The Simplest Planar Tetracoordinate Carbon: CLi3H+

Although numerous planar hypercoordinate atoms have been reported, we describe the first planar tetracoordinate carbon (ptC) stabilized by a distinct electronic mechanism. In the global minimum C2v structure of CLi3H+, the ptC is bonded to one hydrogen and three lithium atoms. This structure lies over 39 kcal/mol below the next isomer on the potential energy surface. Its stabilization arises solely from direct carbon ligand interactions, without contributions from ligand-ligand bonding. The carbon atom adopts a 2s22pσ12pπ²2pπ'¹ configuration, forming a strong C-H σ bond, while the C-Li interactions are weak and polar. CLi3H+ is therefore the smallest electron-deficient planar hypercoordinate species known, revealing a new bonding motif in cluster chemistry.