Synthetic approaches to cobalt(iii) complexes [Co(L)(Lâ ²)2] containing the bidentate dialkylating nitrogen mustard N,N-bis(2-chloroethyl)-1,2-ethanediamine (L = dce) together with anionic ancilliary ligands (Lâ ²) which are either carbonato (CO3^2â), oxalato (ox^2â), bis(2-hydroxyethyl)dithiocarbamato (bhedtc^â), 2-pyridine carboxylato (pico^â) or 2-pyrazine carboxylato (pyzc^â) were investigated. Synthetic routes were developed using the related amines N,N-diethyl-1,2-ethanediamine (dee) and 1,2-ethanediamine (en). The complexes [Co(CO3)2(L)]^â (L = dee 1, dce 2), [Co(ox)2(L)]^â (L = dee 3, dce 4), [Co(bhedtc)2(dee)]⁺ 5, [Co(bhedtc)2(en)]⁺ 6, mer-[Co(pico)3], mer-[Co(pyzc)]3 7 and [Co(pico)2(dee)]⁺ 8 were prepared and were characterised by IR, UV-Vis, ¹H and ¹³C{¹H} NMR spectroscopy, mass spectrometry and cyclic voltammetry. [Co(bhedtc)2(en)]BPh4 6b and trans(O)-[Co(pico)2(dee)]ClO4 8 were characterised by X-ray crystallography. In vitro biological tests were carried out on complexes 1â 4 in order to assess the degree to which coordination of the mustard to cobalt attenuated its cytotoxicity, and the differential toxicity in air vs. nitrogen.