OH 2 ) 4 ]2`, [BeF(OH 2 ) 3 ]`, [BeF 2 (OH 2 ) 2 ], [BeF 3 (OH 2 )]~ and [BeF 4 ]2~.Fluoride exchange is slow on the NMR time-scale at room temperature and so sharp signals are observed in both 9Be and 19F spectra.139 Treatment of Be[N(SiMe 3 ) 2 ] 2 with PhSH resulted in acid–base reactions between NH(SiMe 3 ) 2 and PhSH so that in the presence of pyridine and 18-crown-6 ether, [Be(SPh) 2 (py)(NH 3 )] 2 ·18-crown-6 was formed.The structure consists of two [Be(SPh) 2 (py)(NH 3 )] units hydrogen bonded, via NH 3 , to either side of a central crown.140 Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 146 Magnesium Carbon-donor ligands Magnesium atoms generated by laser ablation have been reacted with methyl halide, MeX, in an argon matrix. The Grignard reagents, CH 3 MgX as well as MgX, MgX 2 , MgH, MgH 2 , CH 4 , C 2 H 6 , CH 2 X, CH 3 MgCH 3 , XMgCH 3 , XMgCH 2 , HMgCH 3 and HMgCH 2 X were also identified, together with associated Grignard species.The data provide the first experimental evidence for CH 3 MgF.141 A new general route to highly functionalized arylmagnesium halides, which proceeds via I–Mg exchange, has been published.142 Reaction of ‘MgBu 2 ’ with 6-methyl-6-phenyl- and 6,6-dicyclopropyl-fulvene gives the b-hydride transfer products 1,1@-bis(1-phenylethyl)- and 1,1@-bis(dicyclopropylmethyl)- magnesocene, respectively, whereas with MgMe 2 deprotonation occurs to give unstable 1,1@-bis(1-phenylethen-1-yl)magnesocene and 6, respectively. In the absence of b-hydrogen, only the addition reaction is possible and MgMe 2 and tetramethylfulvene give dimeric methylmagnesium ethyltetramethylcyclopentadienide in which the bridging methyl groups are shielded from further attack by the fulvene.143 A series of chiral ansa-magnesocene derivatives has been prepared and structurally characterized.The ring hapticities vary between one and five depending on the nature of the intramolecular bridge and the C 5 -ring substituents.144 The preparations and structures of the trigonal bipyramidal molecules [Mg(CH 2 SR) 2 (thf) 3 ] (R\Me, Ph) have been reported; [Mg(CH 2 SMe) 2 ] was also characterized.145 (thf)2Mg 6 Nitrogen- and phosphorus-donor ligands Treatment of N,N@-diphenylethylenediamine and N,N@-dibenzylethylenediamine with ‘MgBu 2 ’ in the presence of thf or hmpa yields [MgN(Ph)CH 2 CH 2 N(Ph)(thf) 2 ] 2 or [MgN(CH 2 Ph)CH 2 CH 2 N(CH 2 Ph)·hmpa] 2 .Both structures contain trans-5.4.5-fused rings with the former possessing a five-co-ordinate distorted trigonal bipyramidal metal centre instead of the more usual four-coordinate distorted tetrahedral geometry found in the latter structure. Replacement of the two thf molecules in the phenyl compound with hmpa molecules leads to the formation of the monomeric [MgN(Ph)CH 2 CH 2 N(Ph)·2hmpa] with tetrahedral magnesium.146 A series of fourco- ordinate magnesium amides, [MgL 2MN(SiMe 3 ) 2N], (L\2,3,5-trimethylpyridine, 2- or 4-methylpyridine, 3,5-dimethylpyridine) have been prepared and structurally characterized.Sublimation of the 2,3,5-trimethylpyridine and 2-methylpyridine derivatives a§ords [MgLMN(SiMe 3 ) 2N]; the 2,6-dimethylpyridine complex was made directly, in solution.147 The syntheses and structures of some g2-pyrazolato complexes Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 15of magnesium, [Mg(3,5-Bu5pz) 2 ] 2 and [Mg(3,5-Bu5pz) 2 L]n (L\tmen, n\1; L\thf, n\2) have been reported.148 The preparations and crystal structures of [ML 2 (4- MeC 6 H 4 NNNC 6 H 4 Me-4) 2 ] (M\Mg, L\thf; M\Ca, L\dme) revealed monomeric units with six-co-ordinate magnesium and eight-co-ordinate calcium.149 Metallation of Ph 3 P––NH with EtMgCl in the presence of hmpa yielded the N-magnesioiminophosphorane complex [Ph 3 P––NMgCl(hmpa)] 2 ; the dimer is centred on a planar Mg 2 N 2 ring with trans-chlorines. Retention of significant P––N double bond character is consistent with an electrostatic interaction between the anions and magnesium cations.150 The first solvent-free magnesium phosphanides have been prepared; Mg[P(SiHPr* 2 ) 2 ] 2 is dimeric in solution and Mg[P(SiMe 3 ) 2 ] 2 exists in a monomer–dimer equilibrium in frozen benzene and in a dimer–trimer equilibrium in toluene at [40 °C.Crystallisation at [30 °C yields the trimer, Mg[(Me 3 Si) 2 PMgMl- P(SiMe 3 ) 2N2 ] 2 , the X-ray structure of which was determined.151 Oxygen- and sulfur-donor ligands Reaction of [M(OMe) 2 ] = (M\Mg, Ca) with SO 2 in MeOH yields [M(O 2 SOMe) 2 (MeOH) 2 ] = ; with COS or CS 2 , [M(OCSOMe) 2 (MeOH)x]n (M\Mg, x\2;M\Ca, x\3, n\2) or [M(S 2 COMe) 2 (MeOH)x]n (M\Mg, x\6;M\Ca, x\4) are obtained.The analogous reactions with CO 2 gave [Mg(O 2 COMe)(OMe)(MeOH) 1.5 ]n and [Ca(O 2 COMe)(OMe)(MeOH)]n; recrystallisation of the magnesium compound from wet hydrocarbon led to the isolation of [Mg 9 (CO 3 )(O 2 COMe) 8 (MeOH) 13 ].The X-ray crystal structure of this compound revealed two Mg 4 O 4 cubes with a remote ninth Mg held in position via two bridging methylcarbonate groups and an unusual l5 -CO 3 ligand.The corresponding reaction of CO 2 with [M(OEt) 2 ] = gave [M(O 2 COEt)(OEt)(EtOH) 2 ]n; CO 2 with [Mg(OR) 2 ] = (R\Pr*, Bu5) in the parent alcohol gave [Mg(O 2 COR)(OR)(ROH)]n.152 A further investigation (NMR, IR, TGA) into the insertion of COS into the M–O bonds of alkaline-earth metal alkoxides has been reported, including the crystal structures of [Mg(OCSOPr*) 2 (Pr*OH) 4 ]·2Pr*OH and [Sr(OCSOPr*) 2 (Pr*OH) 2 ]n.153 A similar study involving CS 2 has also appeared which includes the crystal structures of [Ba(S 2 COEt) 2 ] = and [Ca(S 2 COPr*) 2 (Pr*OH) 3 ] · 2Pr*OH.154 The impetus for this work arises from the potential use of these compounds as MOCVD precursors for electroceramics.The Hauser base reagents MgX(NPr* 2 ) (X\Cl, Br) have been shown to react with a variety of enolisable ketones to give magnesium enolates.These compounds could not be isolated from thf but in ether in the presence of hmpa, [Bu5C(–– CH 2 )OMgBr- (hmpa)] 2 and [Pr*C(––CMe 2 )OMgBr(hmpa)] were isolated; both precipitate as mixtures with [MgBr 2 (hmpa) 2 ]. The results indicate that solvent plays a major role in the dismutation reactions of Hauser bases and halogenomagnesium enolates.155 The first examples of aqua-bridged dimagnesium cores have been described in the crystal structures of [Mg 2 L 4 (l-H 2 O)(l-OAc)(OAc) 2 ] (L\imidazole or benzimidazole).Implications of these structures, particularly the strong hydrogen bonds between H 2 O and ancillary OAc groups, for the hydrolytic activity of dimetallic hydrolases are discussed.156 The preparation and crystal structures of the octahedral cations [Mg(H 2 O) 2 (thf) 4 ][Mg(H 2 O) 4 (thf) 2 ][MnCl 4 ] 2 ·2thf,157 [Mg 2 Br 3 (thf) 6 ]` and [MgBr(thf) 5 ]`,158 have been described.Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 16The syntheses and structural characterization of a series of magnesium thiolates, [Mg(SR) 2 Ln]m (R\Ph, C 6 F 5 , -2-NC 5 H 4 ; L\py, 2,6-Me 2 py; n\1–4; m\1 or 3), and the first magnesiate thiolate, [Mg(SC 6 F 5 ) 4 ]2~, have appeared.159 7 Calcium, strontium and barium Metallocene syntheses using the metal (Ca, Sr or Ba) and pentaisopropylcyclopentadienyl radicals has been reported; the calcium and strontium compounds were stable in air for several weeks whereas the barium compound underwent slow decomposition.The structure of the barium species exhibited staggered, parallel rings.160 The transmetallation of Sn[N(SiMe 3 ) 2 ] 2 with the alkaline-earth metals in thf yields M[N(SiMe 3 ) 2 ] 2 ·2thf (M\Ca, Sr, Ba) which react with 6-methyl-6-phenylfulvene to give the 1,1@-bis(1-phenylethen-1-yl)metallocenes. Reaction of the strontium amide with equimolar amounts of the fulvene and acetophenone leads to the formation of 7 O Ph Me Ph Sr Sr O Ph Ph Me 7 with a planar Sr 2 O 2 ring.161 Reaction of 4-tert-butylacetyl-3-methyl-1-phenylpyrazol- 5-one (HQ) with CaCl 2 in ethanol–KOH yielded [CaQ 2 (EtOH) 2 ].The metal is in a distorted octahedral environment with the two b-diketonates in ‘anti-’ and the ethanol ligands in trans-positions; infinite chains are formed via hydrogen bonding.When R\Me, Et, Pr* complexes of general formula [CaQ 2 (ROH) 2 ] are obtained, whereas when R\Bu5, HC–– – CCH 2 or Pr*(Bu5)CH, [CaQ 2 (H 2 O) 2 ] is isolated.162 Recrystallisation of [(TiCp*F 3 ) 4 CaF 2 ] and [M(TiC 5 Me 4 Et)F 3N4 CaF 2 ] in the presence of hmpa a§orded the soluble 1: 1 adducts; X-ray di§raction of the latter showed calcium co-ordinated to eight fluorines from two [Ti 2 (C 5 Me 4 Et) 2 F 7 ]~ units and by an oxygen of hmpa.The behaviour of the Cp* compound in solution was also reported.163 The preparation and crystal structures of [Ca(SC 6 F 5 ) 2 (py) 4 ], [Ca(18-crown-6)(NH 3 ) 3 ]- [Smes*] 2 · 2thf and [Ca(18-crown-6)(Smes*) 2 ] · thf have been presented; the Ca–S bonds are weak and predominantly ionic.The structure of [CaBr 4 (thf) 2 (l2 -Li) 2 (thf) 4 ] was also given.164 The preparation and structures of [Sr(18-crown-6)(hmpa) 2 ][Smes*] 2 and [Ba(18- crown-6)(hmpa)(Smes*)][Smes*] have been reported; both metals are eight-co-ordinate. The factors a§ecting ion association in alkali and alkaline-earth chalcogenolates were discussed.165 Complexes M(hfac) 2 (Ln) (M\Sr, n\1, L\tetraglyme, dibenzo- 24-crown-8-ether; n\2, L\bipy, phen;M\Ba, n\1, L\tetraglyme, dibenzo-24- crown-8-ether) have been prepared and characterized including the X-ray structure of Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 3–22 17Sr(hfac) 2 (bipy) 2 which contains cis ligands around eight-co-ordinate strontium in the form of a square antiprism.166 Metallation of PH(SiMe 2 Pr*) 2 with Sr[N(SiMe 3 ) 2 ] 2 in thp yields Sr[P(SiMe 2 Pr*) 2 ] 2 ·4thp, which has two trans-phosphine ligands.Partial hydrolysis of this compound leads to the formation of Sr 4 O[P(SiMe 2 Pr*) 2 ] 6 which contains a central oxygen atom surrounded by a metal tetrahedron, the edges of which are bridged by phosphanide ligands; the metals are in trigonal planar environments. 167 The preparation and crystal structure of the linear polymer, [PPh 4 ][Ba(g5-Cp) 3 ] containing tetrahedral barium, have been presented.168 The crystal structure of [Ba(NH 3 ) 7 ]C 60 ·NH 3 revealed a monocapped trigonal antiprism around the metal.169 The synthesis and structure of [Ba 2MCyNC(Me)CHC(Me)NCyN3MN(SiMe 3 ) 2N], in which the diazapentadienyl group adopts three di§erent bonding modes, have been reported.170 Reaction of Ba[P(SiMe 3 ) 2 ] 2 ·4thf with diphenylbutadiyne yields dimeric 8.The alkenide moiety bridges two barium atoms forming a unique three-centre two-electron Ba–C–Ba bond. A mechanism for the formation of 8 is given; 9 is P Ph SiMe3 Ph Ph Ba thf Ba thf Ph P Ph Me3Si Ph Ph Ph Ph (Me3Si)2PMg Ph P(SiMe3)2 8 9 obtained when a similar reaction with magnesium species is carried out.171 Treatment of Ba[N(SiMe 3 ) 2 ] 2 with PH 2 (SiBu5 3 ) yields Ba[PHSiBu5 3 ] which reacts with Sn[N(SiMe 3 ) 2 ] 2 to give BaSn 3 [l-PSiBu5 3 ] 4 which contained a BaSn 3 P 4 cube with toluene g6-co-ordinated to barium.172 Metallation of As(SiMe 2 Bu5) 2 H with Ba[N(SiMe 3 ) 2 ] 2 ·4thf a§ords Ba[As(SiMe 2 Bu5) 2 ] 2 ·4thf; X-ray crystallography revealed a distorted pentagonal bipyramid with apical arsenic atoms and one free equatorial position shielded by the silyl groups.Recrystallisation from toluene gives [BaMAs(SiMe 2 Bu5) 2N2 (thf )] 2 .173 Reaction of BaCO 3 with the appropriate acid in the presence of 18-crown-6 ether a§orded Ba(OAc) 2 (18-crown-6)·nH 2 O and Ba[O 2 C(CH 2 ) 4 CO 2 ](18-crown-6)·nH 2 O.Various hydrates were formed but only those with n\4 (acetate) and n\8 (adipate) were characterized by X-ray di§raction. In both cases the metal sits in the centre of the crown but the acetate is monomeric with axial chelating ligands whereas the adipate is polymeric with one carboxylate chelating to one metal while the second bridges to another metal. Thermal decomposition and reactions with Ti(OPr*) 4 were also discussed. 174 Reaction of BaH 2 with CF 3 CO 2 H and a polydentate ligand leads to isolation of compounds of general formulae [Ba(O 2 CCF 3 ) 2 ]mLn (m\1 or 2; n\1 or 2; L\triglyme, cryptand-222, 12-crown-4, 18-crown-6–py, 2-(hydroxymethyl)-15- crown-5, 2-(hydroxymethyl)-18-crown-6). The bases are bonded to barium through all of their donor atoms and the anions adopt four di§erent bonding modes.All of the compounds decompose to BaF 2 , rather than sublime, between 250 and 600 °C.175 The synthesis and characterization of a series of volatile barium b-ketoimine–polyether Annu. Rep. Prog. Chem., Sect. 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