LIGAND-FIELD AND CHARGE-TRANSFER PHOTOCHEMISTRY OF CR(CO)4(1,10-PHENANTHROLINE), MO(CO)4(1,10-PHENANTHROLINE), W(CO)4(1,10-PHENANTHROLINE) - MECHANISTIC INFORMATION FROM HIGH-PRESSURE EFFECTS

The pressure dependence of the quantum yield for photosubstitution of CO in M(CO)4(1,10-phenanthroline) (M = Cr, Mo, W) was studied at different excitation wavelengths in order to resolve the nature of the substitution mechanism operating from the ligand-field and charge-transfer excited states. Irradiation at 366 nm results in positive volumes of activation (between +6 and +10 cm3 mol“1) for ligand-field photosubstitution, in agreement with a dissociative substitution mode. Irradiation at 546 nm results in significantly negative volumes of activation (between -12 and -14 cm3 mol”1) for charge-transfer photosubstitution in the case of M = Mo and W, which is interpreted as strong evidence for an associative substitution reaction originating from the MLCT excited state. The corresponding value is small positive for the Cr complex and ascribed to a dissociative process operating from the charge-transfer excited state. © 1990, American Chemical Society. All rights reserved.