Chelate complexation of 4,4,5,5-tetramethyl-2-(6-methyl-2-pyridyl)imidazolin-1-oxyl 3-oxide (NITmepy) and 4,4,5,5-tetramethyl-2-(6-methyl-2-pyridyl)imidazolin-1-oxyl (IMmepy) to cobalt(II) chloride (bromide) or zinc(II) chloride afforded tetrahedral four-co-ordinate (T-4) complexes, [MX₂(NITmepy)] and [MX₂(IMmepy)] (MX = CoCl (1 and 2), CoBr (1Br and 2Br) or ZnCl (3 and 4), respectively) containing either a six- or a five-membered chelate ring as determined by single-crystal X-ray analysis. The formation of such T-4 complexes arises from the steric requirements of the methyl group at the o-position of the pyridyl-N donor. The UV-vis-NIR spectra suggest that both complexes 1 and 2 in dichloromethane retain their solid state structure. In acetonitrile, however, 1 exists as an equilibrium mixture due to dissociation of the co-ordinated NITmepy, while 2 is stable. The ligand-field spin-allowed and spin-forbidden d–d transitions of 1 and 2 are not influenced by the coordinated radical ligands. The reaction of nickel(II) chloride and thiocyanate with IMmepy gave a dichloro-bridged dinuclear complex with a square pyramidal five-co-ordinate (SPR-5) geometry and an OC-6 complex, respectively, [{NiCl(IMmepy)}₂(μ-Cl)₂] (5) and [Ni(NCS)₂(IMmepy)₂] (6). The intramolecular magnetic interactions in 1 and 2 are estimated to be antiferromagnetic, whereas 5 and 6 give ferromagnetic interactions. Complex 3, with one-dimensional columns, forms a dimer so that a moderate intermolecular antiferromagnetic coupling is demonstrated, while complex 4 exists as a discrete molecule in the crystal and shows paramagnetic behaviour.