METAL DINITROGEN DELTA-BONDING AND PI-BONDING ROLES IN [FE(DMPE)(2)H(N(2)](+), [FE(DEPE)(2)(N(2))] AND TRANS-[V(DMPE)(2)(N(2))(2)[- (DMPE = ME(2)PCH(2)CH(2)PET(2))

The nature of the M-N and N-N bonding and the activation of co-ordinated N2 towards protonation has been investigated theoretically using the extended Huckel molecular orbital (EHMO) method. Charges on the N2 atoms and the M-N and N-N overlap populations for the model systems trans-[V(PH3)4(N2)2]-, [Fe(PH3)4H(N2)]+ and [Fe(PH3)4(N2)] [the actual phosphine ligand being 1,2-bis(dimethylphosphino)ethane or its diethyl derivative] show an excellent correlation with experiment. The more reactive N2 group of the vanadium complex has a much larger negative charge while both the V-N and N-N bonds in [V(PH3)4(N2)2]- are computed and observed to be relatively weak with the corresponding bonds in the Fe compounds being relatively strong. Both theory and experiment therefore demonstrate that the simple Dewar-Chatt-Duncanson (DCD) picture is not applicable since it predicts a strong M-N bond should be associated with a weak N-N bond and vice versa. For these first-row transition-metal complexes, the M-N bond is sigma dominated while the N-N bond is pi dominated and there is no synergic correlation between them. In contrast, EHMO calculations for [Ru(NH3)5(N2)]2+ and [Os(NH3)5(N2)[2+ correlate both with the experimental IR vibrational data and with the predictions of the DCD model. For these complexes, an increasing M-N interaction is associated with a decreasing N-N bond strength. The factors which lead to the departure away from the DCD picture for the vanadium and iron species are discussed in terms of the metal charges and co-ordination numbers.