Reactions of Cr(V) with phosphate and pyrophosphate were examined as models for ligand exchange binding to DNA and deoxyribonucleotides. The initial Cr(V) complex, [Cr(ehba)(2)O](-), can undergo ligand exchange with either one pyrophosphate, to form [Cr(ehba)(H2P2O7)O](-), or two pyrophosphates, to form [Cr(H2P2O7)(2)O](-), with a concomitant geometry change from trigonal bipyramidal to square pyramidal. Reaction with free phosphate produces two less stable monodentate Cr(V)-phosphate complexes which are assigned as meridional and facial isomers of cis-[Cr(ehba)(2)(H2PO4)O](2-).