CARBONYLATION OF ORGANOPLATINUM DIMERS BRIDGED BY (DIPHENYLPHOSPHINO)CYCLOPENTADIENYL LIGANDS - STRUCTURAL CHARACTERIZATION OF COMPLEXES CONTAINING 1,1-SUBSTITUTED OR 1,2-SUBSTITUTED ETA-1-CYCLOPENTADIENYL GROUPS AND REDUCTIVE ELIMINATION OF KETONES

The complexes [Pt2R2(mu-C5H4PPh2)2] (1a-e: R = Me, Et, neo-Pe (CH2CMe3 ), Ph, Bz (CH2-Ph)) react with carbon monoxide to produce [Pt2R2(CO)2( mu-eta1-C5H4PPh2)2] (2a-e), in which the cyclopentadienyl groups are 1 ,2-substituted by Pt and P. The Rand CO ligands occupy mutually trans positions, and the complexes exist in solution as both syn and anti fo rms. The molecular structures of the methyl and neopentyl derivatives have been determined by X-ray diffraction, and they adopt anti and syn conformations, respectively. Thermolyses of these complexes in the pr esence of CO leads to the platinum(I) dimer [Pt2(CO)2(mu-eta1-C5H4PPh2 )2] (5), in which both cyclopentadienyl rings are 1,1-substituted, and the appropriate ketone. The molecular structure of 5 reveals a Pt-Pt distance of 2.583(1) angstrom. Careful monitoring by NMR spectroscopy of the carbonylation/reductive-elimination sequence allows the identif ication of the intermediates [Pt2R(COR)(mu-eta1-C5H4PPh2)2] (3b,c) and [Pt2R(CO)(COR)(mu-eta1-C5H4PPh2)2] (4b,c). The molecular structure of 4c indicates that the complex contains one 1,1- and one 1,2-substitut ed cyclopentadienyl moiety and a Pt-->Pt dative bond. A mechanism for the carbonylation/reductive-elimination sequence is proposed.