Phosphoranes bond angles

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. 

Phosphole derivatives with CN = 5 and CN = 6 are hypervalent species, which are quite rare. Indeed, the phosphoranes (CN = 5) are stable only when the P-atom bears electronegative substituents such as fluorine atoms or alkoxy groups. The geometry adopted at phosphorus is distorted trigonal bipyramidal, something enforced by the small endocyclic CPC bond angle. Compounds with CN = 6 are anions with octahedral phosphorus atoms. 

A single crystal X-ray structural evaluation of the hydrido-phosphorane (117) reveals that the compound lies on the Berry coordinate apparently 50% between tbp and rp structures with the oxygen atoms in apical positions and an O-P-0 bond angle of 166.9°. 

The barriers to permutational isomerizations in five-membered spiro-phosphoranes have been rationalized in terms of BPR processes involving diequatorial five-membered rings and preferred lone-pair orientations. The energy difference between (22) and (23) is composed of a general strain term due to the increased bond angle at phosphorus, the energy required to rotate the lone pairs on both X and Y from the preferred equatorial orientation to an... 

PCC and CCC bond angles (126.6 and 127.7°) involving the a- and /S-carbon atoms of the allyl group of (191) are considerably increased compared to sp angles. It has been concluded that the ylide character cannot be estimated from the P—C bond length on the basis of complete ylene structure for methylenetriphenylphosphorane. Studies are also reported for the imino-phosphorane (193), (194), monoclinic triphenylphosphine oxide, the... 

A stereochemical possibility unique to the associative mechanism is adjacent displacement in which the bond angle between entering and leaving groups is 90°. This possibility results from the ability of the pen-tacoordinate phosphorane intermediate to undergo pseudorotation (9,10) which, with certain restrictions (11) interchanges axial and equatorial ligands. Nucleophilic displacements on stable phosphoranes appear to proceed via an associative mechanism with hexacoordinated phosphorus intermediates (12, 13). The biochemical relevance of such displacements has not been established. 

O-P-0 found for these two phosphoranes were apical, 162.5° and 163.3° and radial, 107.7° and 112.7° respectively. Probably, compression of the equatorial O-P-O bond angle in the 6-membered ring of (114 RR = CH2CH(Bu )CH2) and related phosphoranes allows the ring to assume a chair conformation more easily than when phosphorus is not part of a bicyclo ring system. 

The first observation is in contrast to the studies on phosphoranes (14, 15, 16) for which the equatorial cycle is highly unfavorable owing to the non-cyclic equatorial C-P-C angle of 120°. The non-cyclic C-S-C bond angle is presumably in the vicinity of the 102° F-S-F angle of SF4. [For relevant data see Table VI of Reference (17).] Also trimethylene-sulfide is a known compound. This is an example of the diflFerence between the pure p-bond picture of sulfuranes and the -hybridized picture of phosphoranes. The only evidence relative to the second observation is the lack of pseudorotation in solvolysis sulfurane intermediates (16) although some authors seem intuitively to believe pseudorotation likely in these compounds (18). 

C.N.-5. Phosphoranes. These are uncommon derivatives, following the usual rules of phos-phoranes that stability requires the presence of multiple electronegative substituents. Unlike the lower coordination numbers, this condition develops two sets of bond angles around phosphorus, 120° in the equatorial plane and 90° in the two apical positions. 

The geometries at the phosphorus atoms in the p-bromophenyl- and. W-toluene-p-suIphonyl- iminotriphenylphosphoranes are very similar. P-C(aryl) distances lie in the range 1.79—1.82 A, and the P=N bonds are 1.567 and 1.579 A. A rather abnormal situation occurs in the crystal structure of bis(triphenylphosphoranylidene)methane (or hexaphenylcarbodi-phosphorane), in which the P=C=P angle is not only bent, but has different values, of 131 and 145°, in the two crystallographically independent molecules (Figure 3). The P=C bond lengths of 1.624 A, by comparison with P=0 values, must represent double bonds. The bonding at the carbon atom is thus difficult to rationalize, but may be connected with use of the phosphorus cf-orbitals. 

---