A series of complexes of general formula [CpMn(CO)(2)(eta(2)-HPR(1)R(2).BCl(3))] has been studied by DFT calculations and topological analyses of the charge density thus derived. The 21 complexes included in this study exhibit closely similar Mn-H-P geometries, in spite of a wide range of substituents (R(1); R(2)) at the phosphorus atom. Topological analysis of the electron density suggests that these are genuine sigma-bond complexes, albeit at a later stage on the oxidative addition reaction coordinate than corresponding silanes, with strong Mn-P and Mn-H bonding and a weak P-H interaction.