The Dichotomy of Mn-H Bond Cleavage and Kinetic Hydricity of Tricarbonyl Manganese Hydride Complexes.

Acid-base characteristics (acidity, p K a, and hydricity, ΔG° _H- or k _H- ) of metal hydride complexes could be a helpful value for forecasting their activity in various catalytic reactions. Polarity of the M-H bond may change radically at the stage of formation of a non-covalent adduct with an acidic/basic partner. This stage is responsible for subsequent hydrogen ion (hydride or proton) transfer. Here, the reaction of tricarbonyl manganese hydrides mer , trans -[L ₂ Mn(CO) ₃ H] ( 1 ; L = P(OPh) ₃ , 2 ; L = PPh ₃ ) and fac -[(L-L')Mn(CO) ₃ H] ( 3 , L-L' = Ph ₂ PCH ₂ PPh ₂ (dppm); 4 , L-L' = Ph ₂ PCH ₂ -NHC) with organic bases and Lewis acid (B(C ₆ F ₅ ) ₃ ) was explored by spectroscopic (IR, NMR) methods to find the conditions for the Mn-H bond repolarization. Complex 1 , bearing phosphite ligands, features acidic properties (p K a 21.3) but can serve also as a hydride donor (ΔG ^≠ _298K = 19.8 kcal/mol). Complex 3 with pronounced hydride character can be deprotonated with KHMDS at the CH ₂ -bridge position in THF and at the Mn-H position in MeCN. The kinetic hydricity of manganese complexes 1 - 4 increases in the order mer , trans -[(P(OPh) ₃ ) ₂ Mn(CO) ₃ H] ( 1 ) < mer , trans -[(PPh ₃ ) ₂ Mn(CO) ₃ H] ( 2 ) ≈ fac -[(dppm)Mn(CO) ₃ H] ( 3 ) < fac -[(Ph ₂ PCH ₂ NHC)Mn(CO) ₃ H] ( 4 ), corresponding to the gain of the phosphorus ligand electron-donor properties.