Sigma bond activation by transition metal ions: the Co(CH4)(n)(+) systems revisited

Measurements are reported for sequential clustering of CH4 to Co+ ions unde r equilibrium conditions. The CH, cluster bond strengths show a pairwise be havior: - DeltaH(0)(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 similar to2 kcal/mol for both n = 5 and 6. This pairwise behavior is well reproduced by large basis set density functi onal theory calculations. These calculations indicate n = 1 and n = 2 add o n opposite sides of the Cot ion in eta (2) configuration and induce signifi cant s/d hybridization on Co+. This hybridization both reduces Pauli repuls ion and fosters sigma donation into the 4s orbital on Cot. Clusters n = 3 a nd n = 4 add at 90 degrees to the n = 1 and 2 line of centers forming a pla nar system. The s/d hybridization is unfavorable for these clusters resulti ng in longer Cot-C bond lengths and substantially reduced binding energies. To n = 5 and 6 ligands probably complete a pseudo octahedral complex and a re very weakly bound, perhaps defining a second solvation shell. An impurit y contributed substantially to the experimental peak at m/z = 123 correspon ding 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 bindin g energy for n = 4 than for n = 3 in contrast to what is reported here.(C) 2001 Elsevier Science B.V.