The structure and the energetics of the model systems CpMX2(PH3) + PH3 reversible arrow CpMX2(PH3)(2) (Cp = cyclopentadienyl; M = Cr, Mo; X = Cl, CH3) are studied by performing Moller-Plesset second order (MP2) and density functional theory (DFT) calculations. Extended basis sets are employed in the geometry optimizations. The results indicate that the structural preference can be traced back to the competition between electron pairing stabilization and M-P bond dissociation energy along the spin doublet surface. At all levels of calculation, the energy splitting, a measure of the cost of pairing the electron during the promotion process from the quartet ground state to the excited doublet state for CpCrX2(PH3), is found to be on average 15-20 kcal/mol greater than the energy gain associated with the formation of the new Cr-PH3 bond along the spin doublet surface. For the analogous Mo chloride system the reverse appears to be true, the products with higher coordination being energetically favored by 10-12 kcal/mol. These data are in agreement with experimental evidence.