6 Oxygen, sulfur, selenium and tellurium P. F. Kelly Department of Chemistry, Loughborough University, Loughborough, UK LE11 3TU 1 Introduction This review highlights new developments in the chemistry of the Group 16 elements (the chalcogens) reported during 1998. Emphasis has been given to results that demonstrate novelty of product or synthetic approach as their main feature. In addition, the products reported have been limited to those in possession of a discrete molecular structure, thus highlighting the extraordinary ability of sulfur, selenium and tellurium to contribute to novel cluster arrangements. 2 Sulfur, selenium and tellurium As with other years we start by looking at the ability of the heavier chalcogens to form compounds with a vast range of other p-block elements, and do so by working from left to right across the Periodic Table starting with Group 13.It has been revealed that a strong base may transform [B 6 H 5 (ECN)]2~ (E\S or Se) to [B 6 H 5 E]3~.1 The X-ray structure of the doubly protonated derivative of the sulfur species, namely [B 6 H 5 H(SH)]~ confirms the expected monsubstituted B 6 octahedron. A novel selenium- based four-membered boracycle B(tbt)Se 2 SnAr 2 (Ar\Ph or mes) forms when B(tbt)Se 3 SnAr 2 (itself the product of Li[B(tbt)H 3 (thf) 3 ] with [TiSe 5 Cp 2 ] and SnAr 2 Cl 2 ) is deselenated with PPh 3 .2 Intriguingly, thermolysis of these products results in the formation of the selenoxoborane (tbt)BSe which may be trapped from the system with, for example, a diene.A noteworthy 90 reference review of boron–chalcogen species has been published.3 The selenium-bridged aluminium compound trans- [MAl(H)(l-Se)(NMe 3 )N2 ] has been shown to react with (PhE) 2 (E\S, Se or Te) to give colourless products of the type trans-[MAl(l-Se)(PhE)(NMe 3 )N2 ],4 while refluxing [Al(mes*)H 2 ] 2 with S(SiMe 3 ) 2 in toluene produces [Al(mes*)S] 2 .5 The latter exists as a dimer with a planar Al 2 S 2 core; interestingly this is in direct contrast to its oxo analogue which exists as a tetramer.Finally amongst Group 13 species, a detailed investigation into the butterfly-shaped [Tl 2 E 2 ]2~ anions (E\Se, Te or a mixture) has been undertaken using a combination of Raman, 205Tl, 203Tl and 77Se (including Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 67enriched samples) NMR. The elegant solution work shows that lability in en or ammonia solutions increases in the order [Tl 2 Se 2 ]2~\[Tl 2 SeTe 2 ]2~ \[Tl 2 Te 2 ]2~.6 The products of the low temperature reaction of tin dichloride with Na[SiBu53 (thf) 2 ] in turn react with P(E)Et 3 (E\Se or Te) to give (Bu5 3 SiSnE) 4 .7 The products, which are yellow for selenium and red for tellurium, have similar structures wherein the tin atoms occupy the corners of a tetrahedron with the chalcogen atoms lying in a plane bisecting the Sn 4 unit.Still in the area of tin chemistry, only the second example of an X-ray crystallographically characterised planer PS 2 Sn ring has been found in [M(Me 3 Si) 2 NN2 Sn 2 P(S)C 6 H 4 OMe] which forms as a pale yellow crystalline material during the reaction of Lawesson’s reagent with [M(Me 3 Si) 2 NN2 Sn].8 The Ge analogue may also be isolated in a similar manner but in much smaller yield (10% compared to 90%).Lead readily forms Zintl ions and this versatility has been demonstrated yet again by the observation that potassium reduction of PbTe 2 in en results in the formation of silver coloured crystals of K 4 [PbTe 3 ]·2en.9 As the formulation would suggest, they contain the [PbTe 3 ]4~ ion which is trigonal pyramidal with additional capping potassium cations.There has been the usual good deal of interest in the chemistry of chalcogen –nitrogen systems, with perhaps the fundamentally most important result coming with the isolation of [Te 6 N 8 (TeCl 4 ) 4 ].10 This pale yellow, non-explosive material forms when TeCl 4 reacts with N(SiMe 3 ) 3 in thf and exhibits a Te 6 N 8 core, stabilised by coordinating TeCl 4 molecules.The latter explains the lack of explosive nature, a surprise given that this is e§ectively the material formerly characterised as Te 3 N 4 and which itself was originally thought to be Te 4 N 4 (and hence to complete the sequence of explosive chalcogen tetranitrides).It is likely that isolation of this material will spark renewed interest in the chemistry of Te–N systems. Amongst S–N systems, sulfur imides of the type SxN(n-C 8 H 17 ) (x\5 or 6) have been generated (as pale yellow oils) by reaction of [TiMS 2 N(n-C 8 H 17 )S 2NCp 2 ] with SCl 2 or S 2 Cl 2 11 while the first examples of salts of arylsulfurdiimides [RNSN]~ (R\C 6 H 4 F-2 or C 6 H 3 F 2 -2,6) have been isolated.12 The latter show terminal S–Nlengths of ca. 1.45Å which are therefore consistent with the presence of S–– – N triple bonds. A range of new perfluoroalkyl derivatives of the –N(SO 2 F) 2 unit have been studied [thus CF 3 N(SO 2 F) 2 forms from ClN(SO 2 F) 2 and CF 3 I]13 while S(NBu5) 3 has been shown to form via heavy halogen oxidation of [Li 4M(NBu5) 3 SN2 ] (a reaction which progresses through the [Li 3M(NBu5) 3 SN2 ]· radical).14 The ternary heterocycle S 3 N 5 C 4 has been prepared and its redox and magnetic properties studied.Chief amongst structural results is the observation that while at ambient temperatures it exists as ribbons of radicals packed into slipped p-stacks, at low temperature an array of dimers forms.15 The most interesting advance in the coordination chemistry of such species comes with the isolation of the first example of a metal selenonitrosyl.When the terminal nitrido complex [OsNCl 2 Tp] is treated with elemental Se at 80 °C over five days a 28% yield of green [Os(NSe)Cl 2 Tp] is obtained. X-Ray di§raction reveals that the NSe ligand is slightly bent (164.7°) and exhibits an N–Se distance of 1.629Å.16 The coordination chemistry of the sulfimides Ph 2 SNH is currently being developed; recent work has revealed that the homoleptic cobalt complex [Co(Ph 2 SNH) 6 ]Cl 2 possesses an unusual alignment of two sets of three N–H bonds, on either side of the coordination octahedron, forming two H-bonding pockets in which the chloride counter ions Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 68sit.17 Finally, a rare example of aM–NCNE(E\S or Se; andM\Pt) metallocycle is found in [PtMN(H)C(Ph)N(H)EN(dppe)]`, which forms upon decomposition of [PtMENC(Ph)NEN(dppe)].18 It has long been known that introducing phosphorus atoms into chalcogen–nitrogen systems induces interesting chemistry. Further progress in this area has come with the isolation of novel thionylphosphazenes of the type R1OP(R2R3)NS(O)(R@) and of hybrid sulfanuric–phosphazene ring systems of the type Ph 4 P 2 N 4MS(O)RN2 (R1,R2,R3,R@,R\alkyl or aryl).The former can be polymerised to the first examples of polythionylphosphazenes in which the side groups are bound by P–C and S–C bonds (such products also constitute the first examples of inorganic polymers with S, P, and 2N repeat units)19 while the latter consist of eight-membered P 2 N 4 S 2 rings in twisted boat configurations.20 Their thermal stability is such that it rules them out as precursors to polymeric systems.Last year witnessed the usual high level of interest in the chemistry of phosphorus –chalcogen systems. Thus the photochemical selenation of P 4 O 6 with red selenium in CS 2 was shown to generate P 4 O 6 Se 2 (which has two terminal selenium atoms).21 The latter has been characterised by a range of techniques as have the related compounds P 4 O 8 and P 4 O 6 S 2 .22 The concave, cyclic [(NiPS 4 ) 3 ]3~ anion forms from the autofragmentation and rearrangement of one-dimensional [NiPS 4 ]~ in dmf.23 31P NMR has been used to follow this unusual reaction, which does not occur in the analogous Pd case.The same PS ligand is found in [Cr 2 (PS 4 ) 4 ]6~, which happens to be the first discrete transition metal cluster unit to have been isolated from a thiophosphate flux reaction (Cr, P 2 S 5 andK 2 S in the ratio 1: 2: 3 at 600 °C for one week24) and in the bright yellow 1-D chain system [CeP 2 S 8 ]3~.25 The latter forms from the reaction of K 2 S with Ce 2 S 3 , P 2 S 5 and sulfur at 300 °C; within its structure each cerium atom coordinates to nine sulfurs.Many similar selenium systems have been shown to form from high temperature reactions. These may be free salts, such as the orange potassium salt of [P 8 Se 18 ]6~ 1 (from K 2 Se, phosphorus and sulfur at 510 °C),26 coordinated to transition metals (as is the case with the 2-D layered K 2 Cu 2 P 4 Se 10 , which interestingly exhibits the first example of a cyclic [P 4 Se 10 ]4~ group),27 to lanthanides or to other main group elements.Examples of the former include M 3 LnP 2 Se 8 and M 2 LnP 2 Se 7 (M\Rb or Cs; Ln\Ce or Gd)28 while indium reacts with molten [PPh 4 ] 2 [Se 5 ] and P 2 Se 5 at 250 °C to give a yellow salt of formulation [PPh 4 ][In(P 2 Se 6 )].29 While the anion in this case is part of a polymeric array, a discrete anion is found with [In(P 2 Se 6 ) 2 ]5~ 2, formed as an air stable caesium salt by the reaction of the elements in the appropriate ratio.Finally among phosphorus systems, the first examples of complexes of the PSe ligand have been discovered.30 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 69Thus reaction of [MCoCpAN3 (l3 -P) 2 ] with selenium at ambient temperature in toluene for a day results in the formation of [MCoCpAN3 (l3 -P)(l3 -PSe)] and [MCoCpAN3 (l3 - PSe) 2 ]. Both have been characterised by X-ray crystallography; in the mono-PSe case, for example, the P–Se distance is 2.09Å.Se As Se Se Se Se Se Se Se As As Te Au Te Te Te As As Te Te Te Te Se As Se Se In Se en en Se As Se Se Se Se Se Se As Se Se Se Se As Se 2– – 5– 5 6 7 – 4 3 Arsenic–chalcogen systems are also popular for their structural versatility and last year saw a number of interesting developments in this area. A common, and very powerful, technique for isolating new anions of this type involves the extraction of elemental alloys with solvents such as en.In this way anions of the type [As 2 Se 6 ]2~ 3, [AsSe 8 ]~ 4 (both from AsSe 4 ) and [AsSe 6 ]~ 5 (from TlAsSe 4 ) have been generated, together with neutral [InMSeAs(Se)Se 2Nen 2 ]·en 6 (from InAsSe 4 extracted with en and 2.2.2.crypt).31 In a similar manner, extraction of K 2 AuSnAs 3 Te 8 with en and addition of [Et 4 N]I results in the formation of the dark red [AuAs 4 Te 8 ]5~ anion 7.As the diagram shows this is in e§ect two [As 2 Te 4 ]4~ anions coordinating to Au cation leading to a square-planar AuTe 4 unit.32 Somewhat di§erent approaches have led to the related anions [AsSe 6~xSx]~ and [Sb 2 Se 14 ]2~ 8 (by a variety of reactions involving AsCl 2 OPh)33 and to the red crystalline salt of [Pd 7 As 10 S 22 ]4~.34 The latter forms in the hydrothermal reaction of PdCl 2 , K 3 AsS 3 and [PPh 4 ]Br in MeOH at 110 °C leading to a product in which both [As 2 S 5 ]4~ and [As 3 S 6 ]5~ units are present.Hydrothermal techniques also generate interesting antimony species in the shape of the anions [Sb 3 S 25 ]3~ and [Sb 2 S 15 ]2~ 9 which form from the elements in a variety of solvents. While the latter contains discrete anions, the former actually consists of [Sb 2 S 17 ]2~ 10 and [Sb 2 S 16 ]2~ 11 units.35 Finally in this area, a noteworthy 115 reference review of Group 15/16 ligands has been published.36 Moving on to ‘pure’ Group 16 species, we find a number of important results, including the isolation of a new allotrope of sulfur, namely S 14 12.This forms as yellow rods (mp\117 °C) via reaction of [ZnS 6 (tmen)] with S 8 Cl 2 .37 This reaction is important not only for the formation of the product itself but for the fact that it confirms the obvious potential that the recently reported zinc species has as a very versatile synthon, along the lines of the well known and much used [Ti(S 5 )Cp 2 ]. The ability of a range of Co, Cu and Zn halide complexes of 2-methylpyridine N-oxide to fix SO 2 in both the solid state and solution has been investigated (and found to be very much dependent upon the nature of the metal, the halide, the medium and the reaction temperature)38 while a single crystal X-ray study upon the [SeSO 3 ]2~ anion has Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 70S S S Sb S S S S S Sb S S S S S S S S S S S Sb S S Sb S S S S S S S S S S S S S S Sb S S Sb S S S S S S S S S S S Se Se Se Se Sb Se Se Se Sb Se Se Se Se Se Se Se 2– 8 2– 10 9 11 2– 2– S S S S S S S S S S S S S S 12 confirmed the presence of a central sulfur bound to all three oxygen atoms and to selenium (S–Se 2.17Å).39 Studies of chalcogen–halogen systems have proved especially fruitful in 1998.The facile synthesis of the new tellurium halides Te 2 Cl 2 and Te 2 Br 2 was one such important breakthrough. They form when elemental tellurium is reduced by superhydride in the presence of TeX 4 . Both are liquids at room temperature (the chloride being yellow, the bromide orange–red) and were characterised by a combination of NMR and mass spectrometry; additional proof of their nature was provided by their reaction with [Ti(E 5 )Cp 2 ] (E\S or Se) to give 1,2-Te 2 E 5 .40 Novel types of [TeCl 9 ]~ anions (in which octahedral and trigonal bipyramidal Te units are bridged at one edge) are found in [H 5 O 2 ][Te 2 Cl 9 ], the colourless crystalline product of TeCl 4 , 1,4-dioxane and HCl,41 while planar TeCl 4 units occur in the green [Mn(CO) 5 (TeCl 4 )]~ anion.42 Moving away from tellurium, iodine atoms are found to bridge two sulfate units in HIS 2 O 8 , which crystallises from a concentrated solution of iodic acid in oleum.43 The I–O distances, which average 1.975Å, correspond to I–O single bonds while longer bonds link neighbouring units into ribbon arrays.Amongst selenium systems, the synthesis of a range of chloroselenates such as [SeCl 6 ]2~, [Se 2 Cl 9 ]~ and [Se 2 Cl 10 ]2~ has been reported44 while treatment of [Se 4 ][MoOCl 4 ] with SOCl 2 has been shown to produce [SeCl 3 ][MoOCl 4 ].45 Finally, studies upon the halogenation of phosphine chalcogenides have revealed the first crystal structure of a 1: 1 charge transfer type product (that of PPh 3 S·I 2 )46 and of trigonal bipyramidal R 3 PSeBr 2 (R\Me 2 N or C 6 H 11 ).47 As usual there have been a large number of reports of metal reactions generating a variety of metal complexes with chalcogenide ligands.Here we will look at some of these, in increasing order of nuclearity, though we start with one of the more unusual observations. There is still considerable interest in the formation of nanoparticulate CdS and it has now been shown that this forms when a mixture of Na 2 [S 2 O 3 ] and Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 71Cd2` is treated with c radiation. Quite why the thiosulfate ion should act as a sulfide source in the presence of such radiation remains something of a mystery, but the technique obviously has great potential.48 It is worth noting that another important advance in this area came with the deposition of zinc and cadmium chalcogenide nanoparticles through thermolysis of bis[methyl(n-hexyl)dichalcogenocarbamato] complexes.49 The gas phase reactions of lanthanide cations with sulfur in an ion cyclotron resonance mass spectrometer have been studied in detail; results indicate that each lanthanide generates [LnSx]` in which x ranges from 2 to 21 and decreases with time.50 The importance of [Zn(S 6 )(tmen)] as a synthon has been demonstrated earlier; work has also revealed that when the tmen is substituted by tridentate pmdien the product ([Zn(S 4 )(pmdien)]) not only reacts with further sulfur (to give [Zn(S 5 )(pmdien)]) but also with DMAD to give [ZnMS 2 C 2 (CO 2 Me) 2N(pmdien)] and with CS 2 to give [ZnMS 3 C(S)N(pmdien)]. It would appear that in such cases the third amine group of the ligand is responsible for the enhanced nucleophility.51A range of aryloxide substituted analogues of the well known [Ti(S 5 )Cp 2 ] have also been studied,52 as have manganese and rhenium complexes of the type [M(OEF 5 )(CO) 5 ] (E\Se or Te).53 We have already mentioned c irradiation as a technique for the formation of sulfides; another study utilising such radiation has shown that glassy solutions of salts of the d0 chalcogenides [ME 4 ]2~ (M\Cr, Mo or W; E\S or O) results in one electron addition to the anions.In such cases EPR results are consistent with the products ([ME 4 ]3~) having the added electron predominantly located on the metal centre.54 Dimeric [MCu(SCN)(l-SCN)N2 ]2~ anions are present in the colourless product of [N(PPh 3 ) 2 ][CuCl 2 ] and K[SCN] in ethanol55 while violet [Fe 2 (S 2 ) 3 (tacn) 2 ] has provided the first example of a M 2 (S 2 ) 3 core.56 Treatment of [MPtS(dppe)N2 ] with Cu[PF 6 ] 2 in MeOH results in red crystalline [CuMPt 2 (l3 -S) 2 (dppe) 2N2 ]2`57 while [M@(l-S) 2MM(S 2 CNEt 2 )N2 (l-S) 2 ] (M@\Pd or Pt; M\Mo or W) results from [M@(PPh 3 ) 4 ] and [M 2 S 2 (l2 -S) 2 (S 2 CNEt 2 ) 2 ].58 Although a thorough review of tetranuclear (and larger) chalcogenide complexes is beyond the scope of this particular work, the following results provide a flavour of the kind of results reported last year.In the [Fe 4 S 4 Cl 4 ]2~ cluster monoprotonation has been shown to catalyse chloro substitution (though addition of one more proton inhibits it)59 while the first cuboidal cluster with an oxygen centre, namely [Nb 4 OTe 4 (CN) 12 ]6~, has been shown to form in the high temperature reaction of NbTe 4 with KCN.60 The [MVI(S)ReI 3 (CO) 9 (l3 -S) 4 ]~ (M\Moor W) anion is notable for the presence of the very di§erent metal oxidation states;61 other tetranuclear systems include [Pd 4 (l3 -Se) 2 (l-SCH 2 Ph) 2 (l-dppm) 2 Cl 2 ] (which exhibits unusual asymmetric coordination of Pd atoms),62 [MRu(CO)Cp*N2MW(CO) 4N(WS 4 )] (which undergoes both thermal and photolytic isomerisation in benzene)63 and [M 2 Ru 2 (l2 - Se) 2 (CO) 4 (CO) 6 (PPh 3 ) 2 ] (M\Mo or W).64 Pentanuclear systems include [WS 4 Cu 4 (SCN) 2 (py) 6 ] (which exhibits promising optical limiting properties)65 and [MIrCp*N4 M(l3 -S) 4 ]n` (M\Fe, Co or Ni; n\1 or 2) which form in ‘bow-tie’ arrangements.66 A series of hexanuclear products are obtained when main-group elements are incorporated into the [Mo 3 S 4 (H 2 O) 9 ]4` unit; corner shared double cubes of the type [Mo 6 XS 8 (H 2 O) 18 ]2` form in which X may be As,67 Sb68 or In.69 A similar nuclearity is observed for [Yb 6 S 6 (SPh) 6 (py) 10 ] (the first example of a lanthanide cubane structure)70 and for a series of complexes of Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 72the general type [Re 6 S 8 (PEt 3 )nBr 6~n](n~4)` (n\2–6) which form by PEt 3 substitution into [Re 6 S 8 Br 6 ]4~.71 The first discrete CuI 8 cubane encapsulating a selenide has been isolated in the form of [Cu 8 (l8 -Se)MSe 2 P(OPr*) 2N6 ]72 while the same number of metal atoms are found in the mixed-valence, double-cubanoid [Fe 8 S 12 (Bu5NC) 12 ]73 and nine are seen in [MWCu 2 S 3 Cp*N3 S 2 ]~.74 Moving to chalcogenide clusters with ten or more atoms, two zinc-based examples were reported last year, namely [Zn 10 S 7 (py) 9 (SO 4 ) 3 ]·3H 2 O (a new type of Zn sulfide cluster with a three-fold Zn 10 S 7 core75) and the telluride system [Zn 10 Te 4 (TePh) 12 (PPh 3 ) 2 ].76 Larger systems include [Mo 12 S 16 (PEt 3 ) 10 ] (from the reaction of [Mo 6 S 8 (PEt 3 ) 6 ] and NO[BF 4 ]),77 [M 4 Cu 10 S 16 E 2 E@]·H 2 O (M\Mo or W; E\E@\Oor S)78 and a range of clusters isolated from the reaction of Bu5SSiMe 3 and [Fe(HgX) 2 (CO) 4 ] (X\Cl or Br), the largest of which was [Fe 8 Hg 39MFe(CO) 4N18 S 8 (SBu5) 14 Br 28 ].79 Examples of even larger systems include [Cu 72 Se 36 (PPh 3 ) 20 ] [from the reaction of Se(SiMe 3 ) 2 with copper(I) acetate and triphenylphosphine]80 and [Ag 172 Se 40 (SeBu/) 92 (dppe) 4 ].81 3 Oxygen One of the most intriguing papers from this, or indeed any other area last year, hinged around the observation that Cu 2 O acts as a mediator/catalyst in the conversion of stirring energy into the decomposition of water to oxygen and hydrogen.The mechanism for this–which in e§ect converts mechanical energy directly into chemical energy –is as yet unclear but the observation is highly unusual and clearly initiates an area of oxide chemistry that will be intensely studied; it will be intriguing to see what next year’s report has to say on the matter!82 Singlet oxygen remains an important area of study and some results from 1998 reflect this.Thus, for example, a new solid state catalyst for its formation from H 2 O 2 has been developed in the form of molybdate immobilised on a Mg,Al-layered double hydroxide matrix. This set up eliminates the usual requirement for the presence of a soluble base and allows for the gradual release of 1O 2 from a hydrophilic source.83 The yield of 1O 2 from the disproportionation of aqueousH 2 O 2 and sodium tungstate catalysts has been shown to be quantitative (and to proceed via a range of intermediates of which the diperoxo species [W(O 2 ) 2 O 2 ]2~ is the actual singlet oxygen precursor)84 while CaO 2 ·2H 2 O 2 has been implicated as the final precursor when Ca(OH) 2 is used as catalyst.85 Finally, quenching of 1O 2 luminescence by a variety of radicals such as PhS·, PhSO·, PhOO· has been investigated leading to the conclusion that the sulfur-bearing radicals were markedly poorer quenchers than the oxygen-based species.86 Many important results from studies of oxidation reactions were reported last year, including the observation that irradiation of [W 10 O 32 ]4~, alkanes and methylcyanoformate in acetonitrile results in nitriles or iminoesters, depending upon the temperature of reaction. Thus while cyclohexane is converted to C 6 H 11 CN in 78% yield at 90 °C, it is formed in minimal amounts at ambient temperatures [the main product then being C 6 H 11 C(NH)CO 2 Me].87 Manganese substituted polyoxometalates such as Li 12 [Mn 2 ZnWMZnW 9 O 34N2 ] have been shown to catalyse the transformation of alkanes to ketones by ozone in water (via, it appears, a green manganese ozonide intermediate)88 while a new photocatalytic system for the oxidation of cyclo- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 73hexane and cyclohexene by oxygen and sunlight has been developed.89 The latter reactions take place at ambient temperatures and pressures and hinge on the use of iron porphyrins inside a Nafion base.Water soluble palladium(II) complexes of diamine ligands, such as bathophenanthroline disulfonate, have proved to be stable, recyclable catalysts for the selective aerobic oxidation of terminal olefins to alkan-2- ones in biphasic systems.90 Thus a 50% conversion of pent-1-ene is achieved with 99% selectivity with respect to alkanone formation.Addition of sodium acetate to the system appears to be the key to recyclability in this system. Methyltrioxorhenium acts as a catalyst for the conversion of N,N-dimethylhydrazones of aldehydes to nitriles by H 2 O 2 91 while the trans-[Ru(OH) 2 O 3 ]2~/[S 2 O 8 ]2~ system has been shown to catalyse the conversion of a wide range of primary benzylic amines to nitriles at room temperature.92 The latter reactions can be achieved on large scales; thus 6.8 g of p-methoxybenzylamine may be converted to 3.2 g of the corresponding nitrile by treatment with 0.1 g of RuCl 3 ·3H 2 O in 500 ml of 1M KOH containing 10.4 g of K 2 [S 2 O 8 ] over 24 h.Finally, when encapsulated in mesoporous cubic Al-MCM-48, both copper(II) acetate dimer and [Mn(bipy) 2 ]2` show high catalytic activities towards, respectively, the oxidation of phenol to catechol (by O 2 activation) and to the conversion of styrene to styrene oxide (by singlet oxygen).93 The first X-ray confirmation of the presence of ‘true’ peroxocarbonate ion, [H(O 2 )CO 2 ]~ has come in the form of its potasium salt.It is, in e§ect, a hydrogencarbonate ion with the OH group replaced by OOH. As one might suspect, there is also extensive hydrogen-bonding present in the system.94 To continue on the carbon theme, though the chemistry of C 60 is very much beyond the scope of this particular report, two aspects of its oxygen chemistry are noteworthy here.Firstly, it has been demonstrated that ambient air oxidation of C 60 produces C 120 O (in which two C 60 units are linked by an oxygen). The clear implication of this result is that it casts doubt upon reports that C 60 could occur naturally.95 The second result of note comes with the observation that the formation of C 60 O in the reaction of C 60 with singlet oxygen proceeds via a triplet excited state of the former.96 Moving down Group 14, we find that the tin oxo species [MSn(g4-Me 8 taa)(O)N2 ] di§ers from its sulfur and selenium analogues (which exhibit terminal Sn–E bonds) by being dimeric (note the contrast with transition metal systems wherein the O group is more likely to be terminal).97 As is normally the case, NO systems have provided a range of interesting results over the past year.For example, a combination of calculations and mass spectrometry has suggested that ammonia oxide, H 3 NO may be isolable (thus the EI spectrum of aqueous ammonia shows a volatile, hitherto uncharacterised NHO compound that may just fit the bill)98 while the reaction of the NH· radical with O 2 in a xenon matrix has been shown to generate imine peroxide, HNOO.99 The latter material, which was characterised by IR, readily photoisomerises to nitrous acid.A number of studies have looked at the chemistry of the peroxynitrite anion and its parent acid. The use of the latter for the oxidation of organic sulfides has been investigated; it is formed in situ from H 2 O 2 and HNO 2 and under optimum conditions the HNO 2 e§ectively acts as a catalyst for Otransfer from H 2 O 2 to R 2 S.100 Work on the anion has shown that when O 2 NOO~ decomposes in solution ca. 50% of it homolyses into O 2 ·~ and NO 2 ,101 and that decomposition in neutral hydrogencarbonate proceeds via CO 2 catalysed formation of nitrate.102 The molecular structure of the related species CF 3 OONO 2 has been determined for the first time; after preparation by photolysis of CF 3 I, NO 2 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 74and O 2 di§raction studies revealed a skew structure with a very long N–O bond (1.52Å) joining the NO 2 and CF 3 COO units. Calculations indicate that its photolytic half life in the troposphere is of the order of 26 days, suggesting that it may have an important role to play in the transport of NO 2 away from industrial zones.103 Ozonolysis of BrNO generates BrNO 2 which in turn yields a mixture of cis and trans BrONO upon photolysis.104 Amongst metal N–O complexes, reduction of the metal centre in [HB(3,5-Me 2 pz) 3 Fe(ONO)Cl 2 ] has been shown to result in the nitrite ligand changing from O-bound nitrito to chelating nitrito105 while a metal peroxynitrite complex has been implicated in the [Co(NH 3 ) 5 (SO 3 )]` catalysed decomposition of the latter anion in basic solutions.106 Finally among nitrogen species, the first nitrosyl complexes of Nb and Ta have been isolated; the products, both of the type [M(NO)(CO) 2 (trimpsi)] may be isolated as air sensitive purple crystalline solids.107 Reaction of AsCl 3 with As 2 O 3 and [PPh 4 ]Cl in MeCN for 16 h generates the [As 4 O 2 Cl 10 ]2~ anion which in turn reacts with further chloride to give [As 2 OCl 6 ]2~.108 Amongst ‘pure’ oxygen species investigated last year we find the superoxide ion, which has had itsO–Obond distance of 1.34Åaccurately determined for the first time.The key to obtaining this was the use of a large, non-spherical, cation ([1,3- (Me 3 N) 2 C 6 H 4 ]2`) to force order into the anion positions.109 Pure sodium ozonide has been isolated as an intensely red, air sensitive crystalline solid–of the Group 1 ozonides it proves to have both the longest O–O distance (1.35Å) and the tightest O–O–O angle (113°)110–while [O 2 ]` salts have been shown to react with Group 1 fluorides in HF to give solvatedO 2 F.111 Not surprisingly the latter can act as a strong oxidising agent.Amongst work upon ozone performed last year came the observation that it can transfer oxygen to several positive ions (including those of the halogens, with reactivity increasing with halogen size)112 and the first observation of isotopic exchange in ionisedO 3 –O 2 mixtures (via an [O 5 ]` intermediate). The latter may be go some way to explaining the fact that stratospheric O 3 is enriched with 18O.113 A number of studies have focused upon hydrogen-bonding HO cations, including the oxonium crown ether complexes [H 7 O 3 ][AuCl 4 ]·15-crown-5 and [H 5 O 2 ][AuCl 4 ]- (benzo-15-crown-5) 2 .The latter species form when gold is dissolved in aqua regia and the crowns added; while the first cation exhibits an infinite H-bonded chain, the latter has discrete cation/crown units in which a pair of crowns sandwich the oxonium ion.114 [H 5 O 2 ][SbF 6 ] forms from phosphorous acid and HF–SbF 5 at [50 °C115 while [H 3 O 2 ][SbF 6 ] and [H 5 O 4 ][SbF 6 ] are generated in the low temperature reaction of (Me 3 SiO) 2 with HF and SbF 5 .116 Finally, systems in which oxygen is bound to main-group elements we have yet to mention include the [M(MeC 6 H 4 ) 2 TeN2 O]2` cation (formed by the action of either [NO][BF 4 ] or (CF 3 SO 2 ) 2 O and O 2 upon the parent telluride)117 and the chelating [H 2 I 2 O 10 ]4~ groups found in H 11 I 2 InO 14 (the colourless product of the reaction of indium(III) nitrate with H 5 IO 6 at low pH).118 As is usually the case, many metal–oxygen systems were investigated last year, and some brief examples are given here.EPR analysis of the products of the reaction of chromate or dichromate with hydrogen peroxide revealed the presence of known [Cr(O 2 ) 4 ]3~ together with other pH/concentration dependant species such as [Cr(O)(O 2 ) 2 (H 2 O)]~ (such results have potential relevance to the carcinogenic properties of CrVI).119 Electrospray mass spectrometry has revealed the products of the reaction of dioxygen with a range of metal bipyridyl complexes120 while the photo- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 75catalytic reduction of O 2 to H 2 O 2 by an immobilised ruthenium bipyridyl cation has also been observed.121 Activation ofO 2 by a coordinatively unsaturated manganese(II) thiolate complex bearing a hindered hydrotris(pyrazolyl)borate ligand has been noted (a reaction which proceeds via a manganese superoxo complex)122 while it has now been shown that bridging O atoms in Bi–O clusters can participate in catalytic oxidations.123 The first example of an unsupported Au–O–Au bridge is found in the [Au 2MN 2 C 10 H 7 (CMe 2 C 6 H 4 )-N,N@,CN2 (l-O)]2` cation (Au–O 1.97Å, Au– O–Au 121.3°)124 while X-ray studies have revealed that, unusually for solid-state oxides, FeVMoO 7 and CrVMoO 7 contain Mo––O double bonds (thus making them good models for the surface O-coordination of molybdate catalysts).125 Metal–oxo cluster systems developed last year include [V 3 OCl 4 (Hmba) 5 ] (bearing the triangular [V 3 (l3 -O)]7` core,126 [MRe 6 S 5 OCl 7N2 O]4~ (intriguingly described as an oxo-bridged ‘Siamese twin cluster’)127 and the super-large [(MoO 3 ) 176 (H 2 O) 63 (CH 3 OH) 17 - Hn](32~n)~ 128 and [(MoO 3 ) 176 (H 2 O) 80 H 32 ], the latter being a ring with a 2.3nm cavity.129 References 1 B.Steuer, S. Zander and W. Preetz, Z. Anorg. Allg. Chem., 1998, 624, 1829. 2 M. Ito, N. Tokitoh and R.Okazaki, Chem. Commun., 1998, 2495. 3 O. Conrad, C. Jansen and B. Krebs, Angew. Chem., Int. Ed., 1998, 37, 3208. 4 W. J. Grigsby, C. L. Raston, V. A. Tolhurst, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans., 1998, 2547. 5 R. J. Wehmschulte and P. P. Power, Chem. Commun., 1998, 335. 6 H. Borrmann, J. Campbell, D. A. Dixon, H. P. A. Mercier, A.M. Pirani and G. J. Schrobilgen, Inorg.Chem., 1998, 37, 1929. 7 M. Weidenbruch, A. Stilter, H. Marsmann, K. Peters and H. G. von Schnering, Eur. J. Inorg. Chem., 1998, 1333. 8 C. J. Carmalt, J. A. C. Clyburne, A. H. Cowley, V. Lomeli and B. G. McBurnett, Chem. Commun., 1998, 243. 9 C.D.W. Jones, F. J. DiSalvo and R. C. Haushalter, Inorg. Chem., 1998, 37, 821. 10 W. Massa, C. Lau, M. Mollen, B. Neumuller and K.Dehnicke, Angew. Chem., Int. Ed., 1998, 37, 2840. 11 R. Steudel, O. Schumann, J. Buschmann and P. Luger, Angew. Chem., Int. Ed., 1998, 37, 492. 12 A. V. Zibarev, E. Lork and R. Mews, Chem. Commun., 1998, 991. 13 B. Krumm, A. Vrij, R. L. Kirchmeier and J. M. Shreeve, Inorg. Chem., 1998, 37, 6295. 14 R. Fleischer, S. Freitag and D. Stalke, J. Chem. Soc., Dalton Trans., 1998, 193. 15 T.M. Barclay, A. W. Cordes, N. A. George, R. C. Haddon, M.E. Itkis, M.S. Mashuta, R. T. Oakley, G.W. Patenaude, R.W. Reed, J. F. Richardson and H. Zang, J. Am. Chem. Soc., 1998, 120, 352. 16 T. H. Crevier, S. Lovell, J. M. Mayer, A. L. Rheingold and I. A. Guzei, J. Am. Chem. Soc., 1998, 120, 6607. 17 P. F. Kelly, A. M. Z. Slawin and K. W. Waring, Inorg. Chem. Commun., 1998, 1, 249. 18 N. Feeder, R. J. Less, J. M. Rawson and J. N. B. Smith, J. Chem. Soc., Dalton Trans., 1998, 4091. 19 V. Chunechom, T. E. Vidal, H. Adams and M. L. Turner, Angew. Chem., Int. Ed., 1998, 37, 1928. 20 M. Brock, T. Chivers and M. Parvez, Inorg. Chem., 1998, 37, 3263 21 M. Jansen and A. Tellenbach, Z. Anorg. Allg. Chem., 1998, 6241, 1267. 22 A. R. S. Valentim, B. Engels, S. D. Peyerimho§, A.Tellenbach, S. Strojek and M. Jansen, Z. Anorg. Allg. Chem., 1998, 624, 642. 23 J. Sayettat, L. M. Bull, J.-C. P. Gabriel, S. Jobic, F. Camerel, A.-M. Marie, M. Fourmigne, P. Batail, R. Brec and R.-L. Inglebert, Angew. Chem., Int. Ed., 1998, 37, 1711. 24 V. Derstro§, V. Ksenofontov, P. Gutlich and W. Tremel, Chem. Commun., 1998, 187. 25 G. Gauthier, S. Jobic, R. Brec and J. Rouxel, Inorg.Chem., 1998, 37, 2332. 26 K. Chondroudis and M.G. Kanatzidis, Inorg. Chem., 1998, 37, 2582. 27 K. Chondroudis and M.G. Kanatzidis, Inorg. Chem., 1998, 37, 2098. 28 K. Chondroudis and M.G. Kanatzidis, Inorg. Chem., 1998, 37, 3792. 29 K. Chondroudis, D. Chakrabarty, E. A. Axtell and M. G. Kanatzidis, Z. Anorg. Allg. Chem., 1998, 624, 975. 30 S. Weigel, G. Wolmershauser and O.J. Scherer, Z. Anorg. Allg. Chem., 1998, 624, 559. 31 D.M. Smith, M. A. Pell and J. A. Ibers, Inorg. Chem., 1998, 37, 2340. 32 C. Wang, R. C. Haushaletr and M.-H. Whangbo, Inorg. Chem., 1998, 37, 6096. Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 7633 W. Czado and U. Muller, Z. Anorg. Allg. Chem., 1998, 624, 239. 34 J. A. Hanko, J.-H. Chon and M.G. Kanatzidis, Inorg.Chem., 1998, 37, 1670. 35 M. Schur and W. Bensch, Z. Anorg. Allg. Chem., 1998, 624, 310. 36 J. Wachter, Angew. Chem., Int. Ed., 1998, 37, 751. 37 R. Steudel, O. Schumann, J. Buschmann and P. Luger, Angew. Chem., Int. Ed., 1998, 37, 2377. 38 M. Fondo, M. R. Bermejo, A. Sousa, J. Sanmartin, M. I. Fernandez and C. A. McAuli§e, Polyhedron, 1998, 17, 413. 39 A. J. Blake, V. Consterdine, M.F. A. Dove, S. Lammas and L. H. Thompson, J. Chem. Soc., Dalton Trans., 1998, 3. 40 J. Pietikainen and R. S. Laitinen, Chem. Commun., 1998, 2381. 41 O. Reich, S. Hascher, S. Bonmann and B. Krebs, Z. Anorg. Allg. Chem., 1998, 624, 411. 42 W.-F. Liaw, S.-J. Chiou, G.-H. Lee and S.-M. Peng, Inorg. Chem., 1998, 37, 1131. 43 M. Jansen and R. Muller, Angew. Chem., Int. Ed., 1998, 37, 1312. 44 W. Czado, M. Maurer and U. Muller, Z. Anorg. Allg. Chem., 1998, 624, 1871. 45 A. Baumann and J. Beck, Z. Anorg. Allg. Chem., 1998, 624, 1725. 46 D. C. Apperley, N. Bricklebank, S. L. Burns, D. E. Hibbs, M. B. Hursthouse and K. M. Abdul Malik, J. Chem. Soc., Dalton Trans., 1998, 1289. 47 S. M. Godfrey, S. C. Jackson, C. A. McAuli§e and R. G. Pritchard, J. Chem. Soc., Dalton Trans., 1998, 4201. 48 Y. Yin, X. Xu, X. Ge, C. Xia and Z. Zhang, Chem. Commun., 1998, 1641. 49 B. Ludolph, M. A. Malik, P. O’Brien and N. Revaprasada, Chem. Commun., 1998, 1849. 50 K. Fisher, I. Dance and G. Willett, J. Chem. Soc., Dalton Trans., 1998, 975. 51 R. J. Pa§ord and T. B. Rauchfuss, Inorg. Chem., 1998, 37, 1974. 52 A. V. Firth and D. W. Stephan, Inorg. Chem., 1998, 37, 4726. 53 M.C. Crossman, E. G. Hope and L. J. Wootton, J. Chem. Soc., Dalton Trans., 1998, 1813. 54 T. Ecclestone, S. H. Lawie, M. C. R. Symons and F. A. Taiwo, Polyhedron, 1998, 17, 1435. 55 R. Hehl and G. Thiele, Z. Anorg. Allg. Chem., 1998, 624, 1736. 56 A. C. Moreland and T. B. Rauchfuss, J. Am. Chem. Soc., 1998, 120, 9376. 57 M. Capdevila, Y. Carrasco, W. Clegg, R. A. Coxall, P. Gonzalez-Duarte, A.Lledos, J. Sola and G. Ujaque, Chem. Commun., 1998, 597. 58 T. Ikada, S. Kuwata, Y. Mizobe and M. Hidai, Inorg. Chem., 1998, 37, 5793. 59 R. A. Henderson and K. E. Oglieve, J. Chem. Soc., Dalton Trans., 1998, 1731. 60 V. P. Fedin, I. V. Kalinina, A. V. Virovets, N. V. Podberezskaya, I. S. Neretin and Y. L. Slovokhotov, Chem. Commun., 1998, 2579. 61 F.M. Hornung, K. W. Klinkhammer and W.Kaim, Chem. Commun., 1998, 2055. 62 R. Cao, W. Su, M. Hong, W. Zhang, W.-T. Wong and J. Lu, Chem. Commun., 1998, 2083. 63 M. Yuki, M. Okazaki, S. Inomata and H. Ogino, Angew. Chem., Int. Ed., 1998, 37, 2126. 64 D. Cauzzi, C. Grai§, C. Massera, G. Mori, G. Predieri and A. Tiripicchio, J. Chem. Soc., Dalton Trans., 1998, 321. 65 M.K. M. Low, H. Hou, H. Zheng, W. Wong, G.Jin, X. Xin and W. Ji, Chem. Commun., 1998, 505. 66 Z. Tang, Y. Nomwa, S. Kuwata, Y. Ishii, Y. Mizobe and M. Hidai, Inorg. Chem., 1998, 37, 4909. 67 R. Hernandez-Molina, A. J. Edwards, W. Clegg and A. G. Sykes, Inorg. Chem., 1998, 37, 2989. 68 G. Sakane, K. Hashimoto, M. Takahashi, M. Takeda and T. Shibahara, Inorg. Chem., 1998, 37, 4231. 69 R. Hernandez-Molina, V. P. Fedin, M.N.Sokolov, D. M. Saysell and A. G. Sykes, Inorg. Chem., 1998, 37, 4328. 70 D. Freedman, J. H. Melman, T. J. Emge and J. G. Brennan, Inorg. Chem., 1998, 37, 4162. 71 M.W. Willer, J. R. Long, C. C. McLauchlan and R. H. Holm, Inorg. Chem., 1998, 37, 328. 72 C.W. Liu, H.-C. Chen, J.-C. Wang and T.-C. Keng, Chem. Commun., 1998, 1831. 73 C. Goh, A. Nivorozhkin, S. J. Yoo, E. L. Bominaar, E.Munck and R. H. Holm, Inorg. Chem., 1998, 37, 2926. 74 J.-P. Lang and K. Tatsumi, Inorg. Chem., 1998, 37, 160. 75 B. Ali, I. G. Dance, D. C. Craig and M.L. Scudder, J. Chem. Soc., Dalton Trans., 1998, 1661. 76 A. Eichhofer, D. Fenske, H. Pfistner and M. Wunde, Z. Anorg. Allg. Chem., 1998, 624, 1909. 77 J. Mizutani, S. Amari, H. Imoto and T. Saito, J. Chem. Soc., Dalton Trans., 1998, 819. 78 J. Guo, T. Sheng, W. Zhang, X. Wu, P. Lin, Q. Wang and J. Lu, Inorg. Chem., 1998, 37, 3689. 79 D. Fenske and M. Bettenhausen, Angew. Chem., Int. Ed., 1998, 37, 1291. 80 A. Eichhofer and D. Fenske, J. Chem. Soc., Dalton Trans., 1998, 2969. 81 D. Fenske, N. Zhu and T. Langetepe, Angew. Chem., Int. Ed., 1998, 37, 2640. 82 S. Ikeda, T. Takata, T. Kondo, G. Hitoki, M. Hara, J.N. Kondo, K. Domen, H. Hosono, H. Kawazoe and A. Tanaka, Chem. Commun., 1998, 2185. 83 F. van Laar, D. DeVos, D. Vanoppen, B. Sels, P. A. Jacobs, A. DelGuerzo, F. Pierard and A. Kirsch- DeMesmaeker, Chem. Commun., 1998, 267. 84 V. Nardello, J. Marko, G. Vermeersch and J. M. Aubry, Inorg. Chem., 1998, 37, 5418. 85 V. Nardello, K. Briviba, H. Sies and J.-M. Aubry, Chem. Commun., 1998, 599. 86 A. P. Darmanyan, D. D. Gregory, Y. Guo, W. S. Jenks, L. Burel, D. Eloy and P. Jardon, J. Am. Chem. Soc., 1998, 120, 396. Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 7787 Z. Zheng and C. L. Hill, Chem. Commun., 1998, 2467. 88 R. Neumann and A. M. Khenkin, Chem. Commun., 1998, 1967. 89 A. Maldotti, A. Molinari, L. Andreotti, M. Fogagnolo and R. Amadelli, Chem. Commun., 1998, 507. 90 G. J. tenBrink, I. W. C. E. Arends, G. Papadogianakis and R. A. Sheldon, Chem. Commun., 1998, 2359. 91 H. Rudler and B. Denise, Chem. Commun., 1998, 2145. 92 W. P. Gri¶th, B. Reddy, A. G. F. Shoair, M. Suriaatmija, A. J. P. White and D. J. Williams, J. Chem. Soc., Dalton Trans., 1998, 2819. 93 M. Eswaramoorthy, U. N. Neeraj and C. N. R. Rao, Chem. Commun., 1998, 615. 94 A. Adam and M. Mehta, Angew. Chem., Int. Ed., 1998, 37, 1387. 95 R. Taylor, M. P. Barrow and T. Drewello, Chem. Commun., 1998, 2497. 96 D. I. Schuster, P. S. Baran, R. K. Hatch, A. U. Khan and S. R. Wilson, Chem. Commun., 1998, 2493. 97 M. C. Kuchta, T. Hascall and G. Parkin, Chem. Commun., 1998, 751. 98 M. Bronstrup, D. Schroder, I. Kretzschmar, C. A. Schalley and H. Schwarz, Eur. J.Inorg. Chem., 1998, 1529. 99 S. L. Laursen, J. E. Grace Jr, R. L. Dekock and S. A. Spronk, J. Am. Chem. Soc., 1998, 120, 12 583. 100 S. Vayssie and H. Elias, Angew. Chem., Int. Ed., 1998, 37, 2088. 101 S. Goldstein, G. Czapski, J. Lind and G. Merenyi, Inorg. Chem., 1998, 37, 3943. 102 S. V. Lymar and J. K. Hurst, Inorg. Chem., 1998, 37, 294. 103 R. Kopitzky, H. Willner, H.-G. Mack, A.Pfei§er and H. Oberhammer, Inorg. Chem., 1998, 37, 6208. 104 D. Scheßer and H. Willner, Inorg. Chem., 1998, 37, 4500. 105 N. Arulsamy, D. S. Bohle, B. Hansert, A. K. Powell, A. J. Thomson and S. Wocaldo, Inorg. Chem., 1998, 37, 746. 106 A.M. Al-Ajlouni, P. C. Paul and E. S. Gould, Inorg. Chem., 1998, 37, 1434. 107 P. J. Da§, P. Legzdins and S. J. Rettig, J. Am. Chem. Soc., 1998, 120, 2688. 108 W. Czado and U. Muller, Z. Anorg. Allg. Chem., 1998, 624, 103. 109 H. Seyeda and M. Jansen, J. Chem. Soc., Dalton Trans., 1998, 875. 110 W. Klein, K. Armbruster and M. Jansen, Chem. Commun., 1998, 707. 111 G.M. Lucier, C. Shen, S. H. Elder and N. Bartlett, Inorg. Chem., 1998, 37, 3829. 112 M. A. Mendes, L. A. B. Moraes, R. Sparrapan, M.N. Eberlin, R. Kostiainen and T. Kotiaho, J. Am. Chem. Soc., 1998, 120, 7869. 113 F. Cacace, R. Cipollini, G. dePetris, F. Pepi, M. Rosi and A. Sgamellotti, Inorg. Chem., 1998, 37, 1398. 114 K. Johnson and J. W. Steed, Chem. Commun., 1998, 1479. 115 R. Minkwitz, S. Schneider and A. Kornath, Inorg. Chem., 1998, 37, 4662. 116 R. Minkwitz, C. Hirsch and H. Hartl, Angew. Chem., Int. Ed., 1998, 37, 1681. 117 K. Kobayashi, N. Deguchi, E. Horn and N. Furukawa, Angew. Chem., Int. Ed., 1998, 37, 984. 118 A. L. Hector, W. Levason and M. Webster, J. Chem. Soc., Dalton Trans., 1998, 3463. 119 L. Zhang and P. A. Lay, Inorg. Chem., 1998, 37, 1729. 120 H. Molina-Svedsen, G. Bojesen and C. J. McKenzie, Inorg. Chem., 1998, 37, 1981. 121 J. Premkumar and R. Ramaraj, J. Chem. Soc., Dalton Trans., 1998, 3667. 122 H. Komatsuzaki, Y. Nagasu, K. Suzuki, T. Shibasaki, M. Satoh, F. Ebina, S. Hikichi, M. Akita and Y. Moro-oka, J. Chem. Soc., Dalton Trans., 1998, 511. 123 M. Kinne, A. Heidenriech and K. Rademann, Angew. Chem., Int. Ed., 1998, 37, 2509. 124 M. A. Cinella, G. Minghetti, M. Vittoria Pinna, S. Stoccoro, A. Zucca and M. Manassero, Chem. Commun., 1998, 2397. 125 X. Wang, K. R. Heier, G. L. Stern and K. R. Poeppelmeier, Inorg. Chem., 1998, 37, 3252. 126 G. B. Karet, S. L. Castro, K. Folting, J. C. Bollinger, R. A. Heintz and G. Christou, J. Chem. Soc., Dalton Trans., 1998, 67. 127 F. Simon, K. Boubekeur, J.-C. P. Gabriel and P. Batail, Chem. Commun., 1998, 845. 128 A. Muller, M. Koop, H. Bogge, M. Schmidtmann and C. Beugholt, Chem. Commun., 1998, 1501. 129 A. Muller, E. Krickemeyer, H. Bogge, M. Schmidtmann, C. Beugholt, P. Kogerler and C. Lu, Angew. Chem., Int. Ed., 1998, 37, 1220. Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 67–78 78