With axial halogen

Both tris(pentafluorophenyl)arsenic difluorideand tri(neo-pentyl)arsenic dibrom-ide follow the usual pattern for five-coordinate arsenic compounds with unidentate ligands and have trigonal bipyramidal structures with axial halogen atoms. For the former As—F and As—C distances are, respectively, 1.782 and 1.915 A in the latter, with sterically demanding alkyl groups, the As—C separation is increased to 1.98 A and there is asymmetry in the bromine positions (As—Br 2.530 and 2.597 A). 

The chloro- and bromo-cyclohexane inclusion compounds have been extensively examined by infrared (4000-30 cm-1) spectroscopy65-68 and by Raman (< 1000 cm-1) spectroscopy 69). In the canals both guests are found to exist exclusively in the chair conformation with an axial halogen substituent, while iodocyclohexane 65, 68,69) acj0pts both axial and equatorial conformations in the canal. These results should be contrasted with the familiar situation in the liquid phase where the equatorial arrangement is the lowest energy conformer and is present to the extent of about 65-70% at room temperature. 

NMR has been used to show that the anomeric effect operates in 2-bromo- and 2-chloro-tetrahydropyrans, which exist predominantly with the halogen atom axial (66JOC544), whilst the equatorial preference of a 4-halogen atom is less pronounced than in cyclohexane and decreases with decreasing electronegativity (78SA(A)297). 

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