Metal-to-Ligand Charge Transfer Photochemistry: Homolysis of the Mn-Cl Bond in the mer-Mn(Cl)(CO) 3 (r-diimine) Complex and Its Absence in the fac-Isomer Angela Rosa,* ,† Giampaolo Ricciardi, † Evert Jan Baerends,* ,‡ and Derk J. Stufkens § Dipartimento di Chimica, Universita ` della Basilicata, Via N. Sauro, 85, 85100 Potenza, Italy, Afdeling Theoretische Chemie, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, and Anorganisch Chemisch Laboratorium, Nieuwe Achtergracht 166, Universiteit van Amsterdam, 1018 WV Amsterdam, The Netherlands ReceiVed July 1, 1998 Homolytic breaking of the axial metal-Cl bond is not observed upon irradiation at 488 nm of either fac-Mn(Cl)- (CO) 3 (R-diimine) or the parent Mn(Cl)(CO) 5 . Surprisingly, it does occur for the equatorial Mn-Cl bond in several mer-Mn(Cl)(CO) 3 (R-diimine). Using DFT calculations it is shown that this photochemical homolysis can only be understood if strong relaxation of the metal fragment occurs concurrent with the equatorial Cl departure, releasing sufficient energy to make the photodissociation energetically possible. The unrelaxed metal fragment with an equatorial vacancy would be very unstable (by 116 kJ/mol) with respect to the relaxed fragment with an axial vacancy. The migration of an axial CO to the equatorial site invoked in the proposed photodissociation mechanism does not occur on the potential energy surface of the photoactive excited state, which is bound in the Mn-Cl dissociation coordinate. It is proposed to occur in a continuum state (above the asymptotic energy) of the ground-state potential energy surface that is in resonance with the photoactive excited state. The possible importance of this mechanism for TM complex photochemistry, where rearrangement of ligands may often occur upon photodissociation, is stressed. Introduction The transition metal complexes containing an R-diimine ligand such as bpy (2,2′-bipyridine), phen (1,10-phenanthroline), or R-DAB (substituted 1,4-diaza-1,3-butadiene) have very low- lying metal-to-ligand charge transfer (MLCT) states which belong to transitions from the metal to the lowest π* orbital of the R-diimine ligand. Although MLCT states are usually unreactive and have long lifetimes, the photoreactivity (ho- molysis of M-X or M-R bonds or photodissociation of a carbonyl ligand) of these low-lying MLCT states is now experimentally well-documented, e.g. for M(CO) 4 (R-diimine) (M ) Cr, Mo, W), 1 M′(CO) 3 (R-diimine) (M′ ) Fe, Ru), 2 Ni- (CO) 2 (R-diimine), 3 fac-Mn(X)(CO) 3 (R-diimine) (X ) halide), 4 M(R)(CO) 3 (R-diimine) (M ) Mn, Re; R ) methyl, ethyl, benzyl), 5 Ru(X)(R)(CO) 2 (R-diimine) (X ) halide; R ) isopro- pyl) 6 and the roles of LF versus CT excited states have been carefully considered. 7 Especially the complexes fac-Mn(X)(CO) 3 (R-diimine) show a remarkable photochemical behavior. Upon irradiation into the lowest-energy MLCT band these complexes undergo release of a carbonyl ligand. According to the reaction sequence of Scheme 1, the CO-loss photoproduct reacts back thermally and photochemically with the carbonyl ligand to give the mer- isomer. The photochemical reaction mechanism, which in this case involves photodissociation of an equatorial CO with a concurrent movement of the axial X to the equatorial position, has been elucidated in ref 10. The mer-isomer, however, exhibits a very different photochemistry 4 for R-diimine ) bpy and i Pr-PyCa (pyridine-2-carbaldehyde N-isopropylimine): it photodecomposes into the radicals X • and [Mn(CO) 3 (R-di- imine)] • . The [Mn(CO) 3 (R-diimine)] • radicals dimerize to give Mn 2 (CO) 6 (R-diimine) 2 . The intriguing difference in primary photoprocess of the fac- and mer-isomers will be the subject of the present paper. Whereas the fac-isomer does not undergo Mn-X homolysis but loses CO, the mer-complexes do exhibit a Mn-X homolysis reaction. This is the first example of such a homolysis reaction for this class of metal carbonyl halide complexes. It has not been observed either in the parent MnX(CO) 5 complexes. 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