| Abstract
| - The cobalt carbonyl carbide clusters [Co6C(CO)n]2− (n = 12, 13, 14, 15, and 16) have been studied by density functional theory. The experimentally known trigonal prismatic structure of [Co6C(CO)15]2− is found to be particularly stable. The lowest energy structures of [Co6C(CO)16]2− and [Co6C(CO)14]2− are thermodynamically unstable with respect to CO loss and disproportionation, respectively. The lowest energy [Co6C(CO)13]2− structure is very similar to a known stable structure with an octahedral Co6 skeleton. The lowest energy [Co6C(CO)12]2− structure contains a carbon-centered Co6 puckered hexagon in the chair form.
- The six-vertex cobalt carbonyl clusters [Co6C(CO)n]2− (n = 12, 13, 14, 15, 16) with an interstitial carbon atom have been studied by density functional theory (DFT). These DFT studies indicate that the experimentally known structure of [Co6C(CO)15]2− consisting of a Co6 trigonal prism with each of its edges bridged by carbonyl groups is a particularly stable structure lying more than 20 kcal/mol below any other [Co6C(CO)15]2− structure. Addition of a CO group to this [Co6C(CO)15]2− structure gives the lowest energy [Co6C(CO)16]2− structure, also a Co6 trigonal prism with one of the vertical edges bridged by two CO groups and the remaining eight edges each bridged by a single CO group. However, this [Co6C(CO)16]2− structure is thermodynamically unstable with respect to CO loss reverting to the stable trigonal prismatic [Co6C(CO)15]2−. This suggests that 15 carbonyl groups is the maximum that can be attached to a Co6C skeleton in a stable compound. The lowest energy structure of [Co6C(CO)14]2− has a highly distorted octahedral Co6 skeleton and is thermodynamically unstable with respect to disproportionation to [Co6C(CO)15]2− and [Co6C(CO)13]2−. The lowest energy [Co6C(CO)13]2− structure is very similar to a known stable structure with an octahedral Co6 skeleton. The lowest energy [Co6C(CO)12]2− structure is a relatively symmetrical D3d structure containing a carbon-centered Co6 puckered hexagon in the chair form.
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