Nickel complexes pseudohalides

When nickel(II) salts containing a coordinating anion (halide and pseudohalide) are reacted with NaBH4 in the presence of np3, trigonal bipyramidal nickel(I) complexes of the type [NiXnp3], in which the coordinating anion occupies die axial position, 39,340 are obtained. When nickel(II) salts with poorly coordinating anions are used, non-stoichiometric hydrido complexes of nickel(II) are obtained (equations 108 and 109). 

Macrocycles with 14-16-membered chelate rings all encircle both high-spin and low-spin nickel(II) in the solid compounds. With anions which have coordination tendency, e.g. halides, pseudohalides and in some cases NOJ, trans octahedral paramagnetic complexes are obtained which are often blue or violet. With anions such as C104, PF6 and BF4 (and I, in some cases) which show little tendency to coordinate, square planar diamagnetic complexes are obtained which are generally yellow. 

M" s for tripeptide complexes to 1.2 x 10 M s" for tetrapeptides. The former value is consistent with NMR line-broadening experiments where an upper limit of 800 M s is determined for the rate. Addition of halide or pseudohalide anions enhances the rate by axial binding to the nickel(III) complexes and allowing a bridged, inner-sphere pathway to operate. Ligands such as pyridine which cannot bridge inhibit the reaction. 

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