Adiabatic Electron Detachment Energies, Reaction Barriers, Chemical Balance, and Ligand Effects on the Nucleophilicities of Metal Carbonyl Monoanions

The absolute values of the adiabatic electron detachment energies (ADBEs) of transition metal carbonyl monoanions are equivalent to the adiabatic electron affinities of the corresponding neutral species. The ADBEs, reaction barriers, and thermal balances for the reactions NaM­(CO)n + CH3I → CH3M­(CO)n + NaI and NaM­(CO)mCp + CH3I → CH3M­(CO)mCp + NaI obtained using the density functional theory and the CCSD­(T) method are shown to be related to the nucleophilicities of transition metal carbonyl anions as originally measured by Dessy, Pohl, and King using electrochemical methods. The relatively large ADBEs and higher reaction barriers in combination with the unfavorable chemical balance of the products synergistically lead to the very weak nucleophilicities of the octahedral metal carbonyl anions M­(CO)6– (M = V, Nb, Ta). The much greater nucleophilicities of the pentacoordinated metal carbonyl anions M­(CO)5– mainly result from the relatively lower reaction barriers of 1.93, 1.85, and 1.63 eV predicted at the CCSD­(T)/6-311G++(df,pd) level, consistent with the Re­(CO)5– anion being more nucleophilic than the Mn­(CO)5– anion. The tetrahedral metal carbonyl anion Co­(CO)4– of intermediate nucleophilicity has an intermediate ADBE of 2.25 eV. The tetrahedral second and third row metal analogues M­(CO)4– (M = Rh, Ir), which have not been measured experimentally, are predicted to have much higher ADBEs of 2.57 eV for Rh and 2.87 eV for Ir but with reaction barriers of 1.62 eV for Rh and 1.71 eV for Ir comparable to a value of 1.66 eV for V. The cyclopentadienyl metal carbonyl monoanions are generally more nucleophilic than the homoleptic metal carbonyl anions with the same metal coordination number after considering the three metal coordination positions occupied by the cyclopentadienyl ring. The lowest ADBE of 0.82 eV and the lowest reaction barrier of 0.93 eV are found for the CpFe­(CO)2– anion, which is the most nucleophilic of all of the anions investigated. Comparison of the calculated ADBEs of these metal carbonyl anions with those of halide ions suggests that the metal carbonyl monoanions most commonly used for organometallic syntheses are significantly stronger nucleophiles than any of the halide ions.