Phosphorus bond angles

Attempts at correlating endocyclic phosphorus bond angles with the phosphorus chemical shifts and P-N bond distances with C1 NQR frequencies have been made [314]. It appears that for closely related compounds there exists a narrow relationship between chemical shifts and endocyclic phosphorus angles [96, 358]. No generalized trend has yet emerged. The differences in the P-Cl bond lengths in PCl2 and PRCl units have been correlated with the C1 NQR frequencies [314],... 

Whereas the barrier for pyramidal inversion is low for second-row elements, the heavier elements have much higher barriers to inversion. The preferred bonding angle at trivalent phosphorus and sulfur is about 100°, and thus a greater distortion is required to reach a planar transition state. Typical barriers for trisubstituted phosphines are BOSS kcal/mol, whereas for sulfoxides the barriers are about 35-45 kcal/mol. Many phosphines and sulfoxides have been isolated in enantiomerically enriched form, and they undergo racemization by pyramidal inversion only at high temperature. ... 

The central phosphorus atom has one unshared pair and three bonded atoms. The molecule is of type AX3E it should be a trigonal pyramid (like NH3) with a bond angle somewhat less than 109.5° (actually, 104°). 

So far, we have not considered whether terminal atoms, such as the Cl atoms in PC15, are hybridized. Because they are bonded to only one other atom, we cannot use bond angles to predict a hybridization scheme. However, spectroscopic data and calculation suggest that both s- and p-orbitals of terminal atoms take part in bond formation, and so it is reasonable to suppose that their orbitals are hybridized. The simplest model is to suppose that the three lone pairs and the bonding pair are arranged tetrahedrally and therefore that the chlorine atoms bond to the phosphorus atom by using sp hybrid orbitals. 

Unlike the geometries for other steric numbers, the five positions in a trigonal bipyramld are not all equivalent, as shown in Figure 9-21a. Three positions lie at the comers of an equilateral triangle around the phosphorus atom, separated by 120° bond angles. Atoms in the trigonal plane are In equatorial positions. The other two positions... 

The strained 60° bond angles in tetrahedral P4 make white phosphorus highly reactive. This form of the element must be kept out of contact with air, in which it spontaneously bums. 

The cage-like phosphonium salt (17) with phenyl-lithium in THF gave the phosphorane (18) which probably owes its great stability to the relief of strain in the ring structure on changing the bond angle at phosphorus to 90°. For the photolysis of (18) see Chapter 10, Section 1. 

The dipole moment of tributylpliosphine varies from 1.49 to 2.4 D according to the solvent used. Inductive effects in phosphines have been estimated by comparing the calculated and observed dipole moments, and the apparent dipole moment due to the lone electron pair on phosphorus has been estimated. A method of calculating the hybridization of the phosphorus atom in terms of bond angles is suggested which leads to a linear relationship between hybridization ratio and lone electron pair moment. The difference between experimental and calculated dipole moments for para-substitued arylphosphines, phosphine sulphides, and phosphinimines has been used to estimate mesomeric transfer of electrons to phosphorus. 

Two modifications are known for polonium. At room temperature a-polonium is stable it has a cubic-primitive structure, every atom having an exact octahedral coordination (Fig. 2.4, p. 7). This is a rather unusual structure, but it also occurs for phosphorus and antimony at high pressures. At 54 °C a-Po is converted to /3-Po. The phase transition involves a compression in the direction of one of the body diagonals of the cubic-primitive unit cell, and the result is a rhombohedral lattice. The bond angles are 98.2°. 

---