The electronic structure of some idealized PH5 geometries of D3h, C4" and C, symmetries is analyzed. Each geometry is characterized by a low-lying nodeless orbital, three singly noded orbitals close in energy, and a high-lying doubly noded nonbonding orbital. The latter orbital is the only one significantly stabilized by the inclusion of 3d orbitals in the P basis set and also determines the relative stability of substituted compounds differing in electronegativity from . A potential surface connecting the D3h and C4" geometries through C2" structures is constructed. It shows a small barrier for the Berry pseudorotation process. Optimum C, structures are at higher energy than the C4" geometry. An examination of substituent effects rationalizes favored apical substitution in the trigonal bipyramid and preferred basal substitution in the square pyramid by more electronegative groups. It is predicted that acceptors will prefer axial sites in the trigonal bipyramid, donors equatorial positions. If a substituent has a single system and is located in the equatorial position it will prefer to have its acceptor orbital perpendicular to the equatorial plane or its donor orbital in that plane. In the square pyramid, t donors will favor the apical position, acceptors the basal sites. The concerted fragmentation reaction PR5 PR3 + R2 is symmetry forbidden for the least-motion axial-equatorial departure from a trigonal bipyramid, and allowed for axial-axial or equatorial-equatorial departure.The literature is replete with theories of bonding of pentacoordinate phosphorus.1-31 We add here a molecular orbital description which is (1) simple,

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Ab initio self-consistent field calculations are performed on N02-(H20), and NO;(H20), (n = 1-3) clusters. Optimized geometries, t o t a l energies, binding energies, and net atomic charges are presented for various structures of the clusters.NMR has been employed to measure the orientational order parameters for the nematogenic homologous series trans-4-alkyl(4-cyanophenyl)cyclohexanes by studying the dipole-dipole splittings. The values of the order parameters are compared with those obtained by optical studies.

Ab initio calculations are used to confirm the geometries found experimentally by Klemperer et al. for the CO2–HF and N2O–HF complexes.