Molecular complexes of small alkanes with Co⁺

Using ab initio methods, including extensive electron correlation, we studied the molecular complexes of Co⁺ with H₂ and a number of small alkanes (CH₄, C₂H₆, and C₃H₈). We found that the bond energies increase with ligand size, ranging from 17.2 (for Co(H₂)⁺) to 29.3 kcal/mol (for Co(C₃H₈)⁺), in close agreement with recent experiments. For methane, Co⁺ coordinates to either two or three C-H bonds, with the η² geometry more stable by 1.3 kcal/mol out of 21.4 kcal/mol. For ethane, η² and η³ coordinations of Co⁺ to the C-H bonds are also favorable, but these bonds can be on either the same carbon center or different centers (strict coordination to the C-C bond was not found). Adding a second CH₄ to Co(CH₄)⁺, we found that the second bond is stronger than the first by 1.7 kcal/mol. In addition, with both methanes in the η² configuration, we found that the C-H bonds are eclipsed (8.3 kcal/mol better than the staggered conformation). With both methanes in the η³ configuration, however, they are staggered.