169 BIBLIOGRAPHY 174 ACKNOWLEDGEMENTS 179 APPENDIX 180 lattice two-thirds of the A + ions and the MX5 ions occupy the positions of the K + and PtClg" ions in the anti-fluorite structure. In addition there are A* ions at the mid-points of the edges and the body-center of the unit cell. Again, slight distortions from ideal cubic symmetry are common. Since this dissertation is concerned with the prepara tion and properties of 4d^ and 5d^ ions, the following discussion will be primarily devoted to

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... exahalo complexes with this type of electronic configuration. In addition, 19 9 certain 3d , 4d , and 5d^ complexes will be mentioned. Niobium(IV) and tantalum(IV) hexahalo complexes At the present time no pure hexahalo complexes of niobium(IV) or tantalum(IV) have been isolated. The existence of A^NbOl^ (A = Na, K, Rb, Cs) compounds has been demon strated by Korshunov, est al. (21, 22) from phase equilibria studies of niobium(IV) chloride-alkali metal chloride systems. These studies were carried out in sealed, argon filled, quartz vessels. The AgNbCl^ salts were found to be congruently melting compounds with high melting points. Unlike the other AgNbCl^ compounds containing the larger alkali metal cations, Na^NbCl^ shows polymorphism. The a modification of this compound was found to transform into electronic spectrum of this dark green compound consisted of four peaks in the ultraviolet at 243, 275, 305, and 354 mM, which were assigned to charge transfer transitions. A single peak at 415 mM, in the visible region was assigned to the d-d transition ^2g ^Eg ^o r a cubic d^ ion. Some asymmetry in the band at 415 mM, centered roughly around 470 mM, was postulated to be evidence of Jahn-Teller distortion in MoClg. Since the spectrum of this compound was obtained from a pellet of KM0CI6 in KCl, extinction coefficients for the peaks could not be calculated. The magnetic moment of KMoClfc was found to be 1.68 B.M. at room temperature = Molybdenum(IV) and tungsten(IV) hexahalo complexes Peacock, et al. (4, 11) also have investigated the hexa halo complexes of molybdenum(IV) and tungsten(IV). The molybdenum compounds A^MoCl^ (A = K, Rb, Cs, Tl) were pre pared by reacting M0CI5 with AC1 in the presence of ICI while the AgMoBr^ compounds (A = Rb, Cs) were prepared by the oxi dation of MoBrg with IBr in the presence of ABr. The analogous tungsten(IV) hexahalo complexes AgWX^ (A = K, Rb, Cs; X = CI, Br) were prepared by reacting WXfc