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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.

Chloride abstraction from [Ru{HB(pz),}(tmen)Cl] (pz = pyrazolyl, tmen = Me,NCH,CH,NMe,) with NaBPh, in the solvents acetone and dimethylformamide led to the formation of the respective cationic [R~(HB(pz)~}(tmen)(solv)] + complexes. In the presence of phenylacetylene, these are easily transformed into the first example of a ruthenium HB(pz), vinylidene complex. Extended-Huckel molecular-orbital calculations have been performed to establish the nature of the bonding involved. From [Ru{HB(pz),}(py),Cl] (py = pyridine) the corresponding vinylidene complex could be prepared in the same way, although the intermediate solvent complexes could not be isolated. Selected crystal structures were determined.

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