STRUCTURE OF R3PCL2 COMPOUNDS IN THE SOLID-STATE AND IN SOLUTION - DEPENDENCY OF STRUCTURE ON R - CRYSTAL-STRUCTURES OF TRIGONAL BIPYRAMIDAL (C6F5)(3)PCL2, PH-2(C6F5)PCL2 AND OF IONIC (PR3PCL2)-P-N

A number of triorganophosphorus dichloride compounds R3PCl2, (R-3 = su bstituted aryl, mixed aryl-alkyl or triaryl) have been synthesized fro m diethyl ether solution and characterised by analytical and P-31-{H-1 } NMR data in CDCl3 solution. The majority of the compounds are ionic, [R3PCl]Cl in CDCl3 solution, in keeping with analogous species contai ning the heavier halogens [R3PX]X (X = Br or I), according to P-31-{H- 1} NMR studies. In contrast, the compounds R3PCl2 [R-3 = (C6F5)(3) or (C6F5)Ph-2] have a molecular five-co-ordinate trigonal-bipyramidal str ucture both in CDCl3 solution and in the solid state. The crystal stru ctures of these two compounds have been determined and represent the o nly crystallographic studies of trigonal-bipyramidal compounds of stoi chiometry R3PCl2. The compound (C6F5)(3)PCl2 exhibits almost perfect t rigonal-bipyramidal geometry, whereas (C6F5)Ph2PCl2 shows significant distortion. This may be due to the asymmetry of the equatorial groups around the phosphorus atom. Why R3PCl2 [R-3 = (C6F5)(3) or (C6F5)Ph-2] adopt a trigonal-bipyramidal structure is reasoned to be due to the a cidity of the parent tertiary phosphines, which favours this geometry for the dihalogen adducts, a phenomenon previously observed for dihalo gen adducts of tertiary arsines. The crystal structure of (Pr3PCl2)-P- n, the first crystallographically characterised example of an ionic R3 PCl2 compound which does not contain a solvent molecule, has been foun d to contain two (Pr3PCl2)-P-n entities. The first consists of an ioni c [(Pr3PCl)-P-n](+) unit weakly linked by a long Cl ... Cl contact to a Cl-, d(Cl ... Cl) 3.207(3) Angstrom. The second shows a discrete [(P r3PCl)-P-n](+) cation, the Cl- anion being associated with delta(+) H atoms on a [(Pr3PCl)-P-n](+) moiety. This compound was prepared and cr ystallised from diethyl ether and its relation to the solvated complex [Ph3PCl ... Cl ... ClPPh3]Cl . CH2Cl2 is discussed.