Halide-oxygen bond strength

The phosphonitrilic halides are weaker bases than are some related nonaromatic compounds, just as pyridine is a weaker base than piperidine. The bond type in the phosphonitrilic series is probably the same as in the phosphine-imines RaPrNR (80, 81) the stability of these compounds, too, increases with the electronegativity of the substituents on the phosphorus. Tetraphenylphosphine-imine is one of the most stable of these compounds, and forms a hydrochloride. Its base strength is therefore greater than those of the phosphonitrilic halides, presumably because the opportunities for electronic delocalization are less. The Af-ethyl ester of the trimeric phosphonitrilic acid, which has a cyclic structure, but in which resonance of the same type as in the phosphonitrilic halides cannot occur, also forms a hydrochloride ( 1). It is, therefore, a comparatively strong base, in spite of the inductive effect of the two oxygen atoms on the phosphorus. The extreme weakness of the phosphonitrilic halides as bases cannot therefore be wholly due to the inductive effect of the halogens. 

As a consequence of the strength of the R-O bond and the usual aqueous environment, nearly all complexes of the rare earths are derived from oxygen-donor ligands (see chapter 56, section 1, of Volume 8 of this Handbook). However, a number of nitrogen-donor complexes can be prepared but often only in strictly anhydrous conditions. They include also inorganic ligands such as NH3 and SCN (Moeller et al., 1973). Complexes where other bonds are present are less common, with the possible exception of those involving the halides. 

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