SPIN-STATE CHANGE IN ORGANOMETALLIC REACTIONS - EXPERIMENTAL AND MP2 THEORETICAL-STUDIES OF THE THERMODYNAMICS AND KINETICS OF THE CO AND N-2 ADDITION TO SPIN TRIPLET CP-ASTERISK-MOCL(PME(3))(2)

The first comparative kinetic study of the addition of the isolobal an d isosteric CO and N-2 ligands to a spin triplet organometallic compou nd, i.e. CpMoCl(PMe(3))(2), is reported. A fast and quantitative addi tion process occurred when interacting CpMoCl(PMe(3))(2) with CO, whi ch is followed by a subsequent slower process involving PMe(3) replace ment and formation of CpMoCl(CO)(2)(PMe(3)). The N-2 addition, on the other band, is much slower and proceeds incompletely to an equilibriu m position. The temperature dependence of this equilibrium gives the p arameters for the reaction Delta H = -22.8 +/- 2.1 kcal/mol and Delta S = -67 +/- 7 cal . mol(-1). K-1. The activation parameters for the CO addition are Delta H-double dagger = 5.0 +/- 0.3 kcal/mol and Delta S -double dagger = -35 +/- 4 cal . mol(-1). k(-1), while the activation parameters for the N-2 addition are Delta H-double dagger = 14.0 +/- 1 .0 kcal/mol and Delta S-double dagger = -20 +/- 3 kcal/mol. Extrapolat ion of the rates to 25 degrees C indicates a difference of more than t hree orders of magnitude: k(CO) = 29 +/- 3 M(-1) s(-1) and k(N2) = 0.0 14 +/- 0.001 M(-1) s(-1). Theoretical calculations with full geometry optimization at the MP2 level have been carried out on the model syste ms CpMoCl(PH3)(2) + L (L = CO or N-2), the calculated energetics of th e system being in agreement with experiment. The 16-electron CpMoCl(PH 3)(2) molecule is found to be more stable in the spin triplet state, t he excited (1)A' state being 10.9 kcal/mol higher in energy. The Mo-L bond formation is calculated to be exothermic by 27.9 kcal/mol for L = N-2 and by 60.0 kcal/mol for L = CO. Calculations along the L additio n coordinate show an initial ligand rearrangement related barrier for both the spin singlet and the spin triplet surfaces. After overcoming this barrier, the spin singlet curve descends in energy earlier for th e CO vs the N-2 addition as expected from greater diffuseness of the C O donor and acceptor orbitals. As the N-2 ligand continues to approach the metal, the (3)A '' surface becomes increasingly repulsive whereas the addition of CO leads to an attractive interaction and a bound tri plet state.