The band structure of Ni(H 5C 3B 2). An example for energetic stabilization due to dimerization

Abstract

The band structure of Ni(H 5C 3B 2) the first experimentally verified one-dimensional (1D) polydecker system, has been investigated by means of a semi-empirical crystal orbital (CO) formalism based on an improved INDO (intermediate neglect of differential overlap) hamiltonian. In order to allow for an analysis of the electronic structure of a 1D arrangement composed by unit cells with an ungerade number of electrons a grand canonical (GC) averaging scheme has been used for the definition of the crystal hamiltonian. The synthesized 1D material with 13 valence electrons per simplest stoichiometric unit is a semiconductor and shows nuclear distortions into the direction of the stacking axis (i.e. formation of non-equivalent metal-ligand contacts). The tight-binding calculations lead to a transparent theoretical explanation of this formal dimerization. The 1D arrangement built up by one Ni(H 5C 3B 2) half-sandwich per unit cell reproducing the full translational symmetry is unstable towards a dimerization that leads to a unit which is formed by two half-sandwiches. The energy of the 1D column with the doubled cell is stabilized due to a symmetry-violation of the spatial wavefunction (i.e. symmetry-breaking for a symmetry atomic arrangement). A nuclear distortion (formation of non-equivalent metal-ligand distances) causes an additional energy lowering of the 1D system. The band structure properties in the outer valence region are analyzed. The calculated band gap is in line with he experimentally observed semiconducting properties of the 1D chain. The microstates of the valence band contain both admixtures from the cyclic organic π ligand (leading contribution) and from the transition metal centers (3d xz or 3d yz orbitals).