THE PHOTOCHEMISTRY OF TRANSITION-METAL HYDRIDES - A CASSCF/CCI STUDY OF THE PHOTODISSOCIATION OF HMN(CO)5

The photodissociation of HMn(CO)5 has been studied through contracted configuration interaction calculations of the potential energy surfaces for the metal-hydrogen bond homolysis and the dissociation of the carbonyl ligand. The corresponding potential energy curves connect the ground and excited states of the reactant to the ground and excited states of the primary products. The calculations were carried out under C4-nu constraint with a basis set that is at least of double-zeta quality. The multireference CCI calculations that correlate the 3d electrons and the two of the Mn-H bond were based on a unique CASSCF wave function with eight electrons in nine active orbitals (3d(xy) 3d(pi), sigma(Mn-H), sigma*Mn-H, 3d(x2-y2), 4d(xy), 4d(pi)) optimized for the 5A2 State, the principal configuration being (3d(xy)2(3d(x)2(sigma)2(sigma*)1(3d(x2-y2)1. It is proposed that excitation of HMn(CO)5 at 193 nm will bring the molecule from the ground state a1A1 into the c1E d --> pi* excited state. From there, after intersystem crossing to the b3Al at a Mn-H distance of about 1.7 angstrom and internal conversion into the a3Al sigma --> sigma* state, the molecule will dissociate along the a3Al potential energy curve to the products H and Mn(CO)5 in their ground state. Irradiation of HMn(CO)5 at 229 nm will bring the molecule into the b1E state d(pi) --> sigma*. Then the system goes down along the b1E potential energy curve corresponding to the Mn-CO elongation until it reaches a potential well. From there after internal conversion to the a1E state the molecule will dissociate along the corresponding potential energy curve to the products CO and HMn(CO)4 in the a1E excited state with the fragment HMn(CO)4 as a square pyramid with H apical. The main reasons responsible for the different behavior, upon irradiation, of the two hydrides HCo(CO)4 and HMn(CO)5 are outlined on the basis of these results.