The complex formation equilibria involved in binary systems of M(II)-Phen were investigated pH metrically at 303 K and an ionic strength of 0.16 mol L–1 (M = Cobalt (Co), Nickel (Ni) and Copper (Cu); Phen (1, 10-Phenanthroline)). The existence of ML, ML2, ML3 and their stability constants are refined with the computer program MINIQUAD75. The best fit chemical models were selected based on the statistical parameters like crystallographic R-factor and sum of squares of residuals in massbalance equations. The formation and distribution of different species with relative concentrations (M:L = 1:2.5, 1:3.75 and 1:5.0) of metal and ligand with varying pH were represented in the form of distribution diagrams. The influence of cosolvent on speciation was discussed based on the dielectric constant of the medium.

In biological systems, many important roles, such as the catalytic interaction of drugs with biomolecules, uptake of ions by living organisms, interaction with viruses and bacterial cell walls, and transport and storage of oxygen, are played by binary complexes of transition metals (Patel, 1982). In continuation of our research program, we have studied the binary and ternary complexes of biological significance (Rao and Ramakrishna, 2005; Rao and Sudarsan, 2006; Latha et al., 2007; Lavanya et al., 2008; and Srikanth et al., 2009).

The transition metal ions interact with the biomolecules like amino acids, carboxylic acids and proteins and exist as metal complexes in biological systems. Metals are involved in every aspect of biosynthesis, biodegradation, and assembly of macromolecular structure (Schubert, 1954). Cobalt (Co) is a central component of Vitamin B12. It is essential to many living organisms. Cobalt is directly coordinated into the protein structure in a few proteins (Kobayashi and Shimizu, 1999). Nickel (Ni) plays numerous roles in the biology of microorganisms and plants (Hausinger, 1987; and Sigel et al., 2007). Many enzymes contain Ni as their central metal ion (Thornalley, 2003; Szilagyi et al., 2004; and Jaouen, 2006). Copper (Cu) is essential in all the plants and animals. It is a central metal ion of hemocyanin which is the oxygen carrier in most of the molluscs and some arthropods such as horseshoe crab (blue blooded animals). Cu is found in a variety of enzymes including the Cu centers of cytochrome C oxidase and superoxide dismutase.

1, 10-Phenanthroline (Phen) is a bidentate ligand which can bind through the Nitrogen (N) donor atoms. It is a good chelating agent and its metal chelates have biological activity in varied microorganisms. The bacteriostatic effects of Phenor Phen in the presence of metal ions have been demonstrated on rumen bacteria, acid fast bacteria and a number of gram-positive microorganisms (Joshi et al., 1973). Propylene Glycol (PG) is a nontoxic anti-freezing agent. It is used in food, medicines and cosmetics, often as a binding agent.

The present investigation deals with the determination of formation constants of binary complexes of Co(II), Ni(II) and Cu(II) with Phen.

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