Sigma bond activation by transition metal ions: The Co(CH₄)_n/⁺ systems revisited

Measurements are reported for sequential clustering of CH₄ to Co⁺ ions under equilibrium conditions. The CH₄ cluster bond strengths show a pairwise behavior: -ΔH₀/⁰ = 23.1 and 25.3 kcal/mol for n = 1 and 2; 7.3 and 5.2 kcal/mol for n = 3 and 4; and ∼2 kcal/mol for both n = 5 and 6. This pairwise behavior is well reproduced by large basis set density functional theory calculations. These calculations indicate n = 1 and n = 2 add on opposite sides of the Co⁺ ion in η² configuration and induce significant s/d hybridization on Co⁺. This hybridization both reduces Pauli repulsion and fosters sigma donation into the 4s orbital on Co⁺. Clusters n = 3 and n = 4 add at 90° to the n = 1 and 2 line of centers forming a planar system. The s/d hybridization is unfavorable for these clusters resulting in longer Co⁺-C bond lengths and substantially reduced binding energies. To n = 5 and 6 ligands probably complete a pseudo octahedral complex and are very weakly bound, perhaps defining a second solvation shell. An impurity contributed substantially to the experimental peak at m/z = 123 corresponding to Co⁺(CH₄)₄. The impurity was tentatively identified as O₂Co⁺(CH₄)₂ and experimental protocals were developed to eliminate its impact on the data reported here. It is suggested this impurity could be responsible for published guided ion beam results that found a substantially larger binding energy for n = 4 than for n = 3 in contrast to what is reported here.