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)NMe(2)), featuring a strong sigma accepter 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 CH3CN. X-ray crystal structures of the complexes with L = H2O, acetone, CO, N-2, and vinylidene 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 sequence of complex stabilities of [TpRu(pn)L](+) is CF3SO3- < acetone approximate to H2O < CH3CN < 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) Angstrom, 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* (2 sigma(u)) MO in molecular dinitrogen.