"In particular, each of L1, L2, L3 and L4 denotes a phosphine PRR'R, each of R, R' and R independently denoting a C1 -C14 alkyl group which may be substituted by SO3 -, COOH, alkoxy or alkyl-S-- or an aromatic group which may be substituted by at least one substituent chosen especially from halogen such as Cl, Br or F, alkyl, CF3, alkoxy, NO2 or SO3 -, it being possible for at least two of these" . . . .