Kinetics and Mechanism of the Monomerization of a Re(V) Dithiolato Dimer with Monodentate Ligands. Electronic and Steric Effects

The sulfur-bridged dimeric dithiolato rhenium(V) chelate (D), derived from
2-mercaptothiophenol, was monomerized to give (M−L) in benzene upon reaction
with various neutral and anionic monodentate ligands (L) such as pyridine and its substituted derivatives,
triarylphosphines, dimethyl sulfoxide, 4-picoline-N-oxide, and halide ions. The kinetic observations can readily
be interpreted for all ligands by a unified mechanism in which the initial fast formation of a 1:1 (DL) and 1:2
(DL2) adduct is followed by the slow monomerization of each species so formed. The use of different ligands
gave insight into different steps of the same multistep mechanism. The kinetics of ligand exchange between free
L and the monomeric complexes was also studied; an associative pathway has been proposed to interpret the
results. The crystal structures of two new monomeric ML complexes (with L = 4-acetylpyridine and
1,3-diethylthiourea) are reported.