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Nickel(II) complexes of N,N′-dimethyl-N,N′-bis(pyridyl-2yl-methyl)ethylene-diamine (L1), N,N′-dimethyl-N,N′-bis(pyridyl-2-ylmethyl)-1,2-diaminopropane (L2) and N,N′-dimethyl-N,N′-bis(pyridyl-2-ylmethyl)-1,3-diaminopropane (L3) were prepared and their spectroscopic and redox properties studied. The distorted octahedral structure was determined for [NiL3ClCH3OH](ClO4) by using X-ray crystallography. The electronic spectral behavior of the complexes at different pHs was analyzed; it is shown that a new band grew at the expense of the other band intensity in acid media. The redox properties of ligands and their complexes show the peaks of Ni(II) → Ni(III) and Ni(II) → Ni(0) as these were detected at low concentration while Ni(II) → Ni(I) process was detectable clearly at high concentration. Furthermore, the interaction studies of 2-mercaptoethanesulfonic acid as a simulator of coenzyme M reductase (CoM) with NiN4 chromophores are discussed.

1:1 and 1:2 cobalt complexes of bis(benzimidazol-2-ylmethyl)amine (bbma) bis(benzimidazol-2-ylmethyl)sulfide (bbms), bis(benzimidazol-2-ylethyl)sulfide (bbes) and diethylenetriamine (dien) were prepared and their spectral and redox behavior studied. Two geometrical isomers pink-[Co(bbes) 2 ] 2+ and blue-[Co(bbes) 2 ] 2+ were obtained when the complexes were prepared by using with bbes and they were separated manually and recrystallized. The octahedral structure of pink-[Co(bbes) 2 ] 2+ was resolved by X-ray analysis. The electronic spectra show the presence of two geometrical isomers for Co(bbes) 2 2+ in the solid state; for example, the spectral bands of pink-[Co(bbes) 2 ] 2+ differs markedly with those of blue-[Co(bbes) 2 ] 2+ . This is consistent with the results obtained from magnetic measurements (5.10 BM for pink-Co(bbes) 2 2+ and 4.72 BM for blue-[Co(bbes) 2 ] 2+ ). Further, the behavior of the ligands (bbma, bbms, bbes) at different pH conditions was determined on the basis of 13 C NMR studies. The redox potentials [Co(II)/Co(I)] of the complexes follow the trend [Co(bbma) 2 ] 2+ 2 ] 2+ ≈ [Co(bbes) 2 ] 2+ which demonstrates the stabilization of the Co(II) ion is more by both weak σ-donor and weak π-acceptor ligands rather than by σ-donor ligand.

Nickel(II)complexes of N,N,N′N′-tetrakis(2-pyridylmethyl)ethylenediamine (L1), N,N,N′N′-tetrakis(2-pyridylmethyl)-1,2-diaminopropane (L2), N,N,N′N′-tetrakis(2-pyridylmethyl)-1,3-diaminopropane (L3), N,N′-bis(2-pyridylmethyl)ethylenediamine (L4), N,N′-bis(2-pyridylmethyl)-1,2-diaminopropane (L5) and N,N′-bis(2-pyridylmethyl)-1,3-diaminopropane (L6) were prepared and their spectroscopic and redox behaviors studied. The distorted octahedral geometry was determined for [NiL2](ClO4)2 by using X-ray crystallography. The electronic spectral bands (3A2g→3T2g and 3A2g→1Eg transitions) of NiN6 and NiN4O2 were analyzed; it shows that the former complexes possess well-resolved bands compared to the latter compounds. In addition, the ligand field parameter Dq found for NiN4O2 complexes was much lower than that of NiN6 compounds; thereby, a more positive redox potential was detected for NiN4O2 complexes in electrochemistry over the NiN6 compounds. Further, the coordination studies of 2-mercaptoethanesulfonic acid as a simulator of coenzyme M reductase (CoM) with NiN4 or NiN6 chromophores were discussed.

The Cu(ClO4)2 complexes of the two pentadentate ligands, N-(2-hydroxyethyl)-N,N′,N′-tris(benzimidazol-2′-ylmethyl)-1,2- ethanediamine and its 6′-methyl derivative and four sexadentate ligands, N,N,N′,N′-tetrakis(benzimidazol-2′-ylmethyl)-1,2- ethanediamine, N,N,N′,N′-tetrakis(benzimidazol-2′-ylmethyl)-1,2-cyclohexanediamine and their 6′-methyl derivatives have been isolated and studied. They are of the type CuL(ClO4)2·nH2O where n = 0, 1. All the complexes exhibit two ligand field bands and the ν max and ϵ values are lower and higher, respectively, than the range known for CuN4 chromophores. Compared to the latter they also have high g∥, low A∥ and high g∥/A∥ values. These spectral data are consistent with an appreciable displacement and/or tetrahedral twist of copper from the N4 square plane of the complexes. The anchoring of a bulky benzimidazole moiety to N,N′-bis(benzimidazol-2-ylmethyl)-1,2-ethanediamine to incorporate a N-donor axial to the planar CuN4 chromophore decreases CFSE and enhances the CuIICuI redox potential. However, addition of two benzimidazole nitrogens to the N4 ligand to obtain a CuN6 chromophore, depresses the redox potential. Further, the fusion of a cyclohexyl ring into the above CuN6 chromophore dramatically leads to elevation in E 1 2 with increase in reversibility. The introduction of a methyl group in the aromatic ring, far away from the redox centre, depresses E 1 2 .