Numerous comparisons among various structural features in compounds containing 4-and 5-membered phosphorus rings and 5-and 6-membered arsenic rings are made. The molecular structure of (PCF3) 4 indicates that the torsion angle about P-P bonds tends to be different from 0 °, a conclusion confirmed by the results on the other molecules. The observed conformations of (PCFa) 5 and (AsCHa) ~ are very similar, and apparently result mainly from intra-, rather than inter-, molecular forces. The stable form of these five-membered rings is intermediate between symmetry m and 2. The P-C-F bond angles in (PCF3) 4 and (PCFz) 5 are explainable in terms of repulsion between non-bonded atoms. The stable orientation about P-C single bonds is the staggered one. The observed irregularities in the torsion angles about the As-C bonds in the (AsC6Hs) G molecule may be ascribed to efficient molecular packing in the crystal. Some interesting but as yet unexplained differences exist among bond distances and bond angles in arsenic ring compounds.

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