| The 16-electron OsHCl(CO)L2 (L=PR3) possesses the π-donor Cl as the most nucleophilic site, hydrogen bonding to which by acidic alcohols in solution increases the extent of unsaturation of Os. Steric factors preclude concomitant coordination of ROH oxygen to Os, rendering the catalytically-active complex in the coordinatively-unsaturated form.;Selective removal of either Cl− or H− from cis,trans-Os(H)2Cl(NO)L2 is achieved under appropriate conditions, with the former favored kinetically and the latter thermodynamically in the protonation reaction. Both cationic products coordinate CH2Cl2 in solution, while Os(H) 2(NO)L2+ forms an adduct with C2H 4, which is highly fluxional by reversible olefin insertion into one Os-H bond.;A series of 18-electron nitrosyl di- and trihydrides of Ru and Os are significantly distorted from octahedral geometry by compression of the cis H-Os-H angles to ∼75°. The increased H to M σ-donation and exceptional π-acidity of linear NO are responsible for the distortion, which in the case of Ru facilitates the reductive elimination of H2, intramolecular hydride site-exchange, and results in observation of quantum mechanical exchange coupling by 1H NMR, the first example found for a d6 metal polyhydride.;Thermal elimination of R-H from cis,trans-Os(H) 2R(NO)L2 affords a zero-valent derivative trans-OsH(NO)L 2, reactive by intramolecular C(sp3)-H and intermolecular C(sp2)-H and C(sp2)-F oxidative addition and affording clean defluorination of H2C=CHF and H 2C=CF2 with high yields. Intermolecular lithium-bonding interactions of R2NLi with the NO oxygen facilitate both the oxidative addition and reductive elimination reactions, stabilizing the transition state by up to 3 kcal/mol.;The combination of the high electrophilicity of the metal center and sufficient steric protection by the two bulky PR3 ligands in Os(H) 2(NO)L2+ enables an unprecedented Me − abstraction reactivity by Os from conventional organolithium reagents, including (Me3Si)2NLi, (Me3Si) 2CHLi, Me3SiCH2Li and B(CH2SiMe 3)4−. Several mechanistic issues are addressed and the intimate mechanism of the aliphatic C-C and C-Si bond cleavage is formulated as bimolecular electrophilic substitution proceeding via back-side attack by Os electrophile with inversion of the Me group. |
