The first comparative kinetic study of the addition of the isolobal and isosteric CO and N-2 ligands to a spin triplet organometallic compound, i.e. Cp*MoCl(PMe(3))(2), is reported. A fast and quantitative addition process occurred when interacting Cp*MoCl(PMe(3))(2) with CO, which is followed by a subsequent slower process involving PMe(3) replacement and formation of Cp*MoCl(CO)(2)(PMe(3)). The N-2 addition, on the other band, is much slower and proceeds incompletely to an equilibrium position. The temperature dependence of this equilibrium gives the parameters 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. Extrapolation of the rates to 25 degrees C indicates a difference of more than three orders of magnitude : k(CO) = 29 +/- 3 M(-1) s(-1) and k(N2) = 0.014 +/- 0.001 M(-1) s(-1). Theoretical calculations with full geometry optimization at the MP2 level have been carried out on the model systems CpMoCl(PH3)(2) + L (L = CO or N-2), the calculated energetics of the system being in agreement with experiment. The 16-electron CpMoCl(PH3)(2) molecule is found to be more stable in the spin triplet state, the 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 addition 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 the CO vs the N-2 addition as expected from greater diffuseness of the CO 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 triplet state.