Linear metal chain compounds nickel

Nickel(II), palladium(II) and platinum(II) form planar complexes with 1,2-diondioximato ligands. The compounds crystallize in columnar stacks with different angles of inclination between the molecular planes and the stacking direction. The metal-metal distances depend strongly on the electronic and steric properties of the ligands. Upon oxidation with molecular iodine mixed valence compounds can be obtained. The stacking direction becomes perpendicular to the molecular planes, in these solids and the metal-metal distances decrease considerably. I -anions are incorporated into the lattice to form linear arrays parallel to the metal chains (1). 

Two nitrogen atoms are linearly coordinated to nickel atoms in all ANiN. They form o-bonds using the 2pz orbitals of nitrogen through a hybridization of the 3(1 and 4s orbitals of nickel. The valency of nickel is formally Ni+ with 3d9. The unpaired electron is accommodated in an antibonding a or it orbital to make the compound metallic. The jt bond character in the linear chain has been pointed out in YCoC which is isostructural to CaNiN.19 The calcium d-character also contributes to the lowest free-electron-like band at the T point.20... 

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