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

Abstract

Treatment of [{Rh(η-C₅H₅)(CO)}₂(µ-CO)] with PPh₃, in saturated hydrocarbons, affords [(η-C₅H₅)Rh(µ-CO)₂Rh(PPh₃)(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 P, 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(PPh₃)(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.