Bonding cyclic phosphazenes

Bonding. All phosphazenes, whether cyclic or chain, contain the formally unsaturated... 

A number of ab initio molecular orbital calculations have been performed on acyclic and cyclic phosphazenes. These calculations point to a phosphorus-nitrogen bond with a large degree of charge separation and a small but essential contribution from phosphorus d-orbitals. 

Table 19. Average PN Bond Lengths, NPN Bond Angles, and N—N (trans-Ring) Distances in Some Cyclic Phosphazenes and the P4N] Ion...

This d —p,j-bonding is not confined to simple molecules and d orbitals can also take part in delocalized r-bonding in compounds such as the cyclic phosphazenes (/) and the thiazyl compounds (II), Fig. 1. We shall review some of the thermochemical data which... 

The cr-bonds in the cyclic phosphazenes, such as the hexa-chloro compound [PNClgls, are undoubtedly formed by or near-5p -hybrid orbitals of phosphorus, since the phosphorus atoms are four-co-ordinated and X-ray analysis indicates that they are probably tetrahedral. Wilson and Carrol suggest that the ring system is planar, whereas Liquori et conclude that the ring... 

Conformational isomerism can accompany geometrical, cis-trans, optical isomerism and so forth, which may arise from the positioning scheme adopted by the exocyclic substituents bonded to the ring. Four types of isomerism arise in cyclic phosphazenes (Chapter 7.8). 

Faucher, J.-R, Labarre, J.-F., and Shaw, R. A., Bonding in cyclophosphazenes. Quantum and experimental support for Dewar s island model, Z. Naturforsch., 31b, 677 (1976). Mitchell, K. A. R., Noodleman, L., and Paddock, N. L., Xa scattered wave calculations of the ionization energies of N3P3F6 and N4P4F8, Chem. Phys. Lett., 47, 265 (1977). Noodleman, L., Westwood, N. P. C., and Mitchell, K. A. R., Ionization energies and electronic structure of the cyclic phosphazene as determined by UV photoelectron spectroscopy and Xa scattered wave methods. Chem. Phys. Lett., 58, 252 (1978). 

Three independent P-N bond 137 lengths. OPO = 102.7°, exo-cyclic groups twisted at 48° to average plane of phosphazene ring... 

P—S bond is elongated to 196.5 pm and the P—0(H) bond shortened to 148 pm.164 The structures of the m-vinyl carbonate (136)165 and the cyclic phosphonic anhydride (96)123 have been determined, and for the latter compound the ring was found to be almost planar. A -Ray diffraction data of deoxyuridine phosphate,166 phospholipid multilayers,167 nucleotides,168 and skeletal muscle169 have been analysed. The crystal structure of the phosphazene (137) has been established.170... 

Metallophosphazenes with Phosphorus-Metal Bonds. Until recently, the chemistry of cyclic and high polymeric phosphazenes was essentially the chemistry of their organic derivatives (Scheme 1). However, a discovery reported in 1979 (31) opened up a new field of metallophosphazene chemistry in which transition metals form the nucleus of the side group structure and are linked to the skeleton through phosphorus-metal bonds. These species are of theoretical and potentially practical importance, and I will summarize briefly some of the main features known at this time. 

Cyclic and polymeric phosphazenes will form water-soluble platinum(II) complexes. The compounds contain square planar platinum bonded to the phosphazene via nitrogen.1503-1506... 

It is exceedingly difficult to determine the molecular structure of a synthetic macromolecule. X-ray diffraction—the ultimate structural tool for small-molecule studies—yields only limited information for most synthetic high polymers, and crucial data about bond lengths and bond angles are difficult to obtain.47 However, that same information can be obtained relatively easily from single crystal X-ray diffraction studies of cyclic trimers, tetramers, and short-chain linear phosphazene oligomers. The information obtained may then be used to help solve the structures of the high polymeric counterparts. 

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