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)L2 (L = PiPr3) proceeds, via a radical adduct RuHCl(CO)(NO) L2, onward to form RuCl(NO)(CO)L2 (X-ray structure determination) and RuHCl(HNO)(CO)L2, 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)L2 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 ∼140°, which distinguishes it from NO- (bent at <140°) and from NO+ (>170°).