Bull. Soc. fr. Minéral. Cristallogr.

(i97i), 94> 271-299.

The structural chemistry of phosphates

1. — Introduction

When used in the widest sense, the term phosphate may be taken to include all phospho¬ rus compounds which contain P-O linkages. Since the crystal structures of several hundreds of these compounds are now known, it is timely to consider the available data and to discuss some of the structural principles which are involved.

Phosphates may be divided into two broad groups :

a) Normal phosphates — in which the P atoms are connected solely to O atoms.

b) Substituted phosphates — in which the P atoms are connected to other atoms X, in addition to O atoms.

On the basis of the established stereochemis¬ try of phosphorus compounds (51), there are four possible spatial arrangements to consider for phosphates [1].

0 »

p p

/l\ /|\ OR OR OR OR OR OR

RO-RO ■

(•>)

(A)

OR I

� P-OR OR (c)

RO-RO'

OR

I

>C

I

OR id)

■OR

-OR (/)

Compounds of type [1 a~\ involve trivalent phosphorus and an unshared electron pair. They are based on a pyramidal bond configuration, e. g. (EtO)a P.

Compounds of type [1 b] involve pentavalent phosphorus and are based on a tetrahedral bond configuration, e. g. (EtO)3PO.

Compounds of type [1 c] again involve pen¬ tavalent phosphorus and are based on a trigo¬ nal bipyramidal configuration. Only in recent years have oxy-compounds of this kind been made and their structures established [2].

/

,c H 3

X

a p �

OEt

.OEt ■OEt

(.2)