Tetracoordinate Planar Nonmetals

Tetracoordinate planar carbon (that is, carbon atoms coordinated by four other atoms in a square-planar arrangement), first proposed by Hoffmann et al. over thirty years ago, was recently observed in the vibrationally averagedD4h [Al4C] , C2v Al3XC, [Al3XC] (X= Si,Ge), and [CAl4] 2 . Tetracoordinate planar (TP) Si and Ge centers were also discovered in MAl4 and [MAl4] . The TP bonding character of Group 14 elements, contrary to the conventional concept of tetrahedral C, Si, and Ge, is supported by excellent agreements between the measured photoelectron spectra and ab initio vertical detachment energies (VDE) of these molecules in the TP structures. Here we extend the range of TP centers to include B, N, and O and the ligands from p-block elements to d-block transition metals (Cu and Ni). On the basis of ab initio optimization results, we present the first theoretical evidence that, in the form of the X-centered hydrometals M4H4X, these first-row nonmetals X (X=B, C, N, O) are tetracoordinated by four transition-metal ligands M (M=Cu, Ni) in perfect squares. Hydrocopper Cu4H4 and hydronickel Ni4H4 are found to be suitable for hosting these tetracoordinate planar nonmetals (TPNs) with the high symmetry ofD4h. To the best of our knowledge, neither experimental nor theoretical data on these unusual molecules are available to date. Our work was stimulated by the proposal of aromatic hydrocopper Cu4H4 at the DFT level. [7]