SOLUTION CALORIMETRIC AND STOPPED-FLOW KINETIC-STUDY OF LIGAND SUBSTITUTION FOR THE COMPLEXES M(CO)3(PCY3)2(L) (M = CR, MO, W) - COMPARISON OF 1ST-ROW, 2ND-ROW, AND 3RD-ROW TRANSITION-METAL LIGAND BONDS AT A STERICALLY CROWDED METAL CENTER

The substitution of pyridine by trimethyl phosphite in the complexes M(CO)3(PCy3)2(py) (M = Cr, Mo) has been studied by stopped-flow kinetics. Direct reactions of the proposed intermediate complexes M(CO)3(PCy3)2 (M = Cr, Mo) have also been studied. The crystal structure of Cr(CO)3(PCy3)2 has been determined and shows an an agostic M...H-C interaction. Cell parameters: a = 10.127 (2) angstrom, b = 12.524 (3) angstrom, c = 15.329 (3) angstrom, beta = 90.93 (3)degrees, gamma = 103.28 (3)degrees, space group P1BAR, Z = 2. These results are combined with calorimetric measurements, and with earlier data on the isostructural W complex, to give a complete picture of ligand substitution in these sterically crowded complexes. The relative rates of dissociation of the M-py bond are in the order Cr > Mo > W in the approximate ratio 4800/300/1. The relative rates of reaction of the formally coordinatively unsaturated complexes M(CO)3(PCy3)2 with trimethyl phosphite are in the order Mo > W > Cr in the approximate ratio 80/36/1. Due to these changes in relative magnitude for the rate constants, the chromium complex does not obey simple steady-state kinetics. Solution calorimetric measurements show that the enthalpies of binding of pyridine and trimethyl phosphite are as follows: W, -19 and -26 kcal/mol; Mo, -17 and -24 kcal/mol; Cr, -16 and -11 kcal/mol. Enthalpies of activation for dissociation of pyridine: W, -22.6 kcal/mol; Mo, -21.9 kcal/mol; Cr, -1 5.1 kcal/mol. The activation energy for addition of py to M(CO)3(PCy3)2 is about 4 kcal/mol for all three metals and probably involves concerted attack at the agostic bond.