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...

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Novel tripodal ligand 1,1',1''-tris(tetrazol-1-ylmethyl)methane (111tz) and products of its reactions with perchlorate as well as with tetrafluoroborate salts of iron(II) are presented. The isostructural complexes, [Fe(111tz)₂](ClO₄)₂ and [Fe(111tz)₂](BF₄)₂, were isolated as two-dimensional (2D) coordination networks revealing a honeycomb-like pattern with cages occupied by disordered anions. 111tz molecules act as a tridentate ligand bridging three adjacent Fe(II) ions, and the nitrogen N4 atom of six tetrazole rings (tz) is placed in octahedron vertices of FeN₆ chromophores. The complexes, crystallizing in the P3 space group, were characterized by variable-temperature single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. Variable-temperature magnetic susceptibility measurements show that both systems undergo abrupt and complete spin transition with T(1/2)(↑) = T(1/2)(↓) = 176 K for perchlorate and T(1/2)(↑) = 193.8 and T(1/2)(↓) = 192.8 K for the tetrafluoroborate analogue. Change of spin state in both complexes is accompanied by a thermochromic effect. The HS→LS transition in [Fe(111tz)₂](ClO₄)₂ involves shortening of the Fe-N4 bond lengths from 2.171(2) Å (293 K) to 2.002(1) Å (100 K). In [Fe(111tz)₂](BF₄)₂, lowering of temperature from 293 to 100 K is accompanied by shortening of the Fe-N4 distances from 2.179(2) to 1.987(2) Å, respectively. Perchlorate in [Fe(111tz)₂](ClO₄)₂ or tetrafluoroborate anions in [Fe(111tz)₂](BF₄)₂ are engaged in the formation of intermolecular contacts within as well as with the neighboring 2D layer.

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