Synthesis of η4-1,3-diphosphacyclobutadiene(η5-1,2-dicarbaborane)rhodium complexes; crystal structures of the compounds [NEt4][Rh(η4-But2C2P2)(η5-C2B9H11)], [Rh{η4-But2C2P2Au(PPh3)}(η5-C2B9H11)], and [Rh{η4-But2C2P2Co-(CO)2(η4-C4Me4)}(η5-C2B9H11)]

Treatment of a thf (tetrahydrofuran) solution of K[Rh(PPh3)2(η5-C2B9H11)] with ButCP, followed by addition of NEt4Cl, affords the salt [NEt4][Rh(η4-But2C2P2)(η5-C2B9H11)](3), the structure of which has been established by X-ray diffraction. Compound (3) reacts with [AuCl(PPh3)] and with [Co(CO)2(NCMe)(η4-C4Me4)][PF6] to afford, respectively, the complexes [Rh{η4-But2C2P2Au-(PPh3)}(η5-C2B9H11)](4) and [Rh{η4-But2C2P2Co(CO)2(η4-C4Me4)}(η5-C2B9H11)](5). The structures of the products (4) and (5) have also been determined by X-ray diffraction. In all three compounds the rhodium is η5 ligated by a nido-icosahedral C2B9H11 fragment, and by a 1,3-diphosphacyclobutadiene group. Within the latter the P–C distances are equal, and the four-membered CPCP rings are close to planar. In the compounds (4) and (5) a phosphorus atom in the η4-But2CP2 group co-ordinates to the Au(PPh3) and the Co(CO)2(η4-C4Me4) groups, respectively. The n.m.r. data (1H, 13C-{1H}, 11B-{1H}, and 31P-{1H}) are reported, and where appropriate discussed.