trans-Diaquabis(dl-valinato-κ2 N,O)nickel(II)

In the title complex, [Ni(C5H9NO2)2(H2O)2], the NiII atom, located on a centre of inversion, is trans-coordinated by two O atoms and two N atoms from d-bidentate valine and l-bidentate valine ligands and two water O atoms in an octahedral geometry. In the crystal, the discrete mononuclear units are linked into a three-dimensional network via O—H⋯O and N—H⋯O hydrogen bonds. C—H⋯O interactions are also observed.

In the title complex, [Ni(C 5 H 9 NO 2 ) 2 (H 2 O) 2 ], the Ni II atom, located on a centre of inversion, is trans-coordinated by two O atoms and two N atoms from d-bidentate valine and lbidentate valine ligands and two water O atoms in an octahedral geometry. In the crystal, the discrete mononuclear units are linked into a three-dimensional network via O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds. C-HÁ Á ÁO interactions are also observed.

Comment
Complexes of transition metals and amino acids have been extensively studied as models for the metal-binding sites in proteins. Amino acids are versatile ligands showing flexible coordination modes (Loo et al., 2005) and they can coordinate to metal ions by their carboxylate and /or amino groups. Amino acid-metal complexes and their derivatives are of great importance because of their biochemical and pharmacological properties (Patrick et al., 2003). Valine is an essential amino acid (Ooiwa et al.;1995), and it can chelate to metal ions via its amino N atom and carboxylate O atom (Menabue et al.,1998). As a part of our studies on structural and properties of metal ion-amino acid complexes, we are reporting here the synthesis, crystal structure of a new Ni(DL-Val) 2 (H 2 O) 2 . The title compound is mononuclear, Ni(II) metal shows an octahedral geometry, it is in tans coordinated to D-bidentate valinate, L-bidentate valinate ions and two water molecules

Experimental
To a hot solution (333 K) of guanidinoacetic acid (0.2342 g, 2 mmol) and DL-valine (0.2342 g, 2 mmol) in deionized water (100 ml) was slowly added a solution of nickel (II) nitrate (0.1827 g, 1 mmol) in deionized water (5 ml). The reaction mixture was stirred at 333 K for 8 h, cooled slowly to 277 K, and the pH adjusted to 6.0 with KOH (3 M). The white precipitate which formed was filtered off and the filtrate was stored in a covered vessel. Thin blue plate-like crystals began to be formed after the some weeks and were collected and washed with absolute ethanol and dried at 323 K.

Refinement
The title compound crystallizes in the centrosymmetric space group C 2/c. All non-H atoms were refined with anisotropic atomic displacement parameters. H-atoms of water molecules and nitrohen were located in difference Fourier syntheses and not refined. Hydrogen atoms linked to carbon atoms were positioned geometrically and refined with a riding model, fixing the bond lengths at 0.98 and 0.96 A ° for CH and CH 3 groups, respectively. The U iso (H) values were constrained to be 1.2Ueq (parent) or 1.5Ueq(methyl C).
supplementary materials sup-2 Figures   Fig. 1. A view of the molecular structure of (I), showing the atom-numbering scheme and 30% displacement ellipsoids (arbitrary spheres for the H atoms)..

trans-Diaquabis(DL-valinato-κ 2 N,O)nickel(II)
Crystal data [Ni(C 5  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq