Pseudo-rotation in the hydrolysis of phosphate esters

Abstract. The hydrolysis of a phosphate, phosphonate or phosphinate ester gen-eraUy takes place either through a trigonal bipyramidal hydroxyphosphorane as intermediate (with expansion of the coordination number of phosphorus from four to five) or (at least for phosphates) through a moaomeric metaphosphate as inter-mediate (with contraction of the coordination number of phosphorus from four to three). These processes parai.lel the principal mechanisms for the hydrolysis of carboxylic esters, which require either the formation of a tetrahedral interme-diate or that of an acylium cation.