Coordination of donor solvent molecules to nickel square-planar complexes

The basicities of donor solvents were investigated for coordination of the donor solvent molecules to nickel square-planar complexes by spectrophotometry. It was observed that coordination equilibrium constants of the donor solvent molecules to [Ni(2,3,2-tet)](2+) (2,3,2-tet=1,4,8,11-tetraazaundecane) and [Ni(cyclen)](2+) (cyclen=1,4,7,10-tetraazacyclododecane) were larger-than those to [Ni(cyclam)](2+) (cyclam=1,4,8,11-tetraazacyclotetradecane). Specific coordinations of acetonitrile (AN) to [Ni(2,3,2-tet)](2+) and water to [Ni(cyclen)](2+) were observed. The first and second coordination equilibrium constants of [Ni(2,3,2-tet)]2+ and [Ni(cyclen)]2+ are discussed with pi-backdonation of AN, the hydrogen bond of water molecules, and steric hindrance between the nickel tetraamine complexes and the donor solvent molecules. Also, coordination equilibrium constants of the nickel tetraamine complexes are compared with those of [Ni(ttp)](2+)(ttp=1,4,8,11-tetrathiacyclotetradecane).