LABILE COMPLEXES OF THE [RUTP(PN)](+) (TP EQUALS TRIPYRAZOLYLBORATE, PN EQUALS PH(2)PCH(2)CH(2)NME(2)) FRAGMENT INCLUDING THE DINITROGEN LIGAND

The [TpRu(pn)](+) fragment (Tp = tripyrazolyl, pn = Ph(2)PCH(2)CH(2)NM e(2)), featuring a strong sigma accepter and a weak pi donor, forms re versible complexes with a variety of sigma donor ligands L including N -2 which appears to be more strongly bonded than even CH3CN. X-ray cry stal structures of the complexes with L = H2O, acetone, CO, N-2, and v inylidene are included (adding to that for L = CH3CN from former work) , as well as a comparative MO study on the bonding nature of L. The se quence of complex stabilities of [TpRu(pn)L](+) is CF3SO3- < acetone a pproximate to H2O < CH3CN < N-2 < CO < vinylidene as suggested from EH MO calculations and crystallographic data as well as qualitative exper imental results. Despite the notable stability of [TpRu(pn)(eta(1)-N-2 )](+), the N-N bond length is 1.097(5) Angstrom, exactly that in free N-2. Our analysis suggests that the insensitivity of the N-N bond esse ntially originates from the counterbalance of pi bond weakening (throu gh back-donation) and sigma bond strengthening (through reducing the a ntibonding character of the sigma (2 sigma(u)) MO in molecular dinitr ogen.