REDISTRIBUTION OF REDUCED THIOPHENE LIGANDS IN THE CONVERSION OF (C5R5)RH(ETA-4-C4ME4S) TO [(C5R5)RH]3(ETA-4,ETA-1-C4ME4S)2

The thermal decomposition Of (C5Me5)Rh(eta-4-C4Me4S) (1) has been examined by spectroscopic, kinetic, and structural studies. Compound 1 cleanly eliminates free C4Me4S to give [(C5Me5)Rh]3(eta-4,eta-1-C4Me4S)2 (2a). The structure of the analogous compound [(C5Me4Et)Rh]3(eta-4,eta-1-C4Me4S)2 (2b) consists of a pair of (C5Me4Et)Rh(eta-4-C4Me4S) ligands bound to a central (C5Me4Et)Rh(I) unit through the sulfur atoms. The conversion of 1 to 2a occurs via a second-order process (in rhodium) with k(60-degrees-C) = 3.94 x 10(-4) M-1.s-1 which implicates an associative mechanism. Activation parameters are DELTA-H double dagger = 20.1 +/- 0.1 kcal/mol and DELTA-S double dagger = -13.9 +/- 3.1 cal/mol.K. Dynamic H-1 NMR studies demonstrate that 2a maintains its structure in solution but that it experiences restricted rotation about the two Rh-S bonds. Compound 2a decomposes via a first-order process with k(100-degrees-C) = 8.94 X 10(-6) s-1 to give (C5Me5)2Rh2C4Me4S, 3.