Stephen J. Dossett

Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 107. Reactions of diphenylphosphine with molybdenumtungsten dimetal complexes; crystal structures of [MoW(µ-PPh2){µ-C(OH)C(C6H4Me-4)}-(CO)(η7-C7H7)(η5-C2B9H11)]and[MoW(µ-PPh2)(CO)3(η5-C9H7){η5-C2B9H8(CH2C6-H4Me-4)Me2}]

Treatment of the compounds [MoW(µ-CC6H4Me-4)(CO)2(η7-C7H7)(η5-C2B9H10R)](R = H or Me) in CH2Cl2with PHPh2 affords the complexes [MoW(µ-PPh2){µ-C(OH)C(C6H4Me-4)}(CO)(η7-C7H7)-(η5-C2B9H10R)]. The structure of the species with R = H was established by X-ray diffraction. The Mo–W bond [2.713(1)A] is spanned by the PPh2 group [Mo–P 2.439(3), W–P 2.360(3)A], and in a transverse manner by the alkyne. However, the latter is twisted so that the C(OH) atom lies appreciably closer to the tungsten [µ-C–W 2.067(8)A] than the molybdenum [µ-C–Mo 2.352(8)A], whereas the C(C6H4Me-4) atom is essentially symmetrically bridging [µ-C–Mo 2.224(9), µ-C–W 2.251(8)A]. The molybdenum atom carries the η7-C7H7 group (C–Mo av. 2.27 A), and the tungsten atom is η5 co-ordinated by the C2B9H11 cage and a CO group. The reaction between [MoW(µ-CC6H4Me-4)(CO)3(η5-C9H7)(η5-C2B9H9Me2)] and PHPh2in CH2Cl2 gives the complex [MoW(µ-CC6H4Me-4)(CO)2(PHPh2)(η5-C9H7)(η5-C2B9H9Me2)]. The latter on heating in toluene at 70 °C yields the compound [MoW(µ-PPh2)(CO)3(η5-C9H7){η5-C2B9H8(CH2C6H4Me-4)Me2}], the structure of which was established by X-ray diffraction. The Mo–W bond [2.708(1)A] is bridged by the PPh2 group [Mo–P 2.357(2), W–P 2.456(2)A]. The tungsten atom is ligated by two CO groups, and η5 co-ordinated by the C2B9 cage. In the open face of the latter a BH group in the α position to a CMe fragment forms a B–H⇀Mo exopolyhedral bond, while the boron atom β to the CMe groups in the pentagonal ring carries a CH2C6H4Me-4 substituent. The molybdenum atom is ligated by a CO molecule and the indenyl group. The n.m.r. data (1H, 13C-{1H}, 11B-{1H}, and 31P-{1H}) for the new compounds are reported and discussed.

Synthesis and reactivity of η2(4e)-alkyne and η2(3e)-vinyl complexes of rhenium

Reaction of cis-/trans-[ReBr2(CO)2(η-C5H5)] with PhC2Ph and MeC2Ph in refluxing toluene afforded good yields of the η2(4e)-donor alkyne complexes [ReBr2(η2-PhC2Ph)(η-C5H5)]1 and [ReBr2(η2-MeC2Ph)(η-C5H5)]2, respectively. Treatment of 1 and 2 with either AgBF4 or TlPF6 in the presence of PPh3, PMePh2 or P(OMe)3(L) gave monocations [ReBr{η2(4e)-alkyne}L(η-C5H5)]+, whereas a similar reaction with 2 equivalents of AgBF4 and 1 equivalent of Ph2PCH2CH2PPh2(dppe) afforded dications [Re(η2-PhC2Ph)(dppe)(η-C5H5)][BF4]2 and [Re(η2-MeC2Ph)(dppe)(η-C5H5)][BF4]2. The structural identity of [ReBr(η2-PhC2Ph)(PMePh2)(η-C5H5)][PF6] was confirmed by single-crystal X-ray crystallography. The alkyne C–C vector lies parallel to the Re–Br bond and the alkyne C–C bond length [C(1)–C(2) 1.26(4)A] is relatively short. Treatment of [ReBr(η2-PhC2Ph)(PPh3)(η-C5H5)][BF4] and [ReBr(η2-PhC2Ph)(PMePh2)(η-C5H5)][PF6] with K[BHBus3] in dichloromethane at –78 °C led to neutral η2(3e)-vinyl complexes [[graphic omitted]HPh}Br(PPh3)(η-C5H5)] and [[graphic omitted]HPh}Br(PMePh2)(η-C5H5)]. The crystal structure of the latter showed that the C–C vector of the vinyl moiety lies almost parallel to the Re–Br bond. The stereochemistry of these reactions is discussed in the light of extended-Huckel molecular orbital calculations. Reaction (–78 °C) of [Re(η2-PhC2Ph)(dppe)(η-C5H5)][BF4]2 with 1 equivalent of K[BHBus3] in tetrahydrofuran afforded the X-ray crystallographically identified monocationic η2(3e)-vinyl complex [[graphic omitted]HPh}(dppe)(η-C5H5)][BF4], which reacted at room temperature with a further equivalent of K[BHBus3] to give the cis-stilbene-substituted complex [Re(η2-Z-PhCHCHPh)(dppe)(η-C5H5)]. The crystal structure of the latter showed that the alkene phenyl substituents are orientated towards the cyclopentadienyl ring. In contrast, a similar reaction between K[BHBus3] and [Re(η2-MeC2Ph)(dppe)(η-C5H5)][BF4]2 gave initially the η2(3e)-vinyl complex [[graphic omitted]HPh}(dppe)(η-C5H5)][BF4]; a further equivalent of K[BHBus3] led to deprotonation and formation of the η2-allene complex [Re{η2-CH(Ph)CCH2}(dppe)(η-C5H5)], in which the substituted allenic bond is co-ordinated to the rhenium. The dinuclear complex [Re2Br2(PPh3)2(µ-O)(η-C5H5)2][BF4]2 was also prepared and shown crystallographically to possess a single rhenium–rhenium bond [2.731(5)A].

A tungstendiiron cluster [NEt4][WFe2μ3-η3, η5-CHCC(t Bu)C(O)C2B9H8Me2(CO)8] derived from [Fe2(CO)9] and [NEt4][W CCCtBu)(CO)2-(η5-C2B9H9Me2)]

Abstract Treatment of a tetrahydrofuran solution of [NEt4][W(CCCtBu)(CO)2(η5-C2B9H9Me2)] with [Fe2(CO)9] affords the trimetal complex [NEt4][WFe2μ3-η3,η5-CHCC(tBu)C(O)C2B9H8Me2(CO)8], the structure of which has been established by X-ray diffraction. The anion has a core based on an essentially isosceles triangle of metal atoms [FeFe 2.663(3), WFe 2.811(2) and 2.829(2) A]. Each iron atom carries three CO groups, whereas the tungsten atom is co-ordinated by two such groups, and by the open face of a nido-icosahedral C2B9 cage. In the five-membered BBBCC ring forming the face of the cage, the boron atom occupying the β site with respect to the two carbon atoms is σ bonded to the carbon atom of a ketonic group [BC 1.57(3) A]. The latter in turn is linked to an allenyl fragment C(tBu)CCH [CC average 1.35(2) A, CCC 150(1)°] which spans the face of the WFe2 triangle. The terminal CH group is attached to the tungsten and to an iron atom [CW 2.25(1), CFe 1.96(1) A], and the central carbon is linked to the tungsten atom [CW 2.07 (1) A] and also to the second iron atom [CFe 2.05(1) A]. The latter is also bonded to the C(tBu) fragment [CFe 2.20(2) A] which in turn is connected to the ketonic group [C(tBu)C(O) 1.47(2) A] linking the μ3-C3 moiety to the cage. The NMR data (1H, 13C-1H, and 11B-1H) for the new cluster compound are reported and discussed.

Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 104. Trimetal molybdenum and tungsten complexes containing η5-C2B9H9R′2(R′= H or Me) ligands; crystal structure of [NEt4][Mo2W(µ3-CC6H4Me-4)(µ-CO)(CO)7(PMe3)(η5-C2B9H9Me2)]

Solutions of the salts [NEt4][M(CR)(CO)2(η5-C2B9H9R′2)](M = Mo or W, R = C6H4Me-4 or CCBut, R′= H or Me), in thf (tetrahydrofuran) at room temperature, react with the compound [Mo(CO)3(NCMe)3] to afford the trimetal complexes [NEt4][Mo2M(µ3-CR)(µ-CO)(CO)8(η5-C2B9H9R′2)](M = W, R = C6H4Me-4, R′= H or Me; M = Mo, R = C6H4Me-4, R′= H; M = W, R = C CBut, R′= Me). A single-crystal X-ray diffraction study was carried out on the PMe3 derivative [NEt4][Mo2W(µ3-CC6H4Me-4)(µ-CO)(CO)7(PMe3)(η5-C2B9H9Me2)]. The metal triangle [Mo–Mo 2.968(3), Mo–W 2.881 (3) and 2.867(3)A] in the anion is capped on one side by the p-tolylmethylidyne group [µ3-C–Mo av. 2.26(2), µ3-C–W 2.01 (2)A], and is bridged on the other by the C2B9H9Me2 cage. The latter is η5 co-ordinated to the tungsten atom and linked to the molybdenum atoms via two exopolyhedral B–H⇀Mo bonds. The bond to one molybdenum centre involves a BH fragment α to a CMe group in the CCBBB face of the cage, while the bond to the other molybdenum centre employs a BH fragment β to the CMe groups. Moreover, the molybdenum atom linked to the cage via the β BH group carries the PMe3 ligand [Mo–P 2.489(7)A]. This metal atom is also ligated by two terminally bound CO groups, and by a CO molecule that asymmetrically bridges to the other molybdenum [Mo–C–O 156(2) and 124(2)°]. The latter is co-ordinated by three CO groups, and the tungsten atom by two CO molecules. In the formation of [NEt4][Mo2W(µ3-CC6H4Me-4)(µ-CO)(CO)7(PMe3)(η5-C2B9H9Me2)] there was n.m.r. evidence for the formation of a second isomer (ca. 15%) in which the PMe3 group occupies a different site. Attempts to protonate the salts [NEt4][Mo2M(µ3-CR)(µ-CO)(CO)8(η5-C2B9H9R′2)] in order to obtain stable neutral complexes were unsuccessful, but treatment of [NEt4][Mo2W(µ3-CCCBut)(µ-CO)(CO)8(η5-C2B9H9Me2)] in thf with [AuCl(PPh3)] in the presence of TIPF6 gives [Mo(CO)6] and the dimetal complex [WAu(µ-CCCBut)(CO)2(PPh3)(η5-C2B9H9Me2)]. The latter reacts with [Pt(nb)(PMe2Ph)2](nb = bicyclo[2.2.1]heptene) to afford the trimetal complex [WPtAu(µ3-CCCBut)(CO)2(PPh3)(PMe2Ph)2(η5-C2B9H9Me2)]. The n.m.r. data (1H, 13C-{1H}, 11B-{1H}, and 31P-{1H}) for the new compounds are reported and discussed.

Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 110. Synthesis of compounds with tungsten–copper or –gold bonds; crystal structure of [NEt4][W2Cu(µ-CCCBut)2(CO)4(η5-C2B9H9Me2)2]·Et2O

Treatment of the reagents [NEt4][W(CR)(CO)2(η5-C2B9H9Me2)] in CH2Cl2 with 1 equivalent of CuCl or [AuCl(tht)](tht = tetrahydrothiophene) affords the labile complexes [NEt4][WMCl(µ-CR)(CO)2(η5-C2B9H9Me2)](M = Cu or Au, R = C6H4Me-4 or CCBut). Use of 2 equivalents of CuCl yields the stable tungstendicopper species [NEt4][WCu2Cl2(µ-CR)(CO)2(η5-C2B9H9Me2)], which may also be obtained by adding 1 equivalent of CuCl to the tungsten-copper dimetal compounds in CH2Cl2. Reactions of the salts [NEt4][WCuCl(µ-CR)(CO)2(η5-C2B9H9Me2)] with CNR′ in CH2Cl2 in the presence of TIBF4 yields the neutral complexes [WCu(µ-CR)(CO)2(CNR′)(η5-C2B9H9Me2)](R = C6H4Me-4 or CCBut, R′= C6H3Me2-2,6; R = CCBut, R′= But). The trimetal compounds [NEt4][W2M(µ-CR)2(CO)4(η5-C2B9H9Me2)2](M = Cu, R = C6H4Me-4 or CCBut; M = Au, R = CCBut) have been prepared by treating, in CH2Cl2, 2 equivalents of the reagents [NEt4][W(CR)(CO)2(η5-C2B9H9Me2)] with 1 equivalent of CuCl or [AuCl(tht)] in the presence of TIBF4. An X-ray diffraction study of the salt [NEt4][W2Cu(µ-CCCBut)2(CO)4(η5-C2B9H9Me2)2] revealed a novel structure in the crystal. The anion has a W2Cu spine [W–Cu 2.602(1) and 2.764(1)A, W–Cu–W 152.7(1)°] with each metal–metal bond asymmetrically bridged by a CCCBut group [µ-C-W 1.85(1), µ-C-Cu(av.) 2.08(1)A; W-µ-C-CCBut(av.) 169.5(1)°]. The shorter of the two W–Cu separations is also spanned by a C2B9H9Me2 cage. As expected the latter is η5 co-ordinated to the tungsten atom, but it also forms an exopolyhedral B–H ⇀ Cu three-centre two-electron bridge bond employing the boron atom β to the carbon atoms in the pentagonal CCBBB face of the cage [µ-B–W 2.36(1), µ-B–Cu 2.33(1)A]. The other tungsten atom is ligated by a C2B9H9Me2 group in the normal η5 bonding mode, and both tungstens carry two terminally bound CO groups. The reaction between [W(CCCBut)(CO)2(η-C5H5)] and [Pt(nb)3](nb = norbornene = bicyclo[2.2.1] heptene) affords the compound [W2Pt(µ-CCCBut)2(CO)4(η-C5H5)2]. The 1H and 13C-{1H} NMR data for the new compounds are reported and discussed.

Alkyne–carbaborane coupling at a molybdenum centre: crystal structure of [closo-3,3,3-(CO)3-8,3-{σ:η2-C(H)C(H)-SiMe3}-3,1,2-MoC2B9H10]

In CH2Cl2 the complex [closo-1,2-Me2-3-(η-PhC2Ph)-3-(CO)-3-(PPh3)-3,1,2-MoC2B9H9] decomposed to yield a mixture of [closo-1,2-Me2-3,3,3-(CO)3-3-(PPh3)-3,1,2-MoC2B9H9] and [closo-1,2-Me2-3,3-(CO)2-3-(PPh3)-8,3-{σ:η2-C(Ph)C(H)Ph}-3,1,2-MoC2B9H8]. Protonation (HBF4·Et2O) of [NEt4][closo-3-(η-C3H5)-3,3-(CO)2-3,1,2-MoC2B9H11], in CH2Cl2 at – 78 °C, in the presence of an excess of Me3SiCCSiMe3, afforded a chromatographically separable mixture of [closo-3,3-(η-Me3SiC2SiMe3)2-3-(CO)-3,1,2-MoC2B9H11] and [closo-3,3,3-(CO)3-8,3-{σ:η2-C(H)C(H)SiMe3}-3,1,2-MoC2B9H10]. The latter complex forms via the intermediacy of Me3SiCCH, generated by HF cleavage of Me3SiCCSiMe3, and its structure was established by X-ray diffraction. The molybdenum atom is ligated on one side by three CO groups, and on the other by the open pentagonal face of the nido-1,2-C2B9 cage framework. The boron atom located in the β site with respect to the two carbons carries a vinyl substitutent C(H)C(H)SiMe3, and this exopolyhedral group is η2 co-ordinated to the molybdenum atom [Mo-C 2.43(1) and 2.55(1)A]. Treatment of [N(PPh3)2][closo-1,2-Me23-(η-C3H5)-3,3-(CO)2-3,1,2-MoC2B9H9] and Me3SiCCH with HBF4·Et2O gives initially the complex [closo-1,2-Me2-3,3-(η-Me3SiC2H)2-3-(CO)-3,1,2-MoC2B9H9], which subsequently rearranges to [closo-1,2-Me2-3-(η-Me3SiC2H)-3-(CO)-8,3-{σ:η2-C(H)C(H)SiMe3}-3,1,2-MoC2B9H8]. Use of Me3SiCCD in this synthesis, combined with NMR studies, suggests that insertion of the alkyne into the cage B–H bond proceeds via the intermediacy of a molybdenum vinylidene species. The NMR data (1H, 13C-{1H}, 11B-{1H}, and 31P-{1H}) for the new compounds are reported and discussed in relation to the structures proposed.

Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 109. Reactions of tungsten–molybdenum dimetal compounds bearing carbaborane ligands with alkynes; crystal structure of [MoW(µ-C4Me4){σ,η5-CH(C6H4Me-4)C2B9H8Me2}(η-C7H7)]

Treatment of the compounds [MoW(µ-CR)(CO)2(η-C7H7)(η5-C2B9H9Me2)] in thf (tetrahydrofuran) with the alkynes MeCCMe or EtCCEt affords the complexes [MoW(µ-CR)(CO)(η-R′C2R′)(η-C7H7)(η5-C2B9H9Me2)](R = C6H4Me-4, R′= Me; R = CCBut, R′= Me or Et). Similar reactions between [MoW(µ-CC6H4Me-4)(CO)2(η-C7H7)(η5-C2B9H10Me)] and MeCCMe or PhCCPh yield the related species [MoW(µ-CC6H4Me-4)(CO)(η-R′C2R′)(η-C7H7)(η5-C2B9H10Me)](R′= Me or Ph). The reaction of [MoW(µ-CC6H4Me-4)(CO)(η-MeC2Me)(η-C7H7)(η5-C2B9H9Me2)] with MeCCMe in toluene at 100 °C gives a novel dimetal complex [MoW(µ-C4Me4){σ,η5-CH(C6H4Me-4)C2B9H8Me2}(η-C7H7)], the structure of which was established by X-ray diffraction. The Mo–W bond [2.922(1)A] is symmetrically bridged by a µ-C4Me4 fragment, the carbon atoms of which are linked to both metal atoms in the η4 fashion. The molybdenum atom carries the η-C7H7 ligand, and the tungsten atom is ligated by a CH(C6H4Me-4)C2B9H8Me2 fragment. In the latter the CH(C6H4Me-4) moiety forms a bridge between the tungsten [W–C 2.226(8)A] and a boron atom [B–C 1.54(1)A] in the pentagonal face of the cage. This boron atom is in the β site ([graphic omitted]) with respect to the carbon atoms, and all the atoms of the ring are bonded to the tungsten, but the connectivities are somewhat asymmetric [2.227(9)–2.472(9)A]. The 1H and 13C-{1H} NMR data for the new compounds are reported and discussed.

Synthesis and crystal structure of the complex [MoW(OEt)(μ-CH2){μ-C(C6H4Me-4)C(Me)O}(η7-C7H7)(η5-C2B9H10Me)]

Abstract The complex [MoW(μ-CC6H4Me-4)(CO)2(η7-C7H7)(η5-C2B9H10Me)] reacts with diazomethane in Et2O containing EtOH to afford the dimetal compound [MoW(OEt)(μ-CH2){μ-C(C6H4Me-4)C(Me)O}(η7-C7H7)(η5-C2B9H10Me)]. The structure of this product was established by X-ray diffraction. The MoW bond [2.778(4) A] is bridged by a CH2 group [μ-CMo 2.14(3), μ-CW 2.02(3) A] and by a C(C6H4Me-4)C(Me)O fragment [MoO 2.11(3), WO 2.18(2), MoC(C6H4Me-4) 2.41(3), WC(C6H4Me-4) 2.09(3), MoC(Me) 2.26(3) A]. The molybdenum atom is η7-coordinated by the C7H7 ring and the tungsten atom is η5-coordinated by the open pentagonal face of the nido-icosahedral C2B9H10Me cage. The tungsten atom also carries a terminally bound OEt group [WO 1.88(3) A]. The 1H and 13C-{1H} NMR data for the dimetal compound are reported and discussed.

Allyl(carbaborane) complexes of Group 6 metals: preparation and reactivity

Treatment of thf (tetrahydrofuran) solutions of [MBr(CO)2(NCMe)2(η3-C3H5)](M = Mo or W) with Na2[7,8-C2B9H9R′2](R′= H or Me), followed by addition of [N(PPh3)2]Cl or [NEt4]Cl, afforded the salts [Y][M(CO)2(η3-C3H5)(η5-7,8-C2B9H9R′2)][Y = N(PPh3)2, R′= Me, M = Mo 1a or W 1b; Y = NEt4. R′= H, M = Mo 1c] in high yield. These species may be synthesised, but in lower yield, from allyl bromide and Tl2[M(CO)3(η5-7,8-C2B9H9R′2)] generated in situ. Protonation of the salts 1a and 1b with HBF4·Et2O in the presence of CO or PPh3 yielded the compounds [M(CO)2L2(η5-7,8-C2B9H9Me2)](M = Mo 2a or W 2b, L = CO; M = Mo 2c or W 2e, L = PPh3). The synthesis of 2c was accompanied by formation of the tricarbonyl species [Mo(CO)3(PPh3)(η5-7,8-C2B9H9Me2)]2d. Treatment of the salt 1c with HBF4·Et2O in the presence of buta-1,3-diene afforded the species [Mo(CO)2(η4-C4H6)(η5-7,8-C2B9H11)]5. When reactions of 1a–c with HBF4·Et2O were carried out in the presence of alkynes, the bis(alkyne) complexes [M(CO)(RC2R)2(η5-7,8-C2B9H9R′2)](M = Mo, R = R′= Me 6a; M = W, R = Ph, R′= Me 6b; M = Mo, R = Me, R′= H 6c) are formed. The compounds 6a and 6c can also be obtained from reactions between Tl2[Mo(CO)3(η5-7,8-C2B9H9R′2)] generated in situ, MeC≡CMe, and AgBF4. Reaction of the complex 6a with PMe2Ph gives the species [Mo(CO)(PMe2Ph)(MeC2Me)(η5-7,8-C2B9H9Me2)]6e. Treatment of the reagent Tl2[Mo(CO)3(η5-7,8-C2B9H9Me2)] with AgBF4 in the presence of [W(≡CC6H4-Me-4)(CO)2(η-C5H5)] afforded the dimetal complex [MoW(µ-CC6H4Me-4)(CO)3(η-C5H5)(η5-7,8-C2B9H9Me2)]7.

Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 103. Carbaborane molybdenum and tungsten dimetal complexes

Treatment of the complex [Mo(CO)2(NCMe)2(η5-C9H7)][BF4](C9H7= indenyl) with the salts [NEt4][W(CCCBut)(CO)2(η5-C2B9H9Me2)]and [NEt4][Mo(CC6H4Me-4)(CO){P(OMe)3}(η5-C2B9H9Me2)] in CH2Cl2 affords, respectively, the dimetal compounds [MoW(µ-CCCBut)(CO)3(η5-C9H7)(η5-C2B9H9Me2)]and[Mo2(µ-CC6H4Me-4)(CO)2{P(OMe)3}(η5-C9H7(η5-C2B9H9Me2)]. Similarly, reactions between the salts [NEt4][M(CR)(CO)2(η5-C2B9H9R′2)](M = Mo or W, R = C6H4Me-4, R′= H; M = W, R = C6H4Me-4 or C6H4Me-2, R′= Me; M = W, R = CCBut, R′= Me) and [Mo(CO)2(NCMe)(η7-C7H7)][BF4] yield the dimetal complexes [MMo(µ-CR)(CO)2(η7-C7H7)(η5-C2B9H9R′2)](M = W, R = C6H4Me-4, C6H4Me-2, or CCBut, R′= Me; M = Mo or W, R = C6H4Me-4, R′= H). The compounds [MoW(µ-CC6H4Me-4)(CO)2(η7-C7H7)(η5-C2B9H10Me)] and [MoW(µ-CC6H4Me-4)(CO)2(η7-C7H7)(η6-C2B10H10Me2)] have also been prepared using the reagents [NEt4][W(CC6H4Me-4)(CO)2(η5-C2B9H10Me)] and [NEt4][W(CC6H4Me-4)(CO)2(η6C2B10H10Me2)]. All the dimetal compounds have structures in which the metal–metal bonds are bridged by an alkylidyne group and by a B–H⇀Mo three-centre two-electron bond. The latter involves a boron atom lying in the face of the nido-carbaborane fragment which is co-ordinated to the other metal centre in the η5(for C2B9H9R′2) or η6(for C2B10H10Me2) bonding mode. The n.m.r. data (1H, 13C-{1H}, 11B-{1H}, 11B, and 31P-{1H}) for the new compounds are reported and discussed in the context of the structures proposed.