Evidence for ligand non-innocence in a formally ruthenium(I) hydride complex

Formally zerovalent, dinitrogen-bridged ruthenium complex, {[N(3)(Xyl)]Ru}(2)(mu-eta(1):eta(1)-N(2)) (1), where [N(3)(Xyl)] = 2,6-(XylN horizontal lineCMe)(2)C(5)H(3)N, reacts with excess H(2) to give the binuclear hydride species, {[N(3)]Ru(H)}(2)(mu-eta(1):eta(1)-N(2)) (2) bearing a single hydrogen per ruthenium. Complex 2 is an unusual example of a structurally characterized paramagnetic transition metal hydride, and the first such example for ruthenium. Structural data and DFT calculations suggest unpaired electron density is strongly delocalized onto the non-innocent [N(3)] ligand, with a relatively small degree of the metalloradical character implied by the Ru(I) formal oxidation state, and that the [N(3)](-)/Ru(II) formalism may be more informative. Consistent with an effective oxidation state greater than Ru(I), further reaction of 2 with excess H(2) to give metal dihydride species ([N(3)]RuH(2)(L)) is not observed. The magnetic moment of 2 (3.50 mu(B)) in solution is consistent with one unpaired electron per [N(3)]Ru moiety; however, 2 is diamagnetic in the solid due to close (3.26 A) head-to-tail contact between Ru pyridine planes of neighboring molecules. Although the geometry is reminiscent of the weak "pi-stacking" observed for closed-shell aromatic ring systems, DFT calculations indicate the structure and associated spin pairing result from in-phase overlap of the delocalized SOMOs on neighboring molecules-that is the interaction is best viewed as a weak covalent bond delocalized over 22 atoms.