Labile Complexes of the [RuTp(pn)](+) (Tp = Tripyrazolylborate, pn = Ph(2)PCH(2)CH(2)NMe(2)) Fragment Including the Dinitrogen Ligand(1).

The [TpRu(pn)](+) fragment (Tp = tripyrazolyl, pn = Ph(2)PCH(2)CH(2)NMe(2)), featuring a strong sigma acceptor and a weak pi donor, forms reversible complexes with a variety of sigma donor ligands L including N(2) which appears to be more strongly bonded than even CH(3)CN. X-ray crystal structures of the complexes with L = H(2)O, acetone, CO, N(2), and vinylidene are included (adding to that for L = CH(3)CN from former work), as well as a comparative MO study on the bonding nature of L. The sequence of complex stabilities of [TpRu(pn)L](+) is CF(3)SO(3)(-) < acetone approximately H(2)O < CH(3)CN < N(2) < CO < vinylidene as suggested from EHMO calculations and crystallographic data as well as qualitative experimental results. Despite the notable stability of [TpRu(pn)(eta(1)-N(2))](+), the N-N bond length is 1.097(5) Å, exactly that in free N(2). Our analysis suggests that the insensitivity of the N-N bond essentially originates from the counterbalance of pi bond weakening (through back-donation) and sigma bond strengthening (through reducing the antibonding character of the sigma (2sigma(u)) MO in molecular dinitrogen.