Molecular Orbital Theory of Pentacoordinate Phosphorus

The electronic structure of some idealized PH5 geometries of D3h, C4v, 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 H. A potential surface connecting the Dah and C.,, geometries through CZv structures is constructed. It shows a small barrier for the Berry pseudorotation process. Optimum C, structures are at higher energy than the CdV 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 a acceptors will prefer axial sites in the trigonal bipyramid, T donors equatorial positions. If a substituent has a single a 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, a donors will favor the apical position, a acceptors the basal sites. The concerted fragmentation reaction PR5 Ft PR3 + RS is symmetry forbidden for the least-motion axial-equatorial departure from a trigonal bipyramid, and allowed for axial-axial or equatorial-equatorial departure. he literature is replete with theories of bonding of T pentacoordinate phosphorus. 1-3 We add here a molecular orbital description which is (1) simple, (1) I. Langmuir, J . Amer. Chem. Soc., 41, 868 (1919). (2) F. J. Garrick, Phil. Mag., 14, 914 (1932). (3) G. E. Kimball, J . Chem. Phys., 8, 188 (1940). (4) N. V. Sidgwick and H. M. Powell, Proc. Roy. Soc., Ser. A , 176, (5) R. Daudel and A. Bucher, J. Chim. Phys., 42,6 (1945); R. Daudel, (6) G. H. Duffey, J . Chem. Phys., 17, 196 (1949). (7) A.-C. Tang and H.-I<. Lee, J. Chin. Chem. Soc. (Taipei) 17, 252 (8) H. Siebert, 2. Anorg. Allgem. Chem., 265, 303 (1951). (9) D . P. Craig, A. Maccoll, R. S. Nyholm, L. E. Orgel, and L. E. Sutton, J . Chem. Soc., 332 (1954); D. P. Craig and E. A. Magnusson, ibid., 4895 (1956). (10) J. W. Linnett and C. F. Mellish, Trans. Faraday Soc., 50, 665 (1954); J. W. Linnett, “The Electronic Structure of Molecules,” Methuen and Co., London, 1964, p 122. 153 (1940). A. Bucher, and H. Moureu, C. R . Acad. Sci., 218,917 (1944).