A recent computational investigation of Jahn-Teller effects in unsaturated 16-electron d(4)-d(6) [CpMLn] complexes (Abu-Hasanayn, F.; Cheong, P.; Oliff, M. Angew. Chem. 2002, 41, 2120) highlighted the typical presence of two spin-triplet and two singlet states of competing stability in these complexes and pointed out the necessity to account for more than one electronic state in studies thereof. Consequently, we have reinvestigated the addition of N-2 to all the four low-energy states of CpMoCl(PH3)(2), a reaction for which previously only one singlet and one triplet state have been considered (Keogh, D. W.; Poli, R. J. Am. Chem. Soc. 1997, 119, 2516). The present study was performed using density functional theory (DFT) and the thus obtained relative stabilities of the four electronic states of the educt are in good accord with those obtained using a multireference MP2 method. The spin-singlet ground state of the 18e(-) product of N-2 addition turns out to be derived from the fourth lowest state (2(1)A') of the 16e(-) educt, immediately demonstrating the importance of accounting for more than one triplet and one singlet state in such reactions. The barrier to N-2 addition was found to arise from the enthalpic cost of obtaining identical geometries for this singlet state and the spin-triplet ground state of the educt ((3)A") in the minimum energy crossing point (MECP). With a spin-triplet ground-state reactant complex, a triplet-singlet MECP defining the rate-limiting step, and a singlet product, our calculated activation (14.4 kcal/mol) as well as reaction enthalpies (21.2 kcal/mol) of N-2 addition to CpMoCl(PMe3)(2) are found to be within the experimental error bars of those measured for Cp*MoCl(PMe3)(2). For the corresponding reaction with CO, there is a delicate balance between the transition state (TS) of addition on the triplet potential energy surface (PES) and the point of crossing between the triplet and singlet PES. Our optimized TS and MECPs for this reaction suggest that the rate is controlled by the barrier defined by the spin-triplet TS, with spin inversion occurring after this point. Our calculated activation enthalpy (6.7 kcal/mol) based on the spin-triplet TS is in excellent agreement with that measured for Cp*MoCl(PMe3)(2).