Phosphorus bond energies

The bond energy of the P=P bond is 493kI/mol and the P-P bond energy is 209kJ/mol. Use these values to show that form of elemental phosphorus is expected to be different from that of elemental nitrogen. 

In the light of several allotropic modifications known for phosphorus, the relatively high single bond energies and the tendency of phosphorus to catenate it remains mysterious, and that apart from P4 and As4 only very scarce information on isolated E cluster molecules is available from hard experimental data. In contrast, a vast amount of solid theoretical work has been performed [11],... 

Revisions of earlier data on the heat of formation and the bond energies of phosphine have been undertaken The heat of formation of phosphine from white phosphorus and hydrogen is 1.30kcal/mol (the corresponding value for P2H4 is 5.0 1.0 kcal/mol), the average bond energy E (P—H) was found to be 76.8 kcal. 

ATP Regarded as Store House of Energy Adenosine triphosphate (ATP) in a nucleotide consists of purine base adenine, a pentose sugar ribose and three molecules of phosphate. It contains two oxygen to phosphorus bonds between two phosphate units. These phosphorus bonds are called high energy phosphatic 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. 

Some quantitative support for the above qualitative arguments comes from average bond energies of phosphorus, bromine, and oxygen (Appendix E)-. 

I8.lt> The P4 molecule in white phosphorus has extremely strained bonds The bond angles in the Tj molecule are only 60°. Therefore the bonds are weak, only 201 kJ mol-1 for each one. The total bond energy of two moles of Pj (962 kJ. Appendix E)is244 kJ less than that of one mole of Pi (1206 kJ. Appendix E). In contrast, the total bond energy of two moles of N. has been calculated57 to be 77 kJ greater than that of one mole of N, ta hypothetical tetrahedral molecule iscslraclund with P4)... 

As far as the data permit, the dependence of bond energies on the hybridisations of the atoms concerned is allowed for. In the case of F—P, Cl—P and Br—P, the bond energies are based upon data for molecules in which the P atom has (approximately) sp3 hybridisation, e.g. PX3, PX2Y, POX3. These values are inappropriate for PX5 species, or for PX3Y2 etc. The mean bond energies found for, e.g., PF5 are considerably lower than for trivalent phosphorus species. 

The comparison of the bond energies in boron, silicon, phosphorus and arsenic atoms with that of carbon atoms supports this idea (Table 2). 

Molecular orbital (CNDO/2) theoretical calculations have been carried out on [Cr(PF3)6], [Ni(PF3)J, and [Fe(PF3)5] (320), and the results compared with experimental ionization energies determined by UV photoelectron spectroscopic measurements of these complexes in the gas phase. The metal-phosphorus bonds show large ct(P— M) and ji(M—>P) charge transfers but small total charge transfers (M— P) which induce on the metal a small positive charge. 

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