The isomorphism of Sb2Se 3 with Sb2S 3 has been confirmed through a re-determination and refinement of the structure of Sb2Se 3. Antimony selenide is orthorhombic, Pbnm, with a---11.62:t:0.01, b ~-11.77~0.01, c = 3"962:t:0"007/k and z ----4. The 'bonded' Sb-Se distances range from 2.576/~ to 2.777 A, while the 'non-bonded' Sb-Se separations start at 2-98 A. The Se-Sb-Se angles range from 86.6 ° to 96"0 ° and the Sb-Se-Sb angles from 91.0 ° to 98-9 °. The structure consists of chain.s parallel to the c or needle axis. The strongest bonds (shortest separations) are within the chains; however, there are strong interactions between the chains. All important features of the structure reported by Hofmann for Sb2S 3 (stibnite) have been confirmed.

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The dissociation constants of dimethylsulfur diiodide, (CH3)2SI2, and dimethylselenium, diiodide, (CH3)2SeI2, have been determined spectrophotometrically in carbon tetrachloride solution. The values found are K = [(CH3)2S] [Í2]/[(CH3)2-SI2] = 1.40 X 10-8 ± 0.04 X 10~8 at 23 ± 1°and K = [(CH3)2Se][I2]/[(CH3)2SeI2] = 2.12 X 10~1 23 ± 0.02 X 10"3 at 25 ± 1°. Dimethylselenium dibromide was found to have a dissociation constant of less than 4 X 10 7 at 25°. Determination of the dissociation constant of dimethylsulfur dibromide was prevented by bromination of the methyl group with formation of HBr.

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