Density functional studies, based on the local density approximation including nonlocal corrections for correlation and exchange self-consistently, have been carried out for the equilibrium structures of the phosphinidene transition metal complexes MLn=PH, with M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ru, Os, Co, Rh, Ir and L = CO, PH3, Cp. The chemical reactivity of the transition metal-stabilized phosphinidene P-R is influenced by its spectator ligands L. Ligands with strong sigma-donor capabilities on the metal increase the electron density on the phosphorus atom, raise the pi*-orbital energy, and enhance its nucleophilicity. Spectator ligands with strong pi-acceptor capabilities lower the charge concentration on P and stabilize the pi*-orbital, which results in a higher affinity for electron-rich species. The MLn=PH bond is investigated using a bond energy analysis in terms of electrostatic interaction, Pauli repulsion, and orbital interaction. A symmetry decomposition scheme affords a quantitative estimate of the sigma- and pi-bond strengths. It is shown that the investigated phosphinidenes are strong pi-acceptors and even stronger sigma-donors. The metal phosphinidene interaction increases on going from the first to the second- and third-row transition metals.