Reversible Isomerization of a Diphosphine Ligand about a Triosmium Cluster:  Synthesis, Kinetics, and X-ray Structures for the Bridging and Chelating Isomers of Os3(CO)10[(Z)-Ph2PCHCHPPh2]

Substitution of the MeCN ligands in the activated cluster Os3(CO)10(MeCN)2 (1) by the unsaturated diphosphine ligand (Z)-Ph2PCHCHPPh2 proceeds rapidly at room temperature to furnish the ligand-bridged cluster 1,2-Os3(CO)10[(Z)-Ph2PCHCHPPh2] (2b). Heating 2b leads to the formation of the thermodynamically more stable chelating isomer 1,1-Os3(CO)10[(Z)-Ph2PCHCHPPh2] (2c). The molecular structure of each isomer of 2 has been crystallographically determined, and the 31P NMR data have been recorded. The kinetics for the ligand isomerization have been investigated by UV−vis and 31P NMR spectroscopy in toluene solution over the temperature range of 358−383 K. The reversible nature of the diphosphine isomerization is confirmed by 31P NMR measurements, and reported within are the forward (k 1) and reverse (k - 1) first-order rate constants for the bridge-to-chelate rearrangement. On the basis of the activation parameters and lack of CO inhibition on the reaction rate, a nondissociative, intramolecular mechanism involving the migration of one P group from an adjacent osmium center to the other P-substituted osmium center via a μ2-bridged phosphine species is presented.