Oxygen ligands, technetium complexes

As observed from X-ray crystal data, which are available for a variety of complexes [4,5,6], oxotechnetium complexes show the following general feature the metal lies above the equatorial plane of the four basal ligand atoms and the oxo group is at the apex. Tc=0 bond lengths correlate with the displacement of the technetium atom from the basal plane towards the oxygen at the apical position. 

Another type of ligand is the monoanionic, tridentate oxygen donor [(C5H4R)Co-(P(0)R R")3] (Lor), which has been used to prepare the complexes of technetium [37] and rhenium [38] [M03L] and [MOX2L] (X Cl, Br). These complexes are stable in organic solvents but hydrolyse slowly in water. In order to evaluate their usefulness in radioimmunotherapy, the corresponding compounds were also prepared with radioactive rhenium isotopes. 

Tc(IV), the DTPA ligand is undoubtedly coordinated in a hexadentate fashion about the Tc through three nitrogens and three carboxylate oxygens, leaving two uncoordinated carboxylates. Deutsch and Packard [56], however, observed three uncoordinated carboxylates on the complex by titration with base, suggesting that the technetium may be in the 4- 5 oxidation state. If Tc(V) is correct, then a 6-coordinate complex would be present with a carboxylate in the position trans to the Tc = 0 group. 

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