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

The nature of the M-N and N-N bonding and the activation of co-ordinat ed N2 towards protonation has been investigated theoretically using th e extended Huckel molecular orbital (EHMO) method. Charges on the N2 a toms and the M-N and N-N overlap populations for the model systems tra ns-[V(PH3)4(N2)2]-, [Fe(PH3)4H(N2)]+ and [Fe(PH3)4(N2)] [the actual ph osphine ligand being 1,2-bis(dimethylphosphino)ethane or its diethyl d erivative] show an excellent correlation with experiment. The more rea ctive N2 group of the vanadium complex has a much larger negative char ge while both the V-N and N-N bonds in [V(PH3)4(N2)2]- are computed an d observed to be relatively weak with the corresponding bonds in the F e compounds being relatively strong. Both theory and experiment theref ore demonstrate that the simple Dewar-Chatt-Duncanson (DCD) picture is not applicable since it predicts a strong M-N bond should be associat ed 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 i s pi dominated and there is no synergic correlation between them. In c ontrast, EHMO calculations for [Ru(NH3)5(N2)]2+ and [Os(NH3)5(N2)[2+ c orrelate both with the experimental IR vibrational data and with the p redictions of the DCD model. For these complexes, an increasing M-N in teraction is associated with a decreasing N-N bond strength. The facto rs which lead to the departure away from the DCD picture for the vanad ium and iron species are discussed in terms of the metal charges and c o-ordination numbers.