The molecular structures of WF6, ReF6, OsF6, IrF6, and PtF6 have been measured by electron diffraction from the gases, the last from both PtF6 itself and from a vapor assumed to consist of a mixture of O2 and PtF6 obtained by heating the salt O2PtF6. For models of Oh symmetry the bond lengths in the first three members of the series are essentially identical, but the Ir-F and Pt-F bonds are respectively about 0.01 and 0.02 A longer. Models of D4h symmetry were also tested for ReF6, OsF6, and IrF6 in which operation of the Jahn-Teller effect is thought possible. For these models the same trend was seen in the average bond-length values. The effect of three-atom multiple scattering was also investigated, and experimental estimates of the effects of vibrational averaging ("shrinkage") on the distances were obtained. Normal-coordinate analyses based on the observed wavenumbers yielded stretching force constants consistent with the usual inverse bond-length/force-constant relationship. Ab initio molecular orbital optimizations of the molecules constrained to Oh symmetry were carried out at several levels of theory and basis-set size. Less extensive optimizations of ReF6, OsF6, and IrF6 with D4h symmetry were also carried out. The best overall agreement with both the experimental values and the distance trend for Oh symmetry was obtained with the Hay-Wadt (n+1)VDZ basis on the metals and the aug-cc-pVTZ on the fluorines at the MP2 level, but these bases with B3P86 and B3PW91 density functional theory were nearly as good and with B3LYP only slightly worse. The D4h structures for ReF6, OsF6, and IrF6 with the cited bases at the B3P86 level were slightly more stable (respectively 0.8, 2.6, and 1.4 kcal/mol) with the axial bonds shorter by about 0.04 A in ReF6 and 0.07 A in OsF6, but about 0.05 A longer in IrF6. The significance of these values is uncertain. The experimental bond lengths (rg/A) with estimated 2sigma uncertainties for the models of Oh symmetry are W-F = 1.829(2), Re-F = 1.829(2), Os-F = 1.828(2), Ir-F = 1.839(2), and Pt-F = 1.852(2); the Pt-F value from the O2PtF6 sample was 1.851(2) A. Although the experimental data neither confirm nor refute the existence of the Jahn-Teller effect in ReF6, OsF6, and IrF6, they ensure that if present the distortion from Oh symmetry must be small.

Me3SnONMe2 has been prepared from trimethyltin chloride and O-lithio-N,N-dimethylhydroxylamine and characterized by IR and NMR spectroscopy, mass spectrometry, and elemental analysis. Its gas-phase molecular structure has been determined by electron diffraction augmented by restraints taken from ab initio calculations at the MP2/DZ(P) level of theory. A secondary interaction (Sn···N, 2.731(14) Å) between the tin and nitrogen atoms has been detected and makes Me3SnONMe2 the first partially hypercoordinate tin compound studied in the gas phase. A solid-state structure of Me3SnONMe2 has been determined by X-ray diffraction of an in situ grown single crystal. In the crystal lattice the coordination sphere of tin is further enlarged by a weak intermolecular Sn···O contact (2.998(10) Å) and a 4+2 coordination geometry is achieved, which can be deduced from that of a distorted trigonal bipyramid with one of the axial substituents replaced by the two weak contacts. Important geometry parameter values are as follows (gas/solid):  Sn−O 2.006(3)/2.063(6) Å, O−N 1.468(6)/1.440(9) Å, Sn−C(in plane) 2.148(4)/2.163(14) Å, Sn−C(out of plane) 2.146(3)/2.062(9) and 2.185(10) Å, Sn−O−N 102.5(8)/101.5(4)°, O−Sn−C(in plane) 99.6(10)/99.7(4)°, O−Sn−C(out of plane) 108.1(6)/119.6(4)° and 116.6(4)°.

Dedicated to Professor Jack D. Dunitz. K. H. has known Jack for about 55 of his 80 years beginning when we were both post-doctoral fellows at Caltech. Those were fun times!The bridged homotropilidines have been of interest for decades because their molecules offer the potential for homoaromaticity. Although many of these have been shown not to be homoaromatic, the energy differences of the delocalized (homoaromatic) forms and the localized (nonhomoaromatic) ones, and the barriers to the interconversion of the localized forms via a Cope rearrangement, have been found to vary greatly. The title compound is a strong candidate for homoaromaticity, and, since the structures of the possible localized and delocalized forms could differ significantly, we have carried out an electron-diffraction investigation of it augmented by quantum-mechanical calculations with different basis sets at several levels of theory. Three models were explored: one representing a localized form of C s symmetry, one a delocalized form of C 2v symmetry, and one a 2 : 1 mixture of the localized/delocalized forms. Although none of the models could be ruled out, the experimental evidence slightly favors the C s form. These results are consistent with those from the DFT B3PW91 calculations with basis sets ranging from 6-31G(d) to cc-pVTZ, which, surprisingly, predict essentially equal thermally corrected free energies for each. The results are discussed.Introduction. ± The molecular structures of the bridged homotropilidines, of which bullvalene and semibullvalene (1) are important examples, have been of great interest for well over three decades. One reason for this interest is that the molecules might be homoaromatic [1] as a consequence of a −negative× barrier to a Cope rearrangement [2] as illustrated below for the skeleton of semibullvalene (1). The homoaromatic, or delocalized, form of the molecule, 1b, is expected to have C 2v symmetry, and the nonhomoaromatic, or localized, forms 1a and 1c should have C s symmetry. The most obvious structural differences between the homoaromatic and localized forms lie in the nearest-neighbor distances between atoms in the six-membered ring of 1b, and the corresponding atoms in 1a and 1c: in 1b the C 2 ÀC 3 , C 3 ÀC 4 , C 6 ÀC 7 , and C 7 ÀC 8 distances should be characteristic of C,C aromatic bonds (ca. 1.4 ä), while the C 2 ÀC 8 and C 4 ÀC 6 distances are expected to be elongated (ca. 2 ä), but in 1a (1c) their values as double, single, and nonbonds are anticipated [3].There is abundant experimental and theoretical evidence that the ground state of semibullvalene itself is not homoaromatic (for a summary of current information about