title: Hexanuclear cobalt carbonyl carbide clusters: the interplay between octahedral and trigonal prismatic structures.
abstract: 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.
authors.affiliation: State Key Laboratory of Materials Modification by Laser, Electron, and Ion Beams, School of Physics and Optoelectronic Technology & College of Advanced Science and Technology, Dalian University of Technology, Dalian 116024, China.
authors.forename: Jijun
authors.initials: J
authors.lastname: Zhao
authors.forename: Jingcheng
authors.initials: J
authors.lastname: Xu
authors.forename: R Bruce
authors.initials: RB
authors.lastname: King