PXaR1 b(OR2)c or POXdR3 e(OR4)f where X is a halogen atom, and R1,R2,R3 and R4 are hydrocarbyls having 1???20 carbon atoms, each of which is the same or different from one another, and a, b, c, d, e and f are integers between 0 and 3 satisfying a+b+c=3 and d+e+f=3.