The dissociation of PH3 from the is-electron system CpMoX(PH3)(3) to afford the corresponding 16-electron CpMoX(PH3)(2) fragment has been investigated theoretically by density functional theory for X = H, CH3, F, Cl, Br, I, OH, and PH2. The product is found to prefer a tripler spin state for all X ligands except PH2, the singlet-tripler gap varying between 1.7 kcal/mol for OH to 8.7 kcal/mol for F. The Mo-PH3 bond dissociation energy to the 16-electron ground state varies dramatically across the series, from 4.5 kcal/mol for OH to 23.5 kcal/mol for H, and correlates with experimental observations on trisubstituted phosphine derivatives. Geometry-optimized spin doublet CpMo(PH3)(3), on the other hand, has a Mo-PH3 bond dissociation energy of 24.3 kcal/mol. The modulation of the Mo-PH bond dissociation energy by the introduction of X is analyzed in terms of three effects that stabilize the 16-electron product relative to the 18-electron starting complex: (i) adoption of the higher (triplet) spin state by release of pairing energy; (ii) Mo-X pi interactions; (iii) release of steric pressure. A computational model for the approximate separation and evaluation of these three stabilizing effects is presented. According to the results of these calculations, the relative importance of the three effects depends on various factors related to the nature of X. For double-sided pi-donor X ligands, the larger triplet-singlet gap is provided by the more electronegative atoms (F > Cl > Br > I), whereas single-sided pi donors favor the singlet state. The pi-stabilization ability goes in the order PH2 > OH > F > other halogens > H. Finally, the major steric interaction appears to be associated with the presence of inactive lone pairs and by their orientation/proximity to the PH3 ligands (Cl, Br > I, OH > F, PH2, H, CH3). The 16-electron methyl system establishes a marked alpha-agostic interaction in the singlet state, which nevertheless remains unfavored relative to an undistorted tripler configuration.