Zn and Ni complexes of pyridine-2,6-di-carboxyl-ates: crystal field stabilization matters!

Six reaction products of Zn and Ni with pyridine-2,6-di-carb-oxy-lic acid (HLig), 4-chloro-pyridine-2,6-di-carb-oxy-lic acid (HLig) and 4-hy-droxy-pyridine-2,6-di-carb-oxy-lic acid (HLig) are used to pinpoint the structural consequences of crystal field stabilization by an incomplete shell. The pseudo-octa-hedral Zn coordination sphere in bis-(6-carb-oxy-picolinato)zinc(II) trihydrate, [Zn(CHNO)]·3HO or [Zn(HLig)]·3HO, (), is significantly less regular than that about Ni in the isostructural compound bis-(6-carb-oxy-picolinato)nickel(II) trihydrate, [Ni(CHNO)]·3HO or [Ni(HLig)]·3HO, (). The Zn complexes poly[(4-chloro-pyridine-2,6-di-carboxyl-ato)zinc(II)], [Zn(CHClNO)] or [Zn(Lig)] , (), and poly[[(4-hy-droxy-pyridine-2,6-di-carboxyl-ato)zinc(II)] monohydrate], {[Zn(CHNO)]·HO} or {[Zn(Lig)]·HO} , (), represent two-dimensional coordination polymers with chelating and bridging pyridine-2,6-di-carboxyl-ate ligands in which the coordination polyhedra about the central cations cannot be associated with any regular shape; their coordination environments range between trigonal-bipyramidal and square-pyramidal geometries. In contrast, the corresponding adducts of the diprotonated ligands to Ni, namely tri-aqua-(4-chloro-pyridine-2,6-di-carboxyl-ato)nickel(II), [Ni(CHClNO)(HO)] or [NiLig(OH))], (), and tri-aqua-(4-hy-droxy-pyridine-2,6-di-carboxyl-ato)nickel(II) 1.7-hydrate, [Ni(CHNO)(HO)]·1.7HO or [NiLig(OH))]·1.7HO, (), feature rather regular octa-hedral coordination spheres about the transition-metal cations, thus precluding the formation of analogous extended structures.