Planar Pentacoordinate Carbon in CAl₅⁺: A Global Minimum

We report evidence for the first global-minimum structure having a planar pentacoordinate carbon. High-level ab initio computations and quantum molecular dynamics simulations at 300 and 400 K reveal that the most stable CAl5(+) isomer has D5h symmetry and is approximately 3.80 kcal/mol lower in energy than the second most stable alternative. The latter has a nonplanar structure based on a tetrahedral CAl4 moiety. The unexpectedly high proclivity for two-dimensional chemical bonding of the carbon in D5h CAl5(+), the robust thermal stability indicated computationally, and its mass spectrometric detection suggest that experimental characterization of this planar pentacoordinate carbon cation at room temperature is a likely prospect.