Photochemical ligand substitution mechanisms

Monstad, L. Monstad, G. Mechanism of Thermal and Photochemical Ligand Substitution Reactions of Chromium(III) and other Octahedral Metal Complexes, Coord. Chem. Revs. 1989,94,109-150. 

The cis-trans isomerization of PtCl2(Bu P)2 and similar Pd complexes, where the isomerization is immeasurably slow in the absence of an excess of phosphine, is very fast when free phosphine is present. The isomerization doubtless proceeds by pseudorotation of the 5-coordinate state. In this case an ionic mechanism is unlikely, since polar solvents actually slow the reaction. Similar palladium complexes establish cis/trans equilibrium mixtures rapidly. Halide ligand substitution reactions usually follow an associative mechanism with tbp intermediates. Photochemical isomerizations, on the other hand, appear to proceed through tetrahedral intermediates. 

Since the molydenum compounds described above demonstrated mechanistic variation in photochemically initiated substitution reactions, the molybdenum dimetallacycle [(CO)4Mo(p-PMD)2Mo(CO)4] was selected for a study of the nature of the substitution mechanism associated with MC and MLCT excitation.285 PMD is pentamethylenediazirine. The substituting ligands were bpy, phen and l,2-bis(dip-henylphosphino)ethane (dppe) for the reaction in toluene solution. For reaction with... 

A photophysical study of cis-[Ru(bipy)2L2] , where L = pyridine, pyridazine (35), 7V-methylimidazole (28), and related ligands is of relevance to photosubstitution at ruthenium(II) as well as to photoredox processes. Irradiation of an aqueous solution of [Ru(bipy)2Cl(NO)] at pH 5 leads to addition of OH" to the nitrosyl ligand, albeit with a low quantum yield. The mechanism of ligand substitution in [Ru(pc)(py)2] depends on the solvent (whether this is a potential ligand or not) substitution here is thought to be photoredox induced, via an Ru(I)(pc ) species. Photochemical studies on substitution in related diimine complexes include those at [Ru(LL)3] , where LL = biquinolyl (36), " or the pyrazole derivative (37). " Both complexes are considerably more photolabile than [Ru(bipy)3] the photodissociation excited state of the biquinolyl complex is easily populated at room temperature. On the other hand, the dinuclear complex [(H3N)4Ru(bipym)Ru(NH3)4) , bipym = bipyrimidine (38),... 

The mechanism summarized in equations (40)—(42) is applicable, with some modifications,92 94 to the CTTM photochemistry of Crm and Rh111 acidoammine complexes. Thus the primary photochemical step is formation of a substitutionally labile reduced metal species and an oxidized ligand radical. In most systems, however, no permanent redox chemistry occurs owing to the facile reoxidation of the metal. The only net photoprocess observed in these cases is substitution of one or more ligands.70 95... 

---