PHOTOSUBSTITUTION OF D(6) METAL-CARBONYLS M(CO)(5)L REVISITED - A THEORETICAL-STUDY

A theoretical study of the photosubstitution reactions of d(6) metal carbonyls M(CO)(5)L, characterized by a high degree of cis stereospecificity, is reported. The possible excited-state isomerization pathways of the five-coordinate species M(CO)(4)L (square pyramid (SP), with L basal or apical, trigonal bipyramid regular (TBP) or distorted (d-TBP)) have been considered on the basis of CASSCF/CASPT2 calculations of the relative stabilities of the different structures of Mn(CO)(4)Cl in the lowest excited states (singlet or triplet). The basis set effect on the relative stabilities of the various structures of Tc(CO)(4)Cl and Mo(CO)(4)NH3 have been ascertained at the SCF level with double-zeta-quality basis sets. The results obtained for Mn(CO)(4)Cl have been used to generate a state correlation diagram that connects the ground state and lowest excited states of the reactant Mn(CO)(5)Cl with those of the five-coordinate species with different structures. On the basis of this diagram, we propose the following mechanism. (i) Excitation of Mn(CO)(5)Cl occurs into the (1)E ligand field state followed by elimination of a carbonyl ligand with formation of Mn(CO)(4)Cl either as an SP apical in the 1E state (axial elimination) or as an SP basal in the (1)A' state (equatorial elimination). (ii) Through a Berry pseudorotation, followed by a B-1(2) --> (1)A(1) internal conversion, the SP structures evolve either to the (1)A(1) ground state of the regular TBP or to the d-TBP in the (1)A(1) state. (iii) The molecule gets trapped in the potential well corresponding to the (1)A(1) ground state of the d-TBP (after a downhill rearrangement in the case of the regular TBP), until it reacts with an incident nucleophile, leading to the cis derivatives.