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.