Department of Chemistry, University of Calcutta, University College of Science, 92 Acharya Prafulla Chandra Road Kolkata-700 009, India E-mail : gurunath@cucc.ernet.in Fax : 91-033-351-9755 Manuscript received 4 September 2000, revised 23 February 2001, accepted 12 May 2001 Equilibrium study on the mixed ligand complex formation of CoII, NiII and ZnII, hereafter M(II), with boric acid and some (N,N) bidentate ligands, viz. 2,2'-bipyridine 1,10-phenanthroline and ethylenediamine provides evidence of formation of binary M(N,N)2+, M(H2BO4)- and ternary mononuclear M(N,N)(H2BO4)- and binuclear M2(N,N)2(BO4)- complexes in solution. Formation constants of the complexes at 25 ± 0.1° have been determined in aqueous solution at a fixed ionic strength. I = 0.1 M (NaNO3) and correlated with electronic structure, stereochemistry of metal ions and metal-ligand n-bonding. Complex formation equilibria have been elucidated with the aid of specification curves.