Phosphorus molecular geometries

All this data has prompted various authors to present a discussion of P-C-H couplings as a function of the molecular geometry and more especially of the dihedral angle H-C-P or H-C-P=X. (1967,77 1968,30) A more complete analysis has been presented by Albrand et al. (1968,62 1969,76) (Fig. 4) to improve the Karplus-like description of 27(P-C-H) values for trivalent phosphorus. As is seen zero coupling is possible with a dihedral angle of about 45° the CH and CH3 resonances appear as singlets in 21(1966,42) and 22(1967,78 1969 77) respectively. ... 

Formula PH3 MW 34.00 molecular geometry trigonal pyramidal. Synonyms phosphorus trdiydride hydrogen phosphide... 

The importance of equilibria in the chemistry of pentacoordinated phosphorus arises from the possibility of passing from a given structure to another, even if the latter has a totally different molecular geometry and distribution of electrons, by a mere shift of hybridization. This has important consequences on the chemical properties of phosphoranes and on the possibilities of interconversion of isomers by bond rupture and recombination processes akin to regular true stereomutations, i.e. without bond cleavage. 

There is much interest in transition-metal carbonyl clusters containing interstitial (or semi-interstitial) atoms in view of the fact that insertion of the encapsulated atom inside the metallic cage increases the number of valence electrons but leaves the molecular geometry essentially unperturbed. The clusters are generally anionic, and the most common interstitial heteroatoms are carbon, nitrogen, and phosphorus. Some representative examples are displayed in Fig. 19.4.3. 

Although the molecular geometries of 2 - and 2 -phosphinins are nearly identical (no significant deviations from planarity or bond alternation are observed with either system), the A -phosphinines as well as the mono-, di-, tri- and tetra-aza-A -phosphinines, the A A -diphosphinines and the A A A -triphosphinines are certainly not Hiickel-aromatic systems. Their description as cyclic phosphorus ylides with ionic resonance structures (26) is probably close to the truth. The H and C NMR evidence is particularly strong on this point. 

Griitzmacher, H. and Pritzkow, H., 3 Independent molecules in the unit-cell of a phosphorus ylide - changes in the molecular geometry on rotation about the phosphorus carbon ylide bond, Angew. Chem. Int. Ed. Engl. 31 (1), 99-101 (1992). 

While the barriers for inversion of pyramidal compounds of first-row elements are normally low so that inversion is fast, the heavier elements have much higher barriers to inversion. The preferred bonding angle at sulfur, phosphorus, and other heavier elements is about 100° for most trivalent derivatives. This means that a much greater distortion of molecular geometry is required to reach the planar transition state. Typical barriers for trisubstituted phosphines are 30-35 kcal/mol, while for sulfoxides the barriers are about 35-45 kcal/mol. Phosphines and sulfoxides can therefore be isolated in optically active form and undergo inversion only at high temperatures. ... 

To describe hybridization schemes that correspond to the 5- and 6-electron-group geometries of VSEPR theory, we need to go beyond the s and p subshells of the valence shell, and traditionally this has meant including d-orbital contributions. We can achieve the five half-filled orbitals of phosphorus to account for the five P—Cl bonds in PCI5 and its trigonal-bipyramidal molecular geometry through the hybridization of the s, three p, and one d orbital of the valence shell into five sp d hybrid orbitals. 

The most recent theoretical study, by Alhrichs and co-workers, deals with the di(phosphino)carbene Id and (phosphino) (phosphonio)carbenes Ie,f.16 The optimized geometry of the di(phosphino)carbene Id is weakly bent (PCP angle 160.5°) and highly unsymmetrical Only one of the phosphorus centers (P1) is in a planar environment, and it is much more closely bonded to the carbenic center than the other one (P1C 1.533 and P2C 1.765 A). The atomic charges (P1 +1.0, C -0.8, P2 +0.6) indicate that the short P bond is a double bond reinforced by Coulombic attraction, while the nature of the molecular orbitals revealed a slight delocalization of the carbene lone pair into the low-lying a (P-N) orbitals of the two phosphino substituents. The distortion from the symmetrical structure can be viewed as a second-order Jahn-Teller effect. 

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