FE-II COMPLEXES WITH TRIPOD PHOSPHINES, PH(2)PCH(2)CH(2)PPH(2) AND NET(3) - STABILITY AND KINETICS OF FORMATION

The formation constants of Fe-II complexes with the;tetradentate phosp hines P(CH(2)CH(2)PPh(2))(3), PP3, and N(CH(2)CH(2)PPh(2))(3), NP3, ha ve been determined spectro-photometrically in acetonitrile-toluene mix tures at 25.0 degrees C and I = 0.05 mol dm(-3) Et(4)NBF(4). The resul ts for mixtures containing 60% v/v acetonitrile are log beta(11) = 6.0 0 (PP3) and 5.37 (NP3). The bidentate phosphine Ph(2)PCH(2)CH(2)PPh(2) , DPPE, and the monodentate amine NEt(3) have also been studied for co mparative purposes. For the case of DPPE, the results are log beta(11) = 4.07 and log beta(12) = 7.10. The kinetics of formation of [FeL(CH3 CN)(x)](2+) complexes from [Fe(CH3CN)(6)](2+) has been studied using t he stopped-flow technique. The rate of complex formation is first orde r with respect to both Fe-II and the incoming ligand: The second order rate constants in 60% v/v acetonitrile at 25.0 degrees C and I = 0.05 mol dm(-3) are (in mol(-1) dm(3) s(-1)): 10.1 10(4) (PP3), 8.6 10(4) (NP3), 0.6 10(4) (DPPE) and 5.4 10(4) (NEt(3)). These results can be r ationalized with the classical Eigen-Wilkins mechanism assuming that t he rate determining step is coordination of the central N or P of the tripod ligand. In all cases, the closure of chelate rings is rapid and the changes of log beta and rate constants observed with different co mpositions of the solvent are small. The formation of trans-[Fe(DPPE)( 2)(CH3CN)(2)](2+) from [Fe(DPPE)(CH3CN)(4)](2+) occurs slowly through a limiting dissociative mechanism.