The photochemistry of Mo(CNPh)6: dissociative photosubstitution and evidence for ‘hot’ electron transfer

The photosubstitution reactivity of Mo(CNPh)₆ was investigated by studying the effects of nucleophile and nucleophile concentration. Substitution quantum yields were identical for pyridine and PPh₃ as nucleophiles. Between 0.1 and 4×10⁻⁴ M pyridine, there was no change in quantum yield. These observations suggest that photosubstitution in Mo(CNPh)₆ is dissociative in character, not associative as previously thought. Photoinduced electron transfer from Mo(CNPh)₆ to chloroform, producing [Mo(CNPh)₆Cl]⁺, was also measured. While photosubstitution is wavelength independent, photoinduced electron transfer quantum yields are wavelength dependent. The electron transfer quantum yield is highest at 313 nm (0.77) and decreases to a constant value of 0.28 at 436 nm. It is proposed that this evidence supports a ‘hot’ electron transfer process, where electron transfer occurs prior to vibrational relaxation into the metal-to-ligand charge transfer (MLCT) manifold.