Stability and reactivity of technetium compounds

Itible 6.4.A Formation constants, force constants, and crystal fietd stabilization energies of Tc(IV) and 

Another example demonstrating that technetium compounds are more reactive than the analogous compounds of rhenium is the racemization of the penicillamine complexes of Tc(V) and Re(V). The complexes are six-coordinate in solution with one tridentate and one bidentatc penicillamine (pen) and one oxo ligand. Both mixed ligand anionic complexes l(/9-pcn)(L-pcn)TcO] and [(D-pcn)(T-pen)ReO] undergo a unimolecular racemization by exchange of carboxylates at the site trans to the oxo ligand  

The kinetics of these processes were measured at 25 °C by complete line-shape analysis of the H NMR spectra. The racemization reaction of [(D-pcn)(Z.-pen) TcO] proceeds nearly 60 times faster than the racemization of the corresponding complex of rhenium [61]. 

The displacement of water in the analogous complexes /ran.v- Tc 0(0H2)(CN)4] and /rans-[Rc 0(0H2)(CN)4] by NCS ions in an aqueous medium was monitored spectrophotomctrically [62] using a stopped-flow technique [63]. The substitution of water by thiocyanate can be represented by the bimolecular reaction  

IR and X-ray structural analyses [64] established the N-bonding of the NCS ligand in both complexes. The rate constant Zc(Tc) for the reaction of trani-[TcO(OH2)(CN)4] with thiocyanate at 25 °C was found to be 22.2 mole s while /t(Re), evaluated under the same conditions, was only 35 10 mole s [62,65]. The rate constant Z (Tc) turns out to be over 6000 times higher than fc(Re) of the rhenium complex. 

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