4-Methylpyridinium nonabromoantimonate(V), (C6H7NH)2SbvBrg, crystallizes as deep red acicular crystals in the monoclinic space group C2/m (C2h3) with unit cell parameters a = 18.39 (1), b = 7.440 (4), c = 9.84 (1) A, and ß = 113.14 (6)°. The observed and calculated densities are 2.64 and 2,760 (7) g/cm3, respectively. The structure was refined by leastsquares methods to a conventional discrepancy index of 0.050 using three-dimensional X-ray diffraction counter data. The structure is comprised of 4-methylpyridinium, SbvBr6", and Br3~ions, the packing of which is similar to that in 2-methylpyridinium nonabromoantimonate(V). The SbBr6" ion possesses crystallographic C2y¡ symmetry and has an average bond length of 2.563 (4) A. The tribromide ion is centrosymmetric with a bond length of 2.561 (4) A. These two ions form nearly linear chains displaying bromine • • • bromine contacts of 3.444 (4) A. The shortest hydrogen • • • bromine separation, 2.9 (1) A, involves the V-H proton of the cation with the SbBr6" anion.to the additional presence of intervalent electron-transfer absorption bands. Those having composition (RH+)2 -(SbvBr6~)(Br3~), however, become variably colored at -196°( the temperature of liquid nitrogen10). Upon cooling, their intense colors disappear, first changing from black to violet (reference here being to reflected light11), then progressively to red, red-orange, and orange, depending upon RH+ and hence structure. This progression of color change is consistent with a sharpening of absorption bands in the near-ultraviolet region whose edges overlap the blue end of the visible spectrum. As is shown by Table I for a series of these salts, the higher the temperature at which the color change starts to occur for a given material the more orange it becomes at -196°. The different colors at any given temperature depend primarily on how far into the visible region the absorption bands extend before tailing off. This has been demonstrated for the 2-methylpyridinium and quinolinium salts by diffuse reflectance spectroscopy;12 spectra of these (1) Research carried out during the spring of 1971 by special permission of Mobil Research and Development Corp.(2) Mobil Research and Development Corp.(3) (a) Senior,

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