Dimerization of lawsone occurs upon reaction with Co(BF(4))(2)·6H(2)O and N,N'-bis(pyridin-2-ylmethyl)ethylenediamine (py(2)en) to produce the mononuclear complex [Co(III)(bhnq)(py(2)en)]BF(4)·H(2)O (1). This complex has been investigated as a prototype of a bioreductive prodrug, where the bhnq(2-) ligand acts as a model for cytotoxic naphthoquinones. Cyclic voltammetry data in aqueous solution have shown a quasi-reversible Co(III)/Co(II) process at E(1/2) = -0.26 V vs Fc/Fc(+). Reactivity studies revealed the dissociation of bhnq(2-) from the complex upon reduction of 1 with ascorbic acid, and a dependence of the reaction rate on the oxygen concentration suggests the occurrence of redox cycling.