Phosphorus Element Single Bonds

The great importance of phosphanes as ligands in organometallic chemistry prompted chemists to search for methods to predict the P-NMR chemical shifts of these compounds not only qualitatively, but quantitatively. To this end, the following set of empirically derived equations was developed. 

In these equations, denotes a shift constant characteristic for a certain substituent. The values are given in Table 5.2. The parameters a and b stand for the number of allyl, benzyl, or cyclohexyl groups (a), and phenyl groups (b), bonded to the phosphorus atom of a phosphonium salt. 

Note The 7t-bonding character of the substituents frequently determines the chemical shift value of the phosphane. 

We note that the equations in Table 5.1 do not cover phosphites, one of the most important classes of phosphanes. We also note that phosphites contain alkoxy and/ or aryloxy substituents that cause h-M effects as substituents on phenyl rings. In other words, they are prone to -bonding. 

A main factor for this change in the order is the fact that in phosphanes, the alkyl group operates on the phosphorus atom (-1 effect, since C is more electronegative 

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The second difference between 0=PF3 and PF is the existence of a formal P=0 double bond (really a P —0 bond subject to hyperconjugation) compared to two P—F single bonds. We will find out in Chap. 5 that a phosphorus element double bond results in a significant downlield shift of the phosphorus resonance. That should not surprise us. We know that a. r-donor interaction from a subsituent results in a significant upheld shift. By the same argument, a double bond to a more electronegative element should result in a signihcant downheld shift, as observed. 

C21-0067. Explain why the known forms of elemental carbon include forms with single, double, and triple bonds, whereas the known fornis of elemental phosphorus all have single bonds. 

The stable form of nitrogen at room temperature is N2, which has an extraordinarily strong (946 kJ mol-1) triple bond In contrast, white phosphorus consists of P4 molecules (see Chapter 16), and the thermodynamically stable form is black phosphorus, a polymer. At temperatures above 800 °C dissociation to P> molecules does take place, but these are considerably less stable than N2 with a bond energy of488 kJ mol 1. In this case. too. in the heavier element several single bonds arc more effective than the multiple bond. 

Note that nitrates have three oxygen atoms surrounding each nitrogen whereas phosphates have four oxygens about each phosphorus. This is probably not a radius ratio effect but is instead connected with the inability of phosphorus to form double bonds of the usual type. When nitrogen, a first-row element, forms four bonds, one or two may be double, but when phosphorus forms four bonds, all must be single. A number of workers feel that there is some overlap between the 3d orbitals of phosphorus and the 2p orbitals of oxygen in phosphates, and that this leads to double-bond character" of a sort, but this question is still open. 

The optimum geometry of tri-t-butylphosphine has been determined by CNDO/2 calculations,234 and the CNDO/S method has been extended to include second-row elements.235 The results for phosphorin (15) satisfactorily explain the observed u.v. transitions, dipole moment, and ionization potentials. An X-ray structure of 1-benzylphosphole (16) points to the presence of a non-planar ring with a mean P—C(ring) distance of 1.783 A 236 the shortening of the latter over the sum of the single-bond radii is consistent with some delocalization of the phosphorus lone pair. A number of new cyclic phosphides of carboxylic acids such as (17) can be... 

Phosphorus exhibits complicated allotropy eleven forms have been reported, of which at least five are crystalline. Crystalline white phosphorus contains tetrahedral P4 molecules (Figure 14.3 a) in which the P—P distances (221 pm) are consistent with single bonds (r ov = 110 pm). White phosphorus is defined as the standard state of the element, but is actually... 

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