KINETICS OF SUBSTITUTION-REACTIONS OF FE-II-PHOSPHINE COMPLEXES WITH CL-, BR- AND SCN- IN ACETONITRILE - A COMPARATIVE-STUDY OF COMPLEXES CONTAINING BIDENTATE AND TRIPODAL PHOSPHINES

The kinetics of formation of FeLX(+) complexes from FeLan(2)(2+) and X -[L=P(CH(2)CH(2)PPh(2))(3), N(CH(2)CH(2)PPh(2))(3), (Ph(2)PCH(2)CH(2)P Ph(2))(2); X=Cl, Br, SCN; an = acetonitrile] has been studied in aceto nitrile at 25.0 degrees C and Et(4)NBF(4) (0.05 mol dm(-3)). All the r eactions seem to go through initial substitution of one coordinated ac etonitrile by X(-) to form FeLX(+), although in some cases the reactio n continues to the formation of FeLX(2). The values obtained for the r ate constants corresponding to the first substitution process do not s how large differences and indicate that the three FeL(an)(2)(2+) compl exes have a very similar behaviour, except that the N(CH(2)CH(2)PPh(2) )(3) complex is intrinsically more labile towards substitution of coor dinated acetonitrile. The stability quotients for the formation of FeL X(+) complexes have been also determined and they suggest some degree of steric selectivity in the reactions of [Fe{P(CH(2)CH(2)PPh(2))(3)}( an)(2)](2+) with X(-). Copyright (C) 1996 Elsevier Science Ltd.