Department of Chemistry. Loyola College (Autonomous), Chennai-600 034, India E-mail : adsgp@ loyolacollege.edu Department of Physical Chemistry, University of Madras, Chennai-600 025, India Manuscript received 23 March 2004, accepted 30 August 2004 The kinetics of chromium(ll) reduction of complexes of the type trans-[Co(dmgH)2(L)X], where dmgH- = dimethylglyoximate; L = pyridine (py), nicotinamide (nic-CONH2) or isonicotinamide (isonic-CONH2 ), and X- = Cl-, Br- or I-, was studied in 10% (v/v)

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... CH3OH-H2O at 27 ± 0.1ºC and I= 0.25 mol dm-3 (LiClO4) in the [H+] range 0.0001-0.100 mol dm-3 under pseudo first-order conditions using an excess of the reductant. The inverse dependence of rate on [H+] suggests an equilibrium between the protonated and unprotonated forms of the complex, the protonatcd form reacting slower than the unprotonated form. The rate constants are in the order Cl- < Br- < I- and py ≈ nic-CONH2 ≈ isonic-CONH2 for the axial ligands. These observations suggest an inner-sphere mechanism for the reduction of the complexes with bridging by CrII at the oxime-oxygen. Product analysis, done by the cation-H+ exchange method further supports this conclusion.