The molecular structures of the stable phosphinyl and arsinyl radicals, .PnR(2) [Pn = P (2); As (4); R = CH(SiMe(3))(2)], have been determined by gas-phase electron diffraction (GED) in conjunction with ab initio molecular orbital calculations. The X-ray crystal structures of the corresponding dipnictines, the "dimers", R(2)PnPnR(2) [Pn = P (1), As (3)], and the chloro derivatives R(2)PnCl [Pn = P (5), As (6)] have also been determined. Collectively, these structural investigations demonstrate that large distortions of the ligands attached to Pn occur when the pnictinyl radicals unite to form the corresponding dipnictine dimers. Principally, it is the shape and flexibility of the CH(SiMe(3))(2) ligands that permit the formation of the P-P and As-As bonds in 1and 3, respectively. However, theoretical studies indicate that in the process of pnictinyl radical dimerization to form 1 and 3, both molecules accumulate substantial amounts of potential energy and are thus primed to spring apart upon release from the solid state by melting, dissolution, or evaporation. The insights gleaned from these unusual systems have permitted a deeper understanding of the functioning of sterically demanding substituents.

A gaseous electron-diffraction investigation of SOF4 has led to the discovery of four models in excellent agreement with experiment, all with molecular symmetry C2υ corresponding to replacement of an equatorial fluorine atom of a trigonal bipyramid with oxygen. The models differ largely in the relative magnitudes of the F(eq)···F(eq) and F(eq)···O distances and the F(ax)···F(eq) and F(ax)···O distances. The favored model has the following distance (ra), angle, and root-mean-square amplitude (la) values (parenthesized errors are 2σ): S=O=1.403 Å (0.0032), S–F(eq) = 1.552 Å (0.0043), S–F(ax) = 1.575 Å (0.0038), F(eq)···F(eq) = 2.545 Å (0.0259), F(ax)···O = 2.121 Å (0.0073), F(eq)···O = 2.621 Å (0.0139), F(ax)···F(eq) = 2.204 Å (0.0050), F(ax)··· = 3.150 Å (0.0076), ∠F(eq)SF(eq) = 110.17°(1.82), ∠F(ax)SO = 90.65°(0.42), ∠F(ax)SF(eq) = 89.63°(0.24), ∠F(eq)SO = 124.91°(0.92), lS = O = 0.0367Å (0.0050), lS–F(eq) = lS–F(ax) = 0.0540Å (0.0029), lF(eq)...F(eq) = 0.0468Å (0.0124), lF(ax)...O = 0.0431Å (0.0055), lF(eq)...O = 0.0758Å (0.0192), lF(ax)...F(eq) = 0.0962Å (0.0074), and lF(ax)...F(ax) = 0.0611Å (0.0122).

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