DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 271–277 271 Generation of (Á2-benzyne)bis(triphenylphosphine)platinum(0): orthometallation of the Pt(PPh3)2 complexes of benzyne (C6H4 ) and cyclohexyne (C6H8) Martin A. Bennett,* Thomas Dirnberger, David C. R. Hockless, Eric Wenger and Anthony C. Willis Research School of Chemistry, Australian National University, GPO Box 414, Canberra, A.C.T. 2601, Australia The benzyne–platinum(0) complex [Pt(PPh3)2(h2-C6H4)] has been generated by treatment of a mixture of [Pt(PPh3)2(h2-C2H4)] and chlorobenzene with 2,2,6,6-tetramethylpiperid-1-yllithium at 0 8C and identified by comparison of its 31P NMR parameters with those of the cyclohexyne analogue, [Pt(PPh3)2(h2-C6H8)].The compounds isolated from the reaction and identified by NMR spectroscopy and X-ray crystallography are the (2,29-biphenyldiyl)platinum(II) complex [Pt(h1 :h1-C6H4C6H4)(PPh3)2], formed by reaction of [Pt(PPh3)2- (h2-C6H4)] with free benzyne, and the orthometallated (h1-phenyl)platinum(II) complex [Pt{C6H4(PPh2)-2}(C6H5)- (PPh3)], formed by internal hydrogen-atom migration in [Pt(PPh3)2(h2-C6H4)].The complex [Pt(PPh3)2(h2-C6H8)] undergoes a similar isomerization on heating in benzene to give the (h1-cyclohexen-1-yl)platinum(II) complex [Pt{C6H4(PPh2)-2}(C6H9)(PPh3)], whose structure has also been determined by X-ray crystallography. Short-lived cyclic alkynes, such as cycloheptyne (C7H10), cyclohexyne (C6H8) and benzyne (C6H4) can be stabilized by complex formation with a variety of transition-metal fragments,1–3 including those of the zerovalent d10 metals ML2 (M = Ni, Pd or Pt; L = various tertiary phosphines).A key compound in this work is the cyclohexyne complex [Pt(PPh3)2- (h2-C6H8)] 1, which was first prepared in high yield by the reduction of 1,2-dibromocyclohexene with 1% sodium amalgam in the presence of [Pt(PPh3)3] or [Pt(PPh3)2(h2-C2H4)].4,5 This reaction is believed to proceed via an undetected intermediate platinum(0) complex of 1,2-dibromocyclohexene (Scheme 1).3,6 More recently, Jones and co-workers 7 have made complex 1 by an alternative method in which a mixture of 1-bromocyclohexene and [Pt(PPh3)3] is treated at room temperature with lithium diisopropylamide, LiNPri 2; a likely intermediate is a platinum(0) complex of 1-bromocyclohexene, which would probably undergo rapid dehydrohalogenation in the presence of LiNPri 2 (Scheme 1).This procedure has been extended to generate the Pt(PPh3)2 complexes of the tropylium analogue of benzyne (tropyne, C7H5), [Pt(PPh3)2(h2-C7H5)]1,8 and the Pt(PPh3)2 complexes of other cyclic alkynes.9 Platinum(0)–benzyne complexes [PtL2(h2-C6H4)] [L2 = dcpe, 2P(C6H11)3, 2PEt3, 2PPri 3; dcpe = 1,2-bis(dicyclohexylphosphino) ethane, (C6H11)2PCH2CH2P(C6H11)2] have been made by reduction of the appropriate (o-halogenoaryl)platinum(II) precursors with 43% sodium amalgam (Scheme 2); 10 the weaker reducing agents 1% sodium amalgam or lithium, which are effective in forming nickel(0) complexes of benzyne and of 2,3-didehydronaphthalene from the corresponding nickel(II) precursors,11–13 do not work.However, all attempts to make the benzyne analogue of complex 1, [Pt(PPh3)2(h2-C6H4)] 2, by this procedure have failed, possibly because of preferential reductive cleavage of the P]Ph bond. Complex 2 also could not be obtained from the reaction of cis-[PtCl2(PPh3)2] with o-Li2- C6H4.14 Early attempts to trap benzyne,15,16 generated by thermal decomposition of benzenediazonium carboxylate or benzo- 1,2,3-thiadiazole-1,1-dioxide, with [Pt(PPh3)4] or [Pt(PPh3)2- (h2-C2H4)] were equally unsuccessful owing to the formation of chelate heterocyclic derivatives of platinum(II), such as compounds 3 and 4, which did not fragment to give complex 2; however, the formation of triphenylene in some of these reactions was believed to indicate the possibility of organoplatinum intermediates.The work described here resulted from attempts to generate [Pt(PPh3)2(h2-C6H4)] 2 by a modification of Jones’s procedure, i.e. by treatment of a mixture of [Pt(PPh3)2(h2-C2H4)] and chlorobenzene with the nonnucleophilic base 2,2,6,6-tetramethylpiperid-1-yllithium, Li[NCMe2CH2CH2CH2CMe2] (LiTMP). This reagent was chosen in the light of its reported reaction with chlorobenzene Scheme 1 (i) [Pt(PPh3)3] or [Pt(PPh3)2 (h2-C2H4)]; (ii) 1% Na–Hg; (iii) LiNPri 2; (iv) [Pt(PPh3)3] Pt Br Br Br Br PPh3 PPh3 Pt PPh3 PPh3 Pt Br Br PPh3 PPh3 ( iv ) possible intermediate possible intermediate 1 ( i ) ( ii ) ( iii ) Pt O Pt O S O O PPh3 PPh3 PPh3 PPh3 3 4272 J.Chem. Soc., Dalton Trans., 1998, Pages 271–277 to generate benzyne, which could be trapped in moderate to good yield in the form of its Diels–Alder adducts with 1,3- diphenylisobenzofuran, 2,5-dimethylfuran, pyrrole, or Nmethylisoindole. 17 This approach has also been used to synthesize a dinickel(0) complex of 1,2,4,5-tetradehydrobenzene (benz-1,4-diyne), [Ni2(dcpe)2(m-1,2-h2 : 4,5-h2-C6H2)] by LiTMP-promoted dehydrohalogenation of the 4-fluorobenzyne –nickel(0) complex [Ni(dcpe)(h2-C6H3F-4)] in the presence of [Ni(dcpe)(h2-C2H4)].18 Results A mixture of [Pt(PPh3)2(h2-C2H4)] and chlorobenzene was treated dropwise with approximately 4 equivalents of LiTMP at 0 8C.Monitoring by 31P NMR spectroscopy showed that only ca. 10% of [Pt(PPh3)2(h2-C2H4)] had undergone reaction.After the remaining LiTMP had been added, the 31P NMR spectrum showed, in addition to the singlet at dP 34.5 [1J(PtP) 3741 Hz] due to unchanged [Pt(PPh3)2(h2-C2H4)], a new singlet at dP 28.2 [1J(PtP) 3325 Hz] together with a small singlet at dP 29.0. The similarity of the 31P NMR parameters of the first formed species to those of the cyclohexyne complex [Pt(PPh3)2- (h2-C6H8)] 1 [ dP 28.3; 1J(PtP) 3406 Hz] 19 suggested that they could arise from the desired benzyne complex [Pt(PPh3)2- (h2-C6H4)] 2.Unfortunately, this species was not stable under the reaction conditions and attempts to isolate it failed; it decomposed to give two compounds whose relative amounts depended on temperature, and, more difficult to control, the amount of base in solution. In one experiment, a solution containing 2 was stored at 278 8C, but after 16 h the main species present was characterized by a singlet in the 31P NMR spectrum at d 29.0 [1J(PtP) 2003 Hz].This compound was isolated in a pure state by preparative thin-layer chromatography and was shown by X-ray structural analysis (see below) to be the (2,29- biphenyldiyl)platinum(II) complex, cis-[Pt(h1 :h1-C6H4C6H4)- (PPh3)2] 5. It showed a parent-ion peak in its electron impact (EI)-mass spectrum. The magnitude of 1J(PtP) is ca. 300 Hz greater than generally observed for neutral bis(tertiary arylphosphine) –h1-aryl complexes of the type cis-[PtX(R)L2],20 e.g.for X = R = C6H5, L = PPh3, values of 1J(PtP) in C6D6 of 1763 Hz20 and 1730 Hz21 have been reported. Compound 5 is believed to arise by reaction of the benzyne complex 2 with free benzyne at 278 8C (see Discussion) and can be isolated in 72% yield from reaction of a mixture of [Pt(PPh3)2(h2-C2H4)] and chlorobenzene with a large excess of LiTMP. At room temperature, a solution containing mainly 2 (and some 5) was stable for at least 1 h, but after heating to 50 8C or, alternatively, after evaporation of the solvent at room temperature, the main species present, 6, showed in its 31P NMR spectrum a pair of doublets at d 255.0 and 21.2 [2J(PP) 10.9 Hz] assignable to inequivalent, mutually cis phosphorus atoms, P(1) and P(2), in a planar platinum(II) complex.22 The shielding of P(1) suggests that this phosphorus atom is part of a fourmembered metallacycle,23 cf.[Pt{C6H4(PPh2)-2}2] (dP 252.3) 24 Scheme 2 (i) 43% Na–Hg Pt L L Br Pt Br L L Br P(C6H11)2 Pt (C6H11)2P Br L = PEt3, PPri 3 or P(C6H11)3 L = PEt3, PPri 3 or P(C6H11)3 2L = dcpe ( i ) and [Pt{C6H4(PPh2)-2}(PPh3)2][CF3SO3] (dP 268.3),25 and the remarkably low value of 1J(PtP1), 1021 Hz, indicates that P(1) is trans to a ligand of high trans influence, presumably s-bonded carbon, cf.[Pt{C6H4(PPh2)-2}2], 1352 Hz.24 It should be noted, however, that 1J(PtP) for the phosphorus atom trans to the s-bonded carbon atom of the cycloplatinated ring in [Pt{C6H4(PPh2)-2}(PPh3)2][CF3SO3] is 2006 Hz,25 so these coupling constants can clearly span a wide range.The magnitude of 1J(PtP2) in complex 6 is 2047 Hz, which allows P(2) to be assigned tentatively to PPh3 trans to a s-bonded carbon atom. The 31P NMR data, therefore, suggest the formulation of 6 as cis-[Pt{C6H4(PPh2)-2}(h1-C6H5)(PPh3)]. This compound was also obtained in a pure state by thin-layer chromatography and its structure was confirmed by X-ray crystallography (see below). It is clearly an isomer of the benzyne complex 2 derived by migration of a hydrogen atom from triphenylphosphine to co-ordinated benzyne.The sequence of reactions occurring on treatment of [Pt(PPh3)2(h2-C2H4)] and chlorobenzene with LiTMP is summarized in Scheme 3. The suggested origin of complex 6 receives additional support from the observation that the cyclohexyne complex [Pt(PPh3)2(h2-C6H8)] 1 undergoes a similar, though much slower, isomerization to the (h1-cyclohexen-1-yl)platinum(II) complex [Pt{C6H4(PPh2)-2}(h1-C6H9)(PPh3)] 7 (Scheme 4).This compound was isolated in good yield as a yellow solid when complex 1 was heated in benzene under reflux for 18 d and its identity was confirmed by X-ray crystallography (see below). Use of unrecrystallized samples of complex 1 also gave small amounts of trans-[PtBr(h1-C6H9)(PPh3)2], presumably derived from NaBr impurity in the original preparation.4,5 The 31P NMR spectrum of complex 7 is similar to that of 6, consisting of a doublet at d 252.8 [2J(PP) 10.9, 1J(PtP) 861 Hz] due to the phosphorus atom P(1) in the cyclometallated fourmembered ring and a doublet at d 20.8 [2J(PP) 10.9, 1J(PtP) Scheme 3 (i) [Pt(PPh3)2(h2-C2H4)], LiTMP, 0 8C; (ii) room temperature Pt Cl Pt PPh3 PPh3 Ph2P Pt P2Ph3 PPh3 PPh3 2 5 6 1 ( i ) ( ii ) Scheme 4 (i) C6H6, reflux Pt PPh3 PPh3 Ph2P Pt C3 P2Ph3 C1 C2 1 7 ( i ) 1J.Chem. Soc., Dalton Trans., 1998, Pages 271–277 273 2144 Hz] due to the phosphorus atom P(2) cis to the cyclohexenyl group.The Pt]P couplings are reproduced in the 195Pt NMR spectrum, which shows the expected doublet of doublets at d 23981 (relative to K2PtCl6). The vinylic proton of the h1- cyclohexen-1-yl group appears as a doublet of multiplets at d 5.26 [J(PH) 11.2, J(PtH) 84 Hz], the chemical shift and coupling constants being similar to those of other (h1-cyclohexen- 1-yl)platinum(II) complexes 5,19 and of [Pt{C6H4(PPh2)-2}{h1- C(CO2Me)]] CHCO2Me}(PPh3)].26 In the 13C NMR spectrum of 7, signals due to the quaternary carbon atoms (numbered as in Scheme 4) were located: a doublet of doublets at d 149.70 [J(PC) 107.4, 9.4, 1J(PtC) 905 Hz] due to a carbon atom s bonded to the metal, either that of the cycloplatinated ring (C1) or of the h1-cyclohexen-1-yl group (C3), a broad doublet at d 153.20 [J(PC) 53.0, 2J(PtC) 32 Hz] due to the remaining carbon atom (C2) of the four-membered ring, and a broad doublet at d 154.51 [J(PC) 117.1, 1J(PtC) 844 Hz] due to C3 or C1.The chemical shifts and coupling constants are similar to those of the cycloplatinated ring in [Pt{C6H4(PPh2)-2}(PPh3)3]- [CF3SO3].25 The magnitudes of 1J(PtP) trans to C6H9 in complex 7 (861 Hz), which is one of the smallest 1J(PtP) values reported for a phosphorus trans to a carbon atom, and of 1J(PtP) trans to C6H5 in complex 6 (1021 Hz) follow the same trend as observed for 1J(PtP) trans to the carbon s-donor in [PtCl(R9)(Ph2PCH2CH2PPh2)] [R9 = C6H9, 1J(PtP) = 1558 Hz; R9 = C6H5, 1J(PtP) = 1613 Hz],27 and indicate that cyclohexen- 1-yl has a slightly higher NMR trans influence than phenyl.However, the difference is too small to be reflected in the Pt]P bond lengths (see below). Complex 7 was also formed when 1 was heated at 80 8C in [2H8]toluene, but prolonged reaction in the refluxing solvent generated two more compounds, 8 and 9, whose 31P and 195Pt NMR parameters were closely similar to, but clearly distinct from, those of 7 (see Experimental section); these compounds clearly contain the cycloplatinated unit Pt[C6H4(PPh2)-2].The final solutions contained approximately equal amounts of compounds 7 and 8 and only minor amounts (5–10%) of 9. Similar changes occurred when 1 was heated over several days in refluxing methylcyclohexane, thus eliminating the obvious possibility that the new compounds were isomeric tolyl complexes resulting from the oxidative addition of toluene to complex 7 and subsequent reductive elimination of cyclohexene.The compounds may be cyclohexen-2-yl or cyclohexen- 3-yl isomers of complex 7 resulting from a metal-catalysed migration of the double bond in the six-membered ring. Unfortunately the compounds could not be separated by fractional crystallization or column chromatography, and attempts to promote a double-bond shift in complex 7 by heating it in the presence of a base (NEt3) were unsuccessful. Molecular structures of [Pt(Á1 :Á1-C6H4C6H4)(PPh3)2] 5, cis- [Pt{C6H4(PPh2)-2}(Á1-C6H5)(PPh3)] 6 and cis-[Pt{C6H4(PPh2)- 2}(Á1-C6H9)(PPh3)] 7 The molecular geometry of complex 5 is shown in Fig. 1 together with atom numbering. Selected interatomic distances and angles are listed in Table 1. The molecule occupies a general position in the unit cell. The metal atom Pt(1) is in a distorted square-planar co-ordination environment, carbon atoms C(1) and C(19) being, respectively, 0.422 Å above and 0.293 Å below the plane defined by Pt(1) and the mutually cis phosphorus atoms.The aromatic rings of the biphenyldiyl ligand are planar, with a dihedral angle of 14.88. The two Pt]C bond lengths [Pt(1)]C(1) 2.068(5), Pt(1)]C(19) 2.092(5) Å] are slightly but significantly different, this difference probably arising from crystal packing. The aromatic C]C bonds in the platinacycle [C(1)]C(2) 1.421(6), C(19)]C(29) 1.427(6) Å] are longer than the remaining C]C bonds in the phenyl rings, which are in the usual range (1.366–1.393 Å).The separation between the linked carbon atoms of the two phenyl rings [C(2)]C(29) = 1.466(7) Å] is similar to those reported for other (2,29-biphenyldiyl)- platinum(II) complexes, i.e. [PtL(PPh3)2] [1.461(11) Å],29 [PtL9- (cod)] [1.486(10) Å],29 and [PtL9(bipy)] [1.493(14) Å] 30 [L = 5,59-bis(trifluoromethyl)-2,29-biphenyldiyl; L9 = 5,59-bis- (tert-butyl)-2,29-biphenyldiyl; cod = cycloocta-1,5-diene; bipy = 2,29-bipyridyl]. The Pt]C and Pt]P distances [Pt(1)]C(1) 2.068(5), Pt(1)]C(19) 2.092(5); Pt(1)]P(1) 2.333(1), Pt(1)]P(2) 2.345(1) Å] are also similar to those found in the 5,59- bis(trifluoromethyl) derivative [Pt]C 2.058(7), 2.065(7); Pt]P 2.328(2), 2.346(2) Å].29 Other bond lengths in complex 5 are unexceptional.The molecular geometries of complexes 6 and 7 are very similar, and are shown in Figs. 2 and 3 together with the atom numbering. Selected interatomic distances and angles are given in Tables 2 and 3, respectively.In both compounds the metal atom lies almost in the co-ordination plane defined by the two mutually cis phosphorus atoms and the s-bonded carbon atoms; the distances from the plane P(1), P(2), C(1) and C(8) Fig. 1 An ORTEP28 diagram of [Pt(h1 :h1-C6H4C6H4)(PPh3)2] 5 with atom labelling and 20% probability ellipsoids Table 1 Selected bond distances (Å) and angles (8) for [Pt(h1 :h1- C6H4C6H4)(PPh3)2] 5 Pt(1)]P(1) Pt(1)]C(1) C(1)]C(2) C(2)]C(29) C(19)]C(69) P(1)]Pt(1)]P(2) P(1)]Pt(1)]C(1) P(2)]Pt(1)]C(1) Pt(1)]C(1)]C(2) 2.333(1) 2.068(5) 1.421(6) 1.466(7) 1.388(6) 94.19(5) 95.1(1) 165.0(1) 115.7(4) Pt(1)]P(2) Pt(1)]C(19) C(19)]C(29) C(1)]C(6) C(1)]Pt(1)]C(19) P(1)]Pt(1)]C(19) P(2)]Pt(1)]C(19) Pt(1)]C(19)]C(29) 2.345(1) 2.092(5) 1.427(6) 1.388(7) 79.7(2) 169.3(1) 92.9(1) 113.4(4) Table 2 Selected bond distances (Å) and angles (8) for cis- [Pt{C6H4(PPh2)-2}(h1-C6H5)(PPh3)] 6 Pt(1)]P(1) Pt(1)]C(1) P(1)]C(7) C(1)]C(2) P(1)]Pt(1)]P(2) P(1)]Pt(1)]C(8) P(2)]Pt(1)]C(8) Pt(1)]P(1)]C(7) C(7)]C(8)]C(9) Pt(1)]C(8)]C(9) 2.330(2) 2.052(7) 1.806(7) 1.40(1) 104.18(7) 68.7(2) 171.1(2) 84.3(3) 117.2(6) 136.3(6) Pt(1)]P(2) Pt(1)]C(8) C(7)]C(8) C(1)]C(6) P(1)]Pt(1)]C(1) P(2)]Pt(1)]C(1) C(1)]Pt(1)]C(8) P(1)]C(7)]C(8) Pt(1)]C(8)]C(7) C(8)]C(7)]C(12) 2.309(2) 2.057(6) 1.41(1) 1.41(1) 159.3(2) 95.9(2) 90.8(3) 100.6(5) 106.5(5) 123.8(6)274 J.Chem. Soc., Dalton Trans., 1998, Pages 271–277 are only 0.092 Å and 0.069 Å, respectively, and the bound carbon atoms C(1) and C(8) in both complexes are less than 0.2 Å below the plane defined by Pt(1), P(1) and P(2).The angle subtended at the metal atom in the orthometallated ring in both compounds is 698 (cf. 698 in [Pt{C6H4(PPh2)-2}2]); 24 there are corresponding increases from the ideal value of 908 in the valence angles P(1)]Pt(1)]P(2) [104 (6), 1078 (7)]. The Pt]P distances in both four-membered rings [Pt(1)]P(1) 2.330(2) (6), 2.336(1) Å (7)] are comparable both to those in the cycloplatinated complexes [Pt{C6H4(PPh2)-2}2] ][2.297(1) Å] 24 and [Pt{C6H4(PPh2)-2}{h1-C(CO2Me)]] CH(CO2Me)}(PPh3)] [2.329(2) Å],31 and to the Pt]P bond length to the unmetallated PPh3 ligand [Pt(1)]P(2) = 2.309(2) (6), 2.296(1) Å (7)].The Pt]C distances in the four-membered ring of all four platinacycles discussed above fall in the narrow range 2.056–2.063 Å, and are similar to the Pt]C6H5 bond length in 6 [Pt(1)]C(1) 2.052(7) Å] and to the Pt]C6H9 bond length in 7 [Pt(1)]C(1) 2.054(5) Å].Discussion Although the benzyne complex [Pt(PPh3)2(h2-C6H4)] 2 is generated by treatment of a mixture of chlorobenzene and [Pt(PPh3)2(h2-C2H4)] with LiTMP, the procedure is evidently not as successful as that used by Jones and co-workers 7 to prepare the cyclohexyne complex [Pt(PPh3)2(h2-C6H8)] 1 (Scheme 1). In principle, there are two possible routes by which complex 2 could have been formed: (i) deprotonation of a transient intermediate dihapto chlorobenzene complex [Pt(PPh3)2(h2- C6H5Cl)], analogous to the intermediate 1-bromocycloheptene complex [Pt(PPh3)2(h2-C7H11Br)] detected by Jones and co- Fig. 2 An ORTEP diagram of cis-[Pt{C6H4(PPh2)-2}(h1-C6H5)(PPh3)] 6 with atom labelling and 20% probability ellipsoids Table 3 Selected bond distances (Å) and angles (8) for cis- [Pt{C6H4(PPh2)-2}(h1-C6H9)(PPh3)] 7 Pt(1)]P(1) Pt(1)]C(1) P(1)]C(7) C(1)]C(2) P(1)]Pt(1)]P(2) P(1)]Pt(1)]C(8) P(2)]Pt(1)]C(8) Pt(1)]P(1)]C(7) C(7)]C(8)]C(9) Pt(1)]C(8)]C(9) 2.336(1) 2.054(5) 1.799(5) 1.361(8) 106.95(4) 68.6(1) 174.5(1) 84.1(2) 117.2(5) 136.6(4) Pt(1)]P(2) Pt(1)]C(8) C(7)]C(8) C(1)]C(6) P(1)]Pt(1)]C(1) P(2)]Pt(1)]C(1) C(1)]Pt(1)]C(8) P(1)]C(7)]C(8) Pt(1)]C(8)]C(9) C(8)]C(7)]C(12) 2.296(1) 2.057(5) 1.409(7) 1.466(8) 159.7(2) 92.8(2) 91.3(2) 101.0(4) 106.2(3) 122.5(5) workers 7 in the preparation of the cycloheptyne complex [Pt(PPh3)2(h2-C7H10)]; (ii) deprotonation of chlorobenzene to give free benzyne, which is trapped by [Pt(PPh3)2(h2-C2H4)].It is plausible that the vinylic halides 1-bromocyclohexene and 1-bromocycloheptene form much stronger p complexes than the aromatic halide chlorobenzene with platinum(0); hence free cyclohexyne or cycloheptyne are not formed whereas under similar reaction conditions free benzyne is readily generated. By whatever route complex 2 is formed, it is clearly capable of reacting rapidly with the highly reactive alkyne benzyne to give [ Pt(h1 :h1-C6H4C6H4)(PPh3)2] 5.This insertion is analogous to the first step, forming a benzonickelacyclopentadiene, of the double insertion of alkynes into nickel(0)–benzyne and nickel(0)–2,3-h-didehydronaphthalene bonds to give, respectively, substituted naphthalenes and anthracenes after reductive elimination of the nickel(0) fragment.3,11–13,32 Fewer reactions of this type are known with platinum(0) complexes: complex 1 is inert towards alkynes, although its derivatives [Pt(R02- PCH2CH2PR02)(h2-C6H8)] (R0 = Me, Et or C6H11) undergo monoinsertion with dimethyl acetylenedicarboxylate to give [Pt{C6H8C(CO2Me)]] C(CO2Me)}(R02PCH2CH2PR02)].33 Benzyne has been reported to insert into the Ni]CH2 bond of the metallacycle [Ni{C6H4(CMe2CH2)-2}(PMe3)2] to give, after reductive elimination, 9,9-dimethyl-9,10-dihydrophenanthrene. 34 It also inserts into the metal–phenylacetylide bond of the trichlorovinylnickel(II) complex trans-[Ni(C2Ph)(C2Cl3)- (PEt3)2] to give trans-[Ni(C6H4C2Ph-2)(C2Cl3)(PEt3)2], together with the product of reductive elimination, C6H4(C2Ph)-1- (C2Cl3)-2.35 A second reason for the failure to isolate the benzyne complex 2 is that it readily isomerizes at or just above room temperature to the orthometallated complex cis-[Pt{C6H4(PPh2)- 2}(h1-C6H5)(PPh3)] 6.The mechanism by which the hydrogen atom is transferred from PPh3 to the unsaturated fragment is not known, but the process is clearly faster than the corresponding isomerizations of the cyclohexyne complex 1 to the cyclohexen-1-yl complex 7 and of the dimethyl acetylenedicarboxylate complex [Pt(PPh3)2(h2-MeO2CC2CO2Me)] to the cis-1,2-bis(methoxycarbonyl)vinyl complex [Pt(C6H4PPh2-2)- {h1-C(CO2Me)]] CH(CO2Me)}(PPh3)],26,31 which require long reaction times at elevated temperatures.The difference may reflect the relatively greater strain and weaker binding of benzyne to the platinum(0) centre. Other orthometallations of platinum(0)–triphenylphosphine complexes also generally require forcing conditions, e.g.irradiation at 254 nm for the Fig. 3 An ORTEP diagram of cis-[Pt{C6H4(PPh2)-2}(h1-C6H9)(PPh3)] 7 with atom labelling and 20% probability ellipsoidsJ. Chem. Soc., Dalton Trans., 1998, Pages 271–277 275 isomerization of [Pt(PPh3)2(h2-C2H4)] to [Pt{C6H4(PPh2)-2}- (C2H5)(PPh3)] 36 and elevated temperatures for the formation of dinuclear or polynuclear cycloplatinated complexes from [Pt(PPh3)n] (n = 2–4).37–41 Experimental General procedures All experiments were performed under an inert atmosphere with use of standard Schlenk techniques, and all solvents were dried and degassed prior to use.All reactions involving benzyne complexes were carried out under argon. The NMR spectra were recorded on the following spectrometers: Varian XL-200E (1H at 200 MHz, 13C at 50.3 MHz, 31P at 80.96 MHz and 195Pt at 42.83 MHz), Varian Gemini-300 BB (1H at 300 MHz, 13C at 75.4 MHz and 31P at 121.4 MHz), Varian VXR-300 (1H at 300 MHz and 13C at 75.4 MHz) and Varian VXR-500 (1H at 500 MHz).The chemical shifts (d) for 1H and 13C are given in ppm relative to residual signals of the solvent, to external 85% H3PO4 for 31P and to external K2PtCl6 for 195Pt. The spectra of all nuclei (except 1H) were 1H decoupled. The coupling constants (J) are given in Hz. Infrared spectra were measured in solid KBr or in solution (KBr cells) on Perkin-Elmer 683 or 1800 FT-IR spectrometers.Mass spectra were obtained by the electron impact (EI) method on a VG Micromass 7070F or a Fisons Instruments VG AutoSpec spectrometer. Starting materials The ethene complex [Pt(PPh3)2(h2-C2H4)] was prepared as described by Nagel.42 The cyclohexyne complex [Pt(PPh3)2- (h2-C6H8)] 1, obtained by a published procedure,5 was washed thoroughly with air-free water and recrystallized from toluene– hexane (1 : 6) before use. Reaction of LiTMP with chlorobenzene in the presence of [Pt(PPh3)2(Á2-C2H4)] In a typical experiment, a solution of LiTMP in tetrahydrofuran (thf) (10 cm3), prepared from 2,2,6,6-tetramethylpiperidine (0.23 cm3, 1.35 mmol) and LiBun (0.79 cm3 of 1.37 M solution in hexane, 1.08 mmol), was added over 1.5 h to a thf solution (10 cm3) at 0 8C containing [Pt(PPh3)2(h2-C2H4)] (200 mg, 0.27 mmol) and chlorobenzene (0.29 cm3, 2.7 mmol).After addition of 1 equivalent of base, monitoring by 31P NMR spectroscopy showed that only 10% of [Pt(PPh3)2(h2-C2H4)] had reacted to form [Pt(PPh3)2(h2-C6H4)] 2.After complete addition of the base and further stirring for 2 h at room temperature, the solution contained a mixture of [Pt(PPh3)2- (h2-C2H4)] [dP 34.5, J(PtP) 3741], [Pt(h1 :h1-C6H4C6H4)(PPh3)2] 5 and 2 in a ratio of 2.1:1:3.9. Attempts to isolate the benzyne complex were unsuccessful. For example, after removal of most of the toluene in vacuo and addition of hexane (20 cm3), the solution was left for 16 h at 278 8C but no crystallization occurred.After evaporation of the solvent, the 31P NMR spectrum of the residue showed the presence of a 3 : 1 mixture of compounds 5 and 2, indicating that further reaction of 2 with free benzyne to give 5 had occurred. In another work-up, the reaction mixture was left at room temperature and the solvent was evaporated. The 31P NMR spectrum of the residue showed the presence of a 1.8:1:2.4 mixture of compounds 5, 2 and 6 with only a trace of [Pt(PPh3)2(h2-C2H4)]; rearrangement of 2 into the cyclometallated product 6 had occurred. Several fractions were combined and complexes 5 and 6 were separated by preparative TLC (silica gel, hexane–diethyl ether 5 : 1); 6 migrated faster than 5.Yellow crystals of 5 and colourless crystals of 6 suitable for X-ray analysis were obtained from toluene–hexane and chlorobenzene–hexane, respectively. The amount of 6 was, however, insufficient for microanalysis. In another experiment, the biphenyldiyl complex 5 was prepared by adding chlorobenzene (0.72 cm3, 6.7 mmol) and a solution of [Pt(PPh3)2(h2-C2H4)] (500 mg, 0.67 mmol) in thf (15 cm3) to a solution of LiTMP (2.68 mmol) in thf (20 cm3) at 260 8C.The solution was stirred for 2.5 h at 0 8C and 2 h at room temperature. As the 31P NMR spectrum of the solution showed that some 2 was still present, further LiBun (1 cm3 of 1.37 M solution in hexane) was added dropwise and the mixture was stirred for 16 h at room temperature.After evaporation of the solvent, the crude product was dissolved in diethyl ether and the solution was filtered through a silica gel column. Removal of the solvent afforded pure 5 (421 mg, 72%). Complex 2: dP(80.96 MHz, C6D6) 28.2 [J(PtP) 3325]. Complex 5: (Found: C, 65.6; H, 4.1. C48H38P2Pt requires C, 66.1; H, 4.4%); dH(300 MHz, CD2Cl2) 7.00–7.70 (m, 34 H); dC(75.43 MHz, CD2Cl2) 127.9–128.4 (m), 128.75, 129.90, 129.99, 132.19, 132.32, 134.2– 135.5 (m); dP(80.96 MHz, C6D6) 29.0 [J(PtP) 2003]; m/z (C48H38P2Pt) 871 (M1, 5%), 262 (100), 228 (66), 183 (44), 154 (44), correct isotopic patterns.Complex 6: dH(500 MHz, CD2Cl2) 6.75–7.40 (m, 34 H); dC(75.43 MHz, CD2Cl2) 126.27, 127.57, 127.69, 128.02, 128.15, 128.24, 129.44 [d, J(PC) 2.2], 129.52, 129.60 [d, J(PC) 2.7], 132.71, 132.86, 133.87, 134.03, 137.58 [J(PtC) 38.5, CH]; dP(121.4 MHz, C6D6) 255.0 [d, J(PP) 10.9, J(PtP) 1021, P(1)], 21.2 [d, J(PP) 10.9, J(PtP) 2047, P(2)]; m/z (C42H34P2Pt) 795 (M1, 38%), 718 (5), 455 (5), 377 (6), 262 (100), 228 (24), 183 (62), 154 (35), correct isotopic patterns.Preparation of cis-[Pt{C6H4(PPh2)-2}(Á1-C6H9)(PPh3)] 7 A solution of [Pt(PPh3)2(h2-C6H8)] 1 (0.16 g, 0.2 mmol) in benzene (5 cm3) was stirred under reflux for 18 d and the solvent was removed by evaporation under reduced pressure. The yellow residue was recrystallized from CH2Cl2–hexane to give 7 as a pale yellow crystalline solid (96 mg, 75%) (Found; C, 63.5; H, 5.1.C42H38P2Pt requires C, 63.1; H, 4.8%); m.p. 234 8C (decomp.); n& max/cm21 (KBr) 3040w, 2980w, 2920m, 2859w, 2810m, 1600w, 1588w, 1560m, 1480s, 1430s, 1308m, 1275m, 1095s, 1035m, 998m, 745s, 735s, 720s, 690s, 530s, 510s, 450m, 440m; dH(200 MHz, CD2Cl2) 0.35–0.55 (br, 4 H, CH2), 1.80– 2.00 (br, 2 H, CH2), 2.05–2.25 (br, 2 H, CH2), 5.26 [dm, 1 H, J(PH) 11.2, J(PtH) 84, ]] CH], 7.00–7.65 (m, 29 H, Harom), 7.65– 7.75 [m, 1 H, J(PtH) 57, PtC]] CHortho]; dC(50.3 MHz, CD2Cl2) 24.39 [C(4)]H2], 26.36 [d, J(PC) 5.7, J(PtC) 59.6, C(5)]H2], 29.55 [d, J(PC) 9.6, J(PtC), 86.4, C(6)]H2], 38.44 [d, J(PC) 4.5, J(PtC) 57.2, C(3)]H2], 125.00 [m, J(PtC) 20, CH], 127.70 (m, CH), 128.15 [d, J(PC) 9.7, CH], 128.26 [d, J(PC) 9.7, CH], 128.62 [d, J(PC) 9.5, CH], 128.75 [d, J(PC) 9.5, CH], 129.97 (br s, CH), 130.25 (CH), 131.45 [d, J(PC) 32, C], 131.70 (m, CH), 133.30 [d, J(PC) 10.7, CH], 133.43 [d, J(PC) 10.7, CH], 134.0– 134.6 (m, C), 134.58 [d, J(PC) 11.7, CH], 134.72 [d, J(PC) 11.7, CH], 138.25 [m, J(PtC) 39, CH], 149.70 [dd, J(PC) 107.4, 9.4, J(PtC) 905, C(112) or C(31)], 153.20 [br d, J(PC) 53.0, J(PtC) 32, C(111)], 154.51 [br d, J(PC) 117.1, J(PtC) 844, C(31) or C(112)]; dP(80.96 MHz, CD2Cl2) 252.8 [d, J(PP) 10.9, J(PtP) 861, P(1)], 20.8 [d, J(PP) 10.9, J(PtP) 2144, P(2)]; dPt(42.83 MHz, CD2Cl2) 23981 [dd, J(PtP) 2144, 861].Under the same conditions, samples of compound 1 that had not been freed from NaBr gave a ca. 4 : 1 mixture of complex 7 and trans-[PtBr(h1-C6H9)(PPh3)2], which could be almost completely separated by fractional crystallization from CH2Cl2– hexane. The latter compound was identified by comparison of its spectroscopic parameters with those of a sample prepared by treatment of complex 1 first with the calculated quantity of 0.1 M HCl in thf to give a mixture of cis- and trans-[PtCl- (h1-C6H9)(PPh3)2] and then with NaBr to give the required product as the colourless trans isomer (Found: C, 56.3; H, 4.5.C42H39BrP2Pt?0.25CD2Cl2 requires C, 55.3; H, 4.5%); m.p. 213 8C (decomp.); n& max/cm21 (KBr) 3070w, 3050w, 2920m, 2850m, 2820m, 1615w, 1585w, 1570w, 1480s, 1430s, 1095s, 740s, 690s, 520s, 510s, 495s; dH(200 MHz, CD2Cl2) 0.25–0.35 (br m, 2 H, CH2), 0.40–0.55 (br m, 2 H, CH2), 1.20–1.30 (br m, 2 H,276 J. Chem. Soc., Dalton Trans., 1998, Pages 271–277 Table 4 Crystal and structure refinement data for [Pt(h1 :h1-C6H4C6H4)(PPh3)2] 5, cis-[Pt(C6H4PPh2-2)(h1-C6H5)(PPh3)] 6 and cis-[Pt(C6H4PPh2-2)- (h1-C6H9)(PPh3)] 7 Compound Chemical formula M Crystal system a/Å b/Å c/Å a/8 b/8 g/8 U/Å3 Space group Dc/g cm23 Z T/K F(000) Colour, habit Crystal size/mm m/cm21 Diffractometer X-Radiation Scan mode w Scan width 2q limits/8 h, k, l Ranges Total reflections Unique reflections Used reflections Corrections (transmission factors) Structure solution Refinement No.of parameters g in Weighting scheme d R (used reflections) R9 (used reflections) Goodness of fit rmax, rmin/e Å23 5 C48H38P2Pt 964.01 Triclinic 12.910(4) 13.077(3) 4.724(3) 74.05(2) 79.12(2) 65.70(2) 2170(1) P1� (no. 2) 1.475 2 293 968 Yellow, irregular 0.32 × 0.24 × 0.16 33.31 Rigaku AFC6S Mo-Ka (graphite monochromated) w–2q 0.80 1 0.34 tan q 50.1 (0, 216, 218) to (15, 16, 18) 8057 7681 (Rint = 0.021) 6137 [I > 3s(I)] Azimuthal scans (0.8806–1.0000) Direct methods a (SHELXS 86,48 DIRDIF 9449) Full-matrix least squares 523 0.002 0.030 0.024 1.59 0.83, 20.79 6 C42H34P2Pt?1.22C6H5Cl?0.28C6H14 795.78 1 162.38 Monoclinic 10.910(3) 22.913(4) 16.877(4) 100.28(2) 4151.0(17) P21/n (no.14) 1.533 4 298 1915 Colourless, trapezoidal 0.06 × 0.09 × 0.12 80.3 Rigaku AFC6R Cu-Ka (graphite monochromated) w–2q 1.21 1 0.30 tan q 120 (0, 0, 218) to (12, 25, 18) 6368 6176 (Rint = 0.015) 4454 [I > 2s(I)] Analytical (0.519–0.706) Direct methods b (SIR 92) 53 Full-matrix least squares with conditions 55 455 0.015 0.035 0.047 1.05 0.91, 20.75 7 C42H38P2Pt 799.79 Monoclinic 11.178(1) 14.892(1) 21.156(1) 98.20(1) 3485.7(4) P21/c (no. 14) 1.524 4 293 1592 Colourless, block 0.24 × 0.15 × 0.26 86.89 Philips PW1100/20 Cu-Ka (graphite monochromated) w–2q 1.2 1 0.142 tan q 128 (213, 0, 0) to (12, 17, 24) 6228 5798 5249 [I > 3s(I)] Analytical (0.155–0.386) Patterson method c (SHELXS 86)48 Full-matrix least squares 406 0.01 0.028 0.046 1.827 0.4, 21.2 a All calculations were performed by use of TEXSAN43 with neutral atom scattering factors from Cromer and Waber,44 Df and Df9 values from ref. 45 and mass attenuation coefficients from ref. 46. Anomalous dispersion effects were included in Fc.47 b Structure solved with TEXSAN,50 data reduction and refinement were performed using XTAL 3.4,51 with neutral atom scattering factors, Df and Df9 values from ref. 52. c Structure solved with SHELXS 86,48 data reduction and refinement were performed using XTAL 3.0,54 with neutral atom scattering factors, Df and Df9 values from ref. 52. d w = 4Fo 2/[s2(Fo 2) 1 (gFo 2)2]. CH2), 1.45–1.55 (br m, 2 H, CH2), 5.21 [br s, 1 H, J(PtH) 69.0, ]] CH], 7.43 (br s, 18 H, Harom), 7.70–7.85 (m, 12 H, Harom); dC(50.3 MHz, CD2Cl2) 22.41 [C(4)]H2], 24.60 [s, J(PtC) 54.2, C(5)]H2], 28.97 [s, J(PtC) 81.0, C(6)]H2], 37.14 [s, J(PtC) 40.1, C(3)]H2], 126.20 [t, J(PC) 4.2, C(2)]H], 127.98 [t, J(PC) 5.2, CH], 130.47 (CH), 131.80 [t, J(PC) 27.7, J(PtC) 21.3, Carom], 135.71 [t, J(PC) 5.8, CH], 138.10 [t, PC) 8.3, PtC(1)], J(PtC) not resolved for C(1) and C(2); dP(80.96 MHz, CD2Cl2) 24.8 [s, J(PtP) 3322]; dPt(42.83 MHz, CD2Cl2) 24448 [t, J(PtP) 3323]; m/z (C42H39BrP2Pt) 880 (M1, 5%), 846 (8), 800 (56), 719 (71), 307 (100), correct isotopic patterns.Isomerization of complex 7. Qualitative NMR experiments showed that when complex 1 (40 mg) was heated in [2H8]toluene (1.5 cm3) at various temperatures for 4 d, the formation of 7 was accompanied by an increasing amount of an isomer 8 and small amounts of a second isomer 9.The proportions as determined by 31P NMR spectroscopy were 1.00 : 0.10 : 0.05 (80 8C), 1.00 : 1.00 : 0.15 (120 8C) and 1.00 : 1.60 : 0.05 (132 8C), respectively. Traces of other unidentified complexes were also observed. Complex 8: dP(80.96 MHz, CD2Cl2) 255.4 [d, J(PP) 11.5, J(PtP) 1052.9, P(1)], 20.3 [d, J(PP) 11.5, J(PtP) 2061.4, P(2)]; dPt(42.83 MHz, CD2Cl2) 23929 [dd, J(PtP) 2061, 1055]. Complex 9: dP(80.96 MHz, CD2Cl2) 253.6 [d, J(PP) 11.0, J(PtP) 849, P(1)], 24.8 [d, J(PP) 11.0, J(PtP) 2154, P(2)]; dPt(42.83 MHz, CD2Cl2) 23985 [dd, J(PtP) 2155, 851].X-Ray crystallography of [Pt(Á1 :Á1-C6H4C6H4)(PPh3)2] 5, cis- [Pt{C6H4(PPh2)-2}(Á1-C6H5)(PPh3)] 6 and cis-[Pt{C6H4(PPh2)- 2}(Á1-C6H9)(PPh3)] 7 Selected crystal data, details of data collection, data processing, structure analysis and structure refinement are in Table 4. The structure of complex 5 was solved by direct methods (SHELXS 86)48 and was expanded using Fourier techniques (DIRDIF 94).49 The calculations were performed using TEXSAN (version 1.6c).43 The structure of 6 was solved by direct methods (SIR 92) 53 using TEXSAN (version 1.7).50 One solvation molecule of chlorobenzene was identified in a general crystallographic position, plus further molecules of solvation about the centre of symmetry ��� , 0, 1 corresponding to disordered chlorobenzene and hexane molecules.The data reduction and refinement computations were performed with XTAL 3.4.51 The structure of 7 was solved by Patterson and Fourierdifference techniques (SHELXS 86).48 Data reduction and refinement computations were performed with XTAL 3.0.54 All non-hydrogen atoms were refined anisotropically by full-matrix least squares, except for the C atoms of the disordered solvation molecules in 6 which were restrained.55 Hydrogen atomsJ.Chem. Soc., Dalton Trans., 1998, Pages 271–277 277 were included at calculated positions (C]H 0.95 Å) and held fixed.CCDC reference number 186/799. See http://www.rsc.org/suppdata/dt/1998/271/ for crystallographic files in .cif format. Acknowledgements We thank the Alexander von Humboldt Foundation for the award of a Feodor von Lynen Fellowship (to Thomas Dirnberger). References 1 M. A. Bennett and H. P. Schwemlein, Angew. Chem., 1989, 101, 1349; Angew. Chem., Int. Ed. Engl., 1989, 28, 1296. 2 S. L. Buchwald and R. B. Nielsen, Chem. Rev., 1988, 88, 1047. 3 M. A. Bennett and E.Wenger, Chem. Ber., 1997, 130, 1029. 4 M. A. Bennett, G. B. Robertson, P. O. Whimp and T. Yoshida, J. Am. Chem. Soc., 1971, 93, 3797. 5 M. A. Bennett and T. Yoshida, J. Am. Chem. Soc., 1978, 100, 1750. 6 M. A. Bennett, Pure Appl. Chem., 1989, 61, 1695. 7 Z. Lu, K. A. Abboud and W. M. Jones, Organometallics, 1993, 12, 1471. 8 Z. Lu, K. A. Abboud and W. M. Jones, J. Am. Chem. Soc., 1992, 114, 10 991. 9 J. Klosin, K. A. Abboud and W. M. Jones, Organometallics, 1995, 14, 2892. 10 M.A. Bennett, J. S. Drage, T. Okano, N. K. Roberts and H.-P. Schwemlein, unpublished work, cited in ref. 1. 11 M. A. Bennett, T. W. Hambley, N. K. Roberts and G. B. Robertson, Organometallics, 1985, 4, 1992. 12 M. A. Bennett and E. Wenger, Organometallics, 1995, 14, 1267. 13 M. A. Bennett, D. C. R. Hockless and E. Wenger, Organometallics, 1995, 14, 2091. 14 H. J. S. Winkler and G. Wittig, J. Org. Chem., 1963, 28, 1733. 15 T. L. Gilchrist, F. J. Graveling and C.W. Rees, Chem. Commun., 1968, 821; J. Chem. Soc. C, 1971, 977. 16 C. D. Cook and G. S. Jauhal, J. Am. Chem. Soc., 1968, 90, 1464. 17 K. L. Shepard, Tetrahedron Lett., 1975, 3371. 18 M. A. Bennett, J. S. Drage, K. D. Griffiths, N. K. Roberts, G. B. Robertson and W. A. Wickramasinghe, Angew. Chem., 1988, 100, 1002; Angew. Chem., Int. Ed. Engl., 1988, 27, 941. 19 M. A. Bennett and A. Rokicki, Aust. J. Chem., 1985, 38, 1307. 20 C. Eaborn, K. J. Odell and A. Pidcock, J. Chem. Soc., Dalton Trans., 1978, 357. 21 V. V. Grushin, I. S. Akhrem and M. E. Vol’pin, J. Organomet. Chem., 1989, 371, 403. 22 P. S. Pregosin and R. W. Kunz, 31P- and 13C-NMR of Transition Metal Phosphine Complexes, Springer Verlag, Berlin, 1979, p. 92. 23 P. E. Garrou, Chem. Rev., 1981, 81, 229. 24 M. A. Bennett, D. E. Berry, S. K. Bhargava, E. J. Ditzel, G. B. Robertson and A. C. Willis, J. Chem. Soc., Chem. Commun., 1987, 1613. 25 C. Scheffknecht, A. Rhomberg, E. P. Müller and P. Peringer, J.Organomet. Chem., 1993, 463, 245. 26 H. C. Clark and K. E. Hine, J. Organomet. Chem., 1976, 105, C32. 27 T. G. Appleton and M. A. Bennett, Inorg. Chem., 1978, 17, 738. 28 C. K. Johnson, ORTEP, Report ORNL-5138, Oak Ridge National Laboratory, Oak Ridge, TN, 1976. 29 T. Debaerdemaeker, R. Hohenadel and H.-A. Brune, J. Organomet. Chem., 1991, 410, 265. 30 T. Debaerdemaeker, R. Hohenadel and H.-A. Brune, J. Organomet. Chem., 1988, 350, 109. 31 N. C. Rice and J. D. Oliver, J.Organomet. Chem., 1978, 145, 121. 32 M. A. Bennett and E. Wenger, Organometallics, 1996, 15, 5536. 33 J. A. Johnson, Ph.D. Thesis, Australian National University, 1991. 34 J. Cámpora, A. Llebaria, J. M. Moretó, M. L. Poveda and E. Carmona, Organometallics, 1993, 12, 4032. 35 R. G. Miller and D. P. Kuhlman, J. Organomet. Chem., 1971, 26, 401. 36 S. Sostero, O. Traverso, M. Lenarda and M. Graziani, J. Organomet. Chem., 1977, 134, 259. 37 D. M. Blake and C. J. Nyman, Chem. Commun., 1969, 483. 38 R. Ugo, G. La Monica, F. Cariati, S. Cenini and F. Conti, Inorg. Chim. Acta, 1970, 4, 390. 39 R. Ugo, S. Cenini, M. F. Pilbrow, B. Deibl and G. Schneider, Inorg. Chim. Acta, 1976, 18, 113. 40 S. Sostero, O. Traverso, R. Ros and R. A. Michelin, J. Organomet. Chem., 1983, 246, 325. 41 N. A. Grabowski, R. P. Hughes, B. S. Jaynes and A. L. Rheingold, J. Chem. Soc., Chem. Commun., 1986, 1694. 42 U. Nagel, Chem. Ber., 1982, 115, 1998. 43 TEXSAN, Single Crystal Structure Analysis Software, version 1.6c, Molecular Structure Corporation, The Woodlands, TX, 1993. 44 D. T. Cromer and J. T. Waber, International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, 1974, vol. 4, p. 72. 45 D. C. Creagh and W. J. McAuley, International Tables for X-Ray Crystallography, Kluwer Academic, Boston, MA, 1992, vol. C, p. 219. 46 D. C. Creagh and J. H. Hubbell, International Tables for X-Ray Crystallography, Kluwer Academic, Boston, MA, 1992, vol. C, p. 200. 47 J. A. Ibers and W. C. Hamilton, Acta Crystallogr., 1964, 17, 781. 48 G. M. Sheldrick, in Crystallographic Computing 3, eds. G. M. Sheldrick, C. Krüger and R. Goddard, Oxford University Press, 1985, p. 175. 49 P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, R. de Gelder, R. Israel and J. M. M. Smits, The DIRDIF 94 Program System, Technical Report of the Crystallographic Laboratory, University of Nijmegen, Nijmegen, 1994. 50 TEXSAN, Single Crystal Structure Analysis Software, version 1.7, Molecular Structure Corporation, The Woodlands, TX, 1995. 51 XTAL 3.4 Reference Manual, eds. S. R. Hall, G. S. D. King and J. M. Steward, University of Western Australia, 1995. 52 International Tables for X-ray Crystallography, Kynoch Press, Birmingham, 1974, vol. 4, pp. 99–101 and 149–150. 53 A. Altomare, M. Cascarano, C. Giacovazzo and A. Guagliardi, J. Appl. Crystallogr., 1993, 26, 343. 54 XTAL 3.0 Reference Manual, eds. S. R. Hall and J. M. Steward, Universities of Western Australia and Maryland, 1990. 55 J. Waser, Acta Crystallogr., 1963, 16, 1091. Received 1st October 1997; Paper 7/