M₄H₄X: Hydrometals (M=Cu, Ni) Containing Tetracoordinate Planar Nonmetals (X=B, C, N, O)

Abstract

Tetracoordinate planar carbon (that is, carbon atoms coordinated by four other atoms in a square-planar arrangement), first proposed by Hoffmann et al. [1] over thirty years ago, was recently observed in the vibrationally averaged D 4h [Al 4 C] À , [2] C 2v Al 3 XC, [Al 3 XC] À (X = Si,Ge), [3,4] and [CAl 4 ] 2À .[5] Tetracoordinate planar (TP) Si and Ge centers were also discovered in MAl 4 and [MAl 4 ] À .[6] 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 M 4 H 4 X, 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 Cu 4 H 4 and hydronickel Ni 4 H 4 are found to be suitable for hosting these tetracoordinate planar nonmetals (TPNs) with the high symmetry of D 4h . 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 Cu 4 H 4 at the DFT level. [7] Various initial structures obtained at the B3LYP/Lanl2dz level were optimized at the B3LYP/6-311 + G(3df,p) level, and imaginary frequencies were checked at the same level. The DFT binding energies of 23.1 eV for D 4h Ni 4 H 4 C and 18.7 eV for C 4v Cu 4 H 4 C demonstrate the stability of these complexes with respect to dissociation. The B3LYP geometries were further refined with the second-order Møller-Plesset (MP2) procedure. We calculated the NMR shielding tensors using the gauge-independent atomic orbital (GIAO) procedure [8,9] at the B3LYP/6-311 + G(3df,p) level, and VDEs, ionization potentials (IPs), and electron affinities (EAs) utilizing the restricted outer valence Green function method (ROVGF) [10,11] with a smaller basis of 6-31 + G(d,p). The optimized MP2 bond parameters, normal vibrational frequencies, and electronic properties of these systems are provided in the Supporting Information. All the calculations were performed with the Gaussian 03 program. À and Cu 4 H 4 C, on the other hand, were found to be transition states with imaginary frequencies of 266i and 135i cm À1 , respectively, and lie 0.962 and 0.124 eV higher in energy than the corresponding tetracoordinate pyramidal C 4v structures. These imaginary frequencies correspond to the A 2u vibrational mode, in which the central atom X moves up and down along the fourfold axis. Distortion of the D 4h structure in the A 2u mode leads to the C 4v global minimum, in which the X atom lies about 0.

Four Decades of the Chemistry of Planar Hypercoordinate Compounds

“…[58] Figure 22). [59] However,all of the double ptC species predicted by Li and co-workers are only highly energetic local minima in as hallow well on the potential- Figure 19.…”

Identifying the Hydrogenated Planar Tetracoordinate Carbon: A Combined Experimental and Theoretical Study of CAl4H and CAl4H–

“…26 Moreover, the simplicity of these penta-atomic species has allowed them to act as building blocks for more complicated complexes. [27][28][29] Clearly, in all the known ptC compounds (except the simplest penta-atomic ptCs), the ptC cores are stabilized by metallic or organometallic ligands. No hydrogenated ptCs have been reported in experiments.…”

D5h Cu5H5X: Pentagonal Hydrocopper Cu5H5 Containing Pentacoordinate Planar Nonmetal Centers (X = B, C, N, O)

“…[6] Even more interesting, the possibility of pentacoordinate planar carbons (PPC) in ϪC 3 B 3 Ϫ, ϪC 2 B 4 Ϫ, and ϪCB 5 Ϫ subgroups and hexacoordinate planar carbons (HPCs) in B 6 C 2Ϫ with D 6h symmetry was also explored by Schleyer et al [7Ϫ9] recently at the density functional theory (DFT) level. In a very recent paper, [10] our group performed an ab initio investigation of tetracoordinate planar nonmetals (TPNs) centered in perfect square hydrometals M 4 H 4 in D 4h M 4 H 4 X complexes (X ϭ B, C, N and O; M ϭ Cu and Ni), and extended the tetracoordinate planar centers from carbon to other first-row nonmetal elements and the ligands from p-block elements to transition metal atoms. However, unlike D 4h TPCs and D 6h HPCs observed in simple molecules, a PPC center with a high D 5h symmetry is still missing.…”