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=CMe)(2)C5H3N, 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]RuH2(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 angstrom) 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.