High-and low-spin cobalt( ti) triazene 1 -oxide bis(chelates) have been isolated. The low-spin complexes possess square-planar structure whilst the high-spin complexes are tetrahedral. The_molecular structure of high-spin [Co(OMeN,C,H,Me-4),] has been determined: triclinic, space group P1, Z = 2, a = 7.970(5), b = 10.1 74(5), c = 11.676(5) A, a = 87.1 8(4), p = 74.31 (4) and y = 74.06(4)". For some complexes the isolation of both planar (low-spin) and tetrahedral (high-spin) isomers or of their conglomerates is possible depending on the synthesis conditions. The crystal structure of square-planar [ Ni(OMeN,C,H,-Me-4),], which is isomorphous wiLh the low-spin isomer of [Co(OMeN3C,H,Me-4),], has been determined: triclinic, space group P1, 2 = 1, with a = 7.495(2), b = 7.694(5), c = 8.61 2(3) A, a = 64.64(5), p = 87.84(2) and y = 78.64(3)". The complexes exhibit a planar-tetrahedral equilibrium in non-co-ordinating solvents, the AH * and AS* values of which, determined from solution magnetic susceptibility measurements, are in the range 1-15 kJ mol-' and 5-30 J K-l moIV, respectively. The electrochemical properties of the complexes are given.The first report on metal complexes of triazene I-oxides appeared as early as in 1938.' Since that time some interest has centred around the formation and properties of these compounds.' The synthesis of the ligands is quite straightforward and involves coupling the diazotised aromatic amines and Nalkylor aryl-hydroxylamines, the latter prepared by reduction of the corresponding nitro compounds. A host of differently substituted starting materials is available. Hence it is possible to introduce into a ligand system a number of substituents of different size and polar effects. In a previous communication 2 j we reported the formation of both high-and low-spin cobalt(I1) complexes, the spin state being dependent on the ligand substituents. The structure of low-spin [Co(OEtN,Ph),] was shown to be square-planar centrosymmetrk2j The formation of low-spin bis(che1ate) cobalt(r1) systems with an N 2 0 2 chromophore indicates rather strong o-donor properties of triazen-1 -olato anions. We also pointed out the possibility of conversion of some of the high-spin complexes into low-spin isomers upon dissolution in non-co-ordinating solvents or by recystallisation from dimethylformamide (dmf)-water.This paper describes the isolation, magnetic, electrochemical and spectroscopic properties of cobalt@) chelates of systematically substituted triazene I-oxides. The molecular structure of high-spin, tetrahedral [Co(OMeN,C6H,Me-4),] has been determined, The formation of high-and low-spin complexes and their interconversions have been found to be due to tetrahedralplanar isomerism. This phenomenon is well documented for four-co-ordinate nickel(I1) bis(che1ates) but is very scarce for analogous cobalt(r1) systems4 Experimental Details of the magnetic, spectroscopic and electrochemical instrumentation are given in refs. 2(k) and 2(n). Magnetic susceptibility measurements in CDCl, and C,D,CD, solutions...