KINETICS OF SUBSTITUTION-REACTIONS OF TRANS-[MO(N2)2-(PPH2ME)4] WITH TRIPODAL PHOSPHINES

The kinetics of substitution reactions of trans-[Mo(N2)2(PPh2Me)4] 1 w ith the tripodal ligands tris(2-diphenylphosphinoethyl)phosphine and t ris(2-diphenylphosphinoethyl)amine has been studied in benzene solutio n at different temperatures. The absorbance vs. time profiles obtained at 25.0-degrees-C are very complicated because of the existence of fo ur consecutive steps. Although the curves can be fitted by consecutive exponentials, the values of the rate constants are very similar, whic h hinders a detailed interpretation of the data. The reactions have al so been monitored by P-31-{H-1} NMR spectroSCopy to determine the orde r of occurrence of the different steps and to propose reasonable struc tures for the reaction intermediates. The proposed mechanism involves initial dissociation of PPh2Me to form an intermediate which is also d etected upon decomposition of 1. This then reacts through two parallel pathways leading to complete decomposition of 1 and to the formation of reaction products with the tripodal phosphines. The second pathway goes through the formation of several intermediates containing one or two molecules of tripodal phosphine acting as mono-, bi-, tri- or tetr a-dentate ligands.