Influence of Ligand Polarizability on the Reversible Binding of O₂ by <i>trans</i>-[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)(O2)(XNC)(PPh3)2] of which [Rh(SC6F5)(O2)(XNC)(PPh3)2] (7) and [Rh(C2Ph)(O2)(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 O2 from the dioxygen adducts of 1−4 were obtained using 31P NMR to monitor changes in the concentration of [Rh(X)(O2)(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 °C, activation parameters were obtained as follows: ΔH⧧ (kJ mol−1): 31.7 ± 1.6 (X = Cl), 52.1 ± 4.3 (X = Br), 66.0 ± 5.8 (X = SC6F5), 101.3 ± 1.8 (X = C2Ph); ΔS⧧ (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 ΔH⧧ and ΔS⧧ are closely correlated (R2 = 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 ΔH⧧ are interpreted in terms of differing polarizabilities of ligands X.