Compounds with Si—O—P Linkages

The Si-O-P linkage can, in principle at least, be formed on condensation of orthophosphoric and silicic acids (9.259). Silicophosphate glasses may contain Si-O-P linkages although they are not readily formed by condensation of simple orthophosphate and orthosilicate anions (Chapter 5.4). 

Simple salts such as Si3(P04)4 and SiP207 contain discrete orthophosphate and diphosphate anions, respectively. In these salts, the Si atoms are octahedrally coordinated by oxygen and the bonds are considerably more ionic than is the case with tetrahedrally coordinated Si atoms. Many crystalline silicophosphates containing condensed tetrahedra as in Equation 9.259 have also been found, and these are dealt with in Chapter 5.4. 

All compounds containing Si-O-P linkages are more liable to hydrolysis than those containing C-O-P and this should be borne in mind in their preparation and handling. 

The first pure P-O-Si organic compound to be isolated was tris(trimethylsilyl) phosphate, bp = 232-234°C, which was made from hexamethyl disiloxane and phosphorus pentoxide (9.260). Other tris(alkylsilyl) phosphates can be made in a similar way, but alternative methods of preparation 

Crystalline methylsilyl phosphonium salts can be easily obtained from trimethylsilyl cobalt tetracarbonyl by low-temperature reaction with trimethylphosphine or trimethylsilyl dimethyl phosphine (9.263). These salts are stable in vacuo at room temperature. 

---