Influence of ligand polarizability on the reversible binding of O2 by trans-[Rh(X)(XNC)(PPh3)2] (X = Cl, Br, SC6F5, C2Ph; XNC = xylyl isocyanide).: Structures and a kinetic study

The complexes trans-[Rh(X)(XNC)(PPh3)(2)] (X = Cl, 1; Br, 2; SC6F5, 3; C2Ph, 4; XNC = xylyl isocyanide) combine reversibly with molecular oxygen to give [Rh(X)(O-2)(XNC)(PPh3)(2)] of which [Rh(SC6F5)(O-2)(XNC)(PPh3)(2)] (7) and [Rh(C2Ph)(O-2)(XNC)(PPh3)(2)] (8) are sufficiently stable to be isolated in crystalline form. Complexes 2, 3, 4, and 7 have been structurally characterized. Kinetic data for the dissociation of O-2 from the dioxygen adducts of 1-4 were obtained using P-31 NMR to monitor changes in the concentration of [Rh(X)(O-2)(XNC)(PPh3)(2)] (X = Cl, Br, SC6F5, C2Ph) resulting from the bubbling of argon through the respective warmed solutions (solvent chlorobenzene). From data recorded at temperatures in the range 30-70 degrees C, activation parameters were obtained as follows: Delta H double dagger (kJ mol(-1)): 31.7 +/- 1.6 (X = Cl), 52.1 +/- 4.3 (X = Br), 66.0 +/- 15.8 (X = SC6F5), 101.3 +/- 1.8 (X = C2Ph); Delta S double dagger (J K-1 mol(-1)): -170.3 +/- 5.0 (X = Cl), -120 +/- 13.6 (X = Br), -89 +/- 18.2 (X = SC6F5), -6.4 +/- 5.4 (X = C2Ph). The values of Delta H double dagger and Delta S double dagger are closely correlated (R-2 = 0.9997), consistent with a common dissociation pathway along which the rate-determining step occurs at a different position for each X. Relative magnitudes of Delta H double dagger are interpreted in terms of differing polarizabilities of ligands X.