SYNTHESES, STRUCTURES AND SPECTROSCOPIC PROPERTIES OF NICKEL(II) CITRATO COMPLEXES, (NH4)2[Ni(Hcit)(H2O)2]2-2H2O AND (NH4)4[Ni(Hcit)2]-2H2O

Dimeric ammonium diaquocitratanickelate (II) dihydrate (NH4)(2)[Ni(Hcit)(H2O)(2)](2).2H(2)O, 1, and its sodium and potassium salts, as well as ammonium dicitratenickelate (II) dihydrate (NH4)(4)[Ni(Hcit)(2)] .2H(2)O, 2, (H(4)cit = citric acid) have been synthesized and characterized by spectroscopic methods. The crystal structures of 1 and 2 were determined by X-ray methods. Compound 1 is triclinic, space group P (1) over bar with a = 6.4071(7), b = 9.4710(7), c = 9.6904(5) Angstrom, alpha = 105.64(5), beta = 91.992(7), gamma = 89.334(8)degrees, V = 567.5(1) Angstrom(3), Z = 1, R = 0.037 for 1714 observed reflections. The structure consists of centrosymmetric dimers, [Ni(Hcit)(H2O)(2)](2-). The principal Ni-O dimensions are Ni-O(hydroxy), 2.074(2)Angstrom, Ni-O(alpha-carboxy), 2.020(3)Angstrom, Ni-O(beta-carboxy), 2.031(2), 2.037(2)Angstrom, Ni-O(water), 2.065(2), 2.072(3)Angstrom. Compound 2 crystallizes in the monoclinic space group P 2(1)/a with a = 9.361(1), b = 13.496(1), c = 9.4238(7)degrees, beta = 115.475(6)degrees, V = 1074.9(3)Angstrom(3), Z=2,R=0.052 for 1507 observed reflections. The two terdentate citrate ligands coordinate symmetrically to one nickel via hydroxyl, alpha- and beta-carboxylato oxygens and the remaining acetato groups are bonded through strong hydrogen bonds with the hydroxy group of another dicitrato nickel anion [2.526(6)Angstrom]. The principal Ni-O dimensions are: Ni-O(hydroxy), 2.021(3)Angstrom, Ni-O (alpha-carboxy), 2.038(3)Angstrom, Ni-O(beta-carboxy), 2.072(3)Angstrom. Nickel atoms in the two compounds have octahedral geometry The two compounds can be transformed easily by reaction with ammonium citrate or nickel chloride.