Bi- and polynuclear complexes with short-bite ligands bridging metal atoms. [(.eta.5-C5H5)Rh(.mu.-CH3O2CC2CO2CH3)(.mu.-Ph2Ppy)Rh(CO)(.mu.-Cl)]2: synthesis, x-ray crystal structure, structural dynamics, and reactions with carbon monoxide and sulfur dioxide.

Reaction of [CpRh(.mu.-CO)(.mu.-Ph2Ppy)Rh(CO)Cl] [Cp = .eta.5-cyclopentadienyl; Ph2Ppy = 2-(diphenylphosphino)pyridine] with MeO2CCCCO2Me (DMA) and EtO2CCCCO2Et (DEA) in CH2Cl2 gives tetranuclear complexes [[CpRh(.mu.-DMA)(.mu.-Ph2Ppy)Rh(CO)(.mu.-Cl)]2CH2Cl2] (I) and [[CpRh(.mu.-DEA)(.mu.-Ph2Ppy)Rh(CO)(.mu.-Cl)]2CH2Cl2], resp. The alkyne in both the complexes adopts the .mu.-.eta.2-|| (cis-dimetalated olefinic) bonding mode. The structure of I was established by X-ray crystallog. The mol. has a crystallog. inversion center originating from the presence of two asym. bridging chloro ligands that link two bimetallic units. Each Rh dimer unit is completely asym. and consists of two Rh atoms bridged by the Ph2Ppy and the DMA ligands. DMA is attached essentially parallel to the Rh(1)-Rh(2) axis in a cis-dimetalated olefinic geometry; the Rh(1)-C(24) and Rh(2)-C(25) bond distances are significantly different (2.022 (5) and 2.040 (4) .ANG., resp.). 31P{1H} and 1H NMR spectra of I in CD2Cl2 soln., at different temps., indicate an equil. between the tetranuclear, I, and the binuclear, [CpRh(.mu.-DMA)(.mu.-Ph2Ppy)Rh(CO)(Cl)] (II), species. The unsatn. of the Rh(II), 16-electron center in II seems to be the key to the reactivity of this species, and it accounts for both the formation of the tetranuclear chloro-bridged species I and the reactions of I with small mols. such as CO and SO2. The reaction of I with CO and SO2 leads to adducts that could not be isolated owing to the reversibility of the process. 31P, 1H, and 13C NMR spectra indicate that in these adducts CO and SO2 are both terminally bonded to Rh.