Indian Journal of Chemistry Vol. 48A, February 2009, pp. 181-188 On the distorted {NiN 6 } octahedron in hexakis(imidazole)nickel(II) bis(4-nitrobenzoate) dihydrate Bikshandarkoil R Srinivasan a, *, Jyoti V Sawant a & Pallepogu Raghavaiah b a Department of Chemistry, Goa University, Goa 403 206 India Email: srini@unigoa.ac.in b School of Chemistry, University of Hyderabad, Hyderabad 500 046, India Received 21 May 2008; revised and accepted 22 December 2008 The reaction of [Ni(H 2 O) 6 ]Cl 2 with the sodium salt of 4-nitrobenzoic acid (4-nbaH) in the presence of imidazole results in the formation of the title compound hexakis(imidazole)nickel(II) bis(4-nitrobenzoate) dihydrate (1). Compound (1) is dehydrated to hexakis(imidazole)nickel(II) bis(4-nitrobenzoate) (2) by heating at 100 o C. Both compounds are characterized by elemental analysis, infrared spectra, X-ray powder pattern and thermal studies. The title compound [Ni(Im) 6 ](4-nba) 2 .2H 2 O (1) crystallizes in the centrosymmetric triclinic space group Pī with the Ni(II) situated on an inversion center. The structure of (1) consists of a distorted octahedral hexakis(imidazole)nickel(II) cation, a free uncoordinated 4-nba anion and a lattice water with half of the molecule accounting for the asymmetric unit. In the complex cation the central metal is bonded to six neutral terminal Im ligands. The distortion of the {NiN 6 }octahedron in (1) is discussed in terms of the difference between the longest and shortest Ni-N bonds. A comparative study of several [Ni(Im) 6 ] 2+ compounds in different structural environments is described. In the crystal structure, the cation, anion and lattice water are linked by three types of H-bonding interactions comprising two O-H···O, three N-H···O and three C-H···O interactions. Each hexacoordinated Ni(II) complex cation is linked to eight symmetry related 4-nba anions and four different lattice water molecules via N-H···O and C-H···O interactions, while each 4-nba anion is H-bonded to four complex cations and two symmetry related lattice water molecules. Pairs of [Ni(Im) 6 ] 2+ cations and 4-nba anions are linked to lattice water molecules via O-H···O and C-H···O interactions. As a result of the hydrogen bonding interactions, the cations and anions are organized into alternating layers. Keywords: Coordination chemistry, Hydrogen bonding interactions; Crystal structure, Imidazoles, Nitrobenzoates, Nickel IPC Code: Int. Cl. 8 : C07F15/04 Compounds containing imidazole and carboxylate are of considerable interest in view of their biological and medicinal applications 1-3 . Imidazole (Im) can function as a neutral monodentate, bridging bidentate or in certain cases as an anionic imidazolate ligand 4 . The octahedral [M(Im) 6 ] 2+ unit (M=bivalent metal) has been structurally characterized for several metals 5-8 . The availability of Im in very pure form, its solubility in a variety of solvents, its ability to function as a ligand for metals, the importance of metal-imidazole compounds in biological applications etc. are some reasons for the study of metal-imidazole compounds. An extensive chemistry of metal-carboxylates has been developed demonstrating the versatile ligational behavior of the carboxylate anion towards metal ions 9-11 . The study of metal-carboxylates and mixed ligand metal carboxylates continues to attract the attention of several research groups. A few years ago we initiated a study of metal carboxylates 12-13 and as part of this research we are currently investigating the synthesis, reactivity characteristics, and structural aspects of nitrobenzoate compounds of transition and s-block metals 14-22 . We report herein the synthesis and characterization of a new Ni(II) compound, namely, hexakis (imidazole) nickel(II) bis(4-nitrobenzoate) dihydrate (1), with a distorted {NiN 6 } octahedron. Materials and Methods All the chemicals were of reagent grade and were used as received. The starting materials and reaction products are air stable and hence were prepared under normal laboratory conditions. MIR spectra of both compounds were recorded in a KBr matrix on a Shimadzu (IR Prestige-21) FT-IR spectrometer while far IR spectrum of (1) was measured on a Bruker IFS-66 infrared spectrometer in a pressed polyethylene disc. TG-DTA measurements were