Synthesis and crystal and molecular structure of [(η-C5H5)Rh(μ-CO)2Rh(PPh3)(η-C5H5)] an asymmetric bimetallic complex with a polar rhodium-rhodium bond

Treatment of [{Rh(η-C5H5)(CO)}2(μ-CO)] with PPh3, in saturated hydrocarbons, affords [(η-C5H5)Rh(μ-CO)2Rh(PPh 3)(η-C5H5)] which has been characterized by i.r., 1H n.m.r., and X-ray diffraction methods. The complex crystallizes in the triclinic space group PT, with a = 9.700(2), b = 11.076(2), c = 13.790(3) Å, α = 105.3(1), β = 109.6(1), γ = 99.1(1)°, and Z = 2. On the basis of 2 697 reflections with I ≥ 2.5 σ(I), the structure has been refined to R 0.053 and R′ 0.059. The molecule possesses a near mirror plane defined by the Rh-Rh bond [2.673(1) Å] and the phosphorus atom. The two carbonyl ligands form two different asymmetric bridges to the rhodium atoms [Rh(1)-C(19) 2.17(1), Rh(2)-C(19) 1.89(1), Rh(1)-C(20) 2.10(1), and Rh(2)-C(20) 1.90(1) Å]. The cyclopentadienyl ligands are in a cis position with an interplanar angle of 66.3° and both are perpendicular to the pseudo-mirror molecular plane. The co-ordination geometry about the two metal centres is different, being formally Rh(1) seven-co-ordinate and Rh(2) six-co-ordinate. The complex [(η-C5H5)Rh(μ-CO)2Rh(PPh 3)(η-C5H5)] reacts with PPh3 to give the known [Rh(η-C5H5)(CO)(PPh3)]; the same product can be obtained directly by reacting [{Rh(η-C5H5)(CO)}2(μ-CO)] with PPh3 in excess, in benzene solution.