Complexes with M-P-Halogen Bonds

Many fluoride addition complexes have been prepared and characterised. Fewer chloride complexes are known and those involving bromine and iodine are comparatively few, although the latter have not been widely studied. 

Particularly stable complexes can be formed with PF3. Although the highly electronegadve F atoms significantly lower the ability of the phosphorus atom to donate its lone-pair electrons in o bond formation this is compensated by the Ji bonding capacity of the vacant d orbitals. Trifluorophosphine forms complexes analogous to those of carbon monoxide, for example, F3P BH3 and Ni(PF3)4 which can be compared with the very stable H3B CO and Ni(CO)4. A whole series of complexes of general formulae [M(PF3)where jc = 1 or 2, are known, which correspond to carbonyl complexes with similar formulae (8.173). 

Tetrakis (trifluorophosphine) nickel, Ni(PF3)4 and Pt(PF3)4 can be obtained by direct synthesis of the gas with metal powder at 100°C and 300 atm. The very stable nickel compound contains zero valent tetrahedral nickel since sufficient lone pairs are donated by the ligand PF3 groups to complete 

A general method of preparation of the zero valent metal trifluorophosphines is by ligand replacement. Under high PF3 pressure, for example, the chromium or molybdenum compound can be prepared from the dibenzene derivatives (8.175). Carbon dioxide is also displaced under high pressure (8.176), although the reaction is reversible. Under some conditions, partial replacement can be effected to produce complexes of type M(CO) j (PFslx- 

Another method of preparation involves the reaction with metal halides in the presence of a reducing agent (8.177). The nickel compound may also be prepared by ligand exchange as in (8.178) where the whole ligand is replaced. In certain cases direct reaction with metal salts can occur (8.179-8.181). 

---