Density functional theory calculations reproduce the changes in geometry which are observed experimentally upon oxidation of a range of metal phosphine complexes. Removal of one electron from the metal leads to an increase in the M–P bond length, along with a decrease in pyramidalization at the phosphorus atom. These changes had previously been used to suggest that π back-bonding from the metal to the P–R σ* antibonding orbitals plays a role even in complexes of simple alkyl- and aryl-phosphines. Similar changes in geometry are found upon one electron oxidation of the simple model species Cr(CO)5(PR3) and Mo(CO)5(PR3) (R = H or Me). The analogous ammonia complexes instead undergo a decrease of the M–N bond length upon ionization, consistent with stronger σ-bonding upon increasing the charge on the metal atom. Analysis of the electron density in the neutral and cationic species, and of the change in density (this is the finite difference Fukui function) also shows evidence of π back-bonding in the neutral species, and much less in the cationic form.
