Magnetic and structural studies on some d3 transitional element halide complexes.

In this thesis the results of a study of the magnetic interactions in series of polynuclear complexes of chromium(III), molybdenum(III), and vanadium(II) are reported. These comprise series of compounds of formula A31V12X9 (M = Cr(III), Mo(III); X = Cl, Br; A = univalent cation) containing binuclear [M2X9]3- entities and the compounds AVX3 (X = Cl, Br) containing infinite linear chains (-VX3-)1-n . Particular emphasis has been placed on the relation-ship between magnetic properties and the structure of the compounds. In this connection a survey has been made of previous investigations of magnetic interactions in poly-nuclear transition element complexes. It is evident that there is the need for a study of the variation of magnetic interactions in series of simple polynuclear complexes of closely related structures. From single crystal and powder x-ray diffraction studies carried out on the A3M2X9 system it has been determined that change of the size of cation A results in a change in the metal-metal separation in the binuclear anion._ From a combination of the x-ray structural data and magnetic susceptibility measurements the variation of magnetic exchange with change of metal-metal distance has been determined. Detailed correlations have also been made between the magnetic properties and the proposed structures for the vanadium complexes. The magnetic data on the compounds of formula AVX3 is consistent with a structural model which involves pairing of vanadium atoms along an infinite chain. For all the compounds studied it has been possible to obtain useful information on the contributions of the different mechanisms for exchange by a comparison of the magnetic properties of related chloride and bromide complexes. It is concluded that in the molybdenum binuclear compounds, the dominant mechanism for spin-spin coupling is direct exchange between the molybdenum atoms. The various superexchange effects via the bridging halogens contribute no more than 10% of the total exchange. The relative contributions of the ferro- and antiferromagnetic superexchange mechanisms are very dependent on the structure of the binuclear anion. The magnetic inter-actions in the corresponding chromium complexes are weaker than in their molybdenum analogues by a factor of fifty. The magnetic properties of the vanadium(II) compounds are similar to those of the molybdenum binuclear complexes. In the former compounds, however, the superexchange via the bridging ligands forms a greater contribution to the total exchange. This is attributed to the shorter metal-ligand bonds in the AVX3 compounds.