Reactivity of the hydrido/nitrosyl radical MHCl(NO)(CO)(PiPr3)2, M = Ru, Os

The reaction of equimolar NO with the 16 electron molecule RuHCl(CO)L-2 (L = (PPr3)-Pr-i) proceeds, via a radical adduct RuHCl(CO)(NO) L-2, onward to form RuCl(NO)(CO)L-2 (X-ray structure determination) and RuHCl(HNO)-(CO)L-2, in a 1:1 mole ratio. The HNO ligand, bound by N and trans to hydride, is rapidly degraded by excess NO. The osmium complex behaves analogously, but the adduct has a higher formation constant, permitting determination of its IR spectrum; both MHCl(CO)(NO)L-2 radicals are characterized by EPR spectroscopy, and DFT calculations on the Ru system show it to have a "half-bent" Ru-N-O unit with the spin density mainly on nitrogen. DFT (PBE) energies rule out certain possible mechanistic steps for forming the two products. A survey of the literature leads to the hypothesis that NO should generally be considered as a (neutral) Lewis base (2-electron donor) when it binds to a 16 electron complex which is resistant to oxidation or reduction, and that the resulting N-centered radical has a M-N-O angle of similar to140degrees, which distinguishes it from NO- (bent at <140degrees) and from NO+ (>170degrees).