The first comparative kinetic study of the addition of the isolobal and isosteric CO and N2 ligands to a spin triplet organometallic compound, i.e. Cp*MoCl(PMe3)2, is reported. A fast and quantitative addition process occurred when interacting Cp*MoCl(PMe3)2 with CO, which is followed by a subsequent slower process involving PMe3 replacement and formation of Cp*MoCl(CO)2(PMe3). The N2 addition, on the other hand, is much slower and proceeds incompletely to an equilibrium position. The temperature dependence of this equilibrium gives the parameters for the reaction ΔH = −22.8 ± 2.1 kcal/mol and ΔS = −67 ± 7 cal·mol-1·K-1. The activation parameters for the CO addition are ΔH⧧ = 5.0 ± 0.3 kcal/mol and ΔS⧧ = −35 ± 4 cal·mol-1·K-1, while the activation parameters for the N2 addition are ΔH⧧ = 14.0 ± 1.0 kcal/mol and ΔS⧧ = −20 ± 3 kcal/mol. Extrapolation of the rates to 25 °C indicates a difference of more than three orders of magnitude:  kCO = 29 ± 3 M-1 s-1 and kN2 = 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 N2), 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 1A‘ 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 = N2 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 N2 addition as expected from greater diffuseness of the CO donor and acceptor orbitals. As the N2 ligand continues to approach the metal, the 3A‘‘ surface becomes increasingly repulsive whereas the addition of CO leads to an attractive interaction and a bound triplet state.