The chemistry of technetium nitrido complexes

Abstract

A characteristic feature of the chemistry of technetium in the +5–+7 oxidation states is the formation of a wide variety of highly stable complexes containing the terminal Tc≡N bond. The Tc≡N bond is short (1.585–1.65 Å) and formally triple with one σ component and two π components formed by overlap of px and py orbitals on N with Tc d _xz , d _yz orbitals. Coordination numbers of 5, 6 and 7 and square pyramidal, trigonal bipyramidal, distorted octahedral and pentagonal pyramidal and bipyramidal geometries have been established by X-ray crystallography. The nitrido ligand exerts a strong trans influence, with the trans ligand either absent or only weakly bound. In octahedral complexes, the NTc-L_trans bond is 0.1–0.3 Å longer than NTc-L_cis. In the IR spectrum v(TcN) is observed as a strong sharp band at 1100–1000 cm⁻¹. [Tc^vN]²⁺ complexes have been prepared with diverse coordination spheres and are monomeric except for one example with the cyclic Tc₄N₄ core. Monomeric [Tc^VIN]³⁺ (d ¹) complexes are readily detected by EPR spectroscopy and this technique has been extensively used to identify new species and study exchange reactions. Substitution of [Tc^VINX₄]⁻ (X=Cl, Br) by thiols, phosphines, amines and nitrogen heterocycles in organic solvents generally yields the reduced [Tc^VN]²⁺ complexes. In aqueous solution dimerization of [Tc^VIN]³⁺ occurs to yield EPR silent [NTc^VI-O-Tc^VIN]⁴⁺ and [NTc^VI(μ-O)₂Tc^VIN]²⁺ complexes, including the structurally characterized (AsPh₄)₄[Tc₄N₄(O)₂(ox)₆] and (AsPh₄)₂[(TcNX₂)₂(μ-O)₂] (X-Cl, Br). Novel [Tc^VIIN]⁴⁺ complexes are Cs[TcN(O₂)₂Cl] and (AsPh₄)₂[{TcN(O₂)₂}₂(ox)]. ^99mTcN complexes have been investigated as potential radiopharmaceuticals.