Phosphonium salts 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. 

Phosphonium salts 1 (m = n = 4) were shown to have tetrahedral geometry with small endocyclic C-P-C bond angles 97-98° <2002JA6126>. 

This extremely air-sensitive compound, which is valence isoelectronic to an olefin, has been structurally characterized by X-ray diffraction. It has a short carbon-phosphorus double bond (1.62 A) the phosphorus and carbon atoms adopt a trigonal planar geometry with a dihedral angle of 60° (Fig. 3). This value is significantly larger than that reported for the most crowded olefin.61 Formally, this compound can be viewed as the product of a car-bene-carbenoid coupling between bis(trimethylsilyl)carbene and bis(diiso-propylamino)phosphenium triflate. Note that another route to methylene-phosphonium salt has been reported by Griitzmacher et al.62... 

Structurally, it is pyramidal (H—P—H angle = 93.7°) like ammonia but does not hydrogen-bond or dissolve in water because the P—H bonds are essentially nonpolar. Phosphine also differs from ammonia in that it is a very weak Bronsted—Lowry base (proton acceptor). It can, however, be forced to react with strong acids to form phosphonium (PH4) salts. 

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