not readily form cations (ions with a net positive charge). Technetium exhibits nine oxidation states from −1 to +7, with +4, +5, and +7 being the most common. Technetium dissolves in aqua regia, nitric acid, and concentrated sulfuric acid, but it is not soluble in hydrochloric acid of any concentration. Metallic technetium slowly tarnishes in moist air and, in powder form, burns in oxygen. Technetium can catalyse the destruction of hydrazine by nitric acid, and this property is due to its multiplicity of valencies. This caused a problem in the separation of plutonium from uranium in nuclear fuel processing, where hydrazine is used as a protective reductant to keep plutonium in the trivalent rather than the more stable tetravalent state. The problem was exacerbated by the mutually enhanced solvent extraction of technetium and zirconium at the previous stage, and required a process modification. Compounds Pertechnetate and derivatives The most prevalent form of technetium that is easily accessible is sodium pertechnetate, Na[TcO4]. The majority of this material is produced by radioactive decay from [99MoO4]2−: [99MoO4]2− → [99mTcO4]− + e− Pertechnetate (tetroxidotechnetate) behaves analogously to perchlorate, both of which are tetrahedral. Unlike permanganate (), it is only a weak oxidizing agent. Related to pertechnetate is technetium heptoxide. This pale-yellow, volatile solid is produced by oxidation of Tc metal and related precursors: 4 Tc + 7 O2 → 2 Tc2O7 It is a molecular metal oxide, analogous to manganese heptoxide. It adopts a centrosymmetric structure with two types of Tc−O bonds with 167 and 184 pm bond lengths. Technetium heptoxide hydrolyzes to pertechnetate and pertechnetic acid, depending on the pH: Tc2O7 + 2 OH− → 2 TcO4− + H2O Tc2O7 + H2O → 2 HTcO4 HTcO4 is a strong acid. In concentrated sulfuric acid, [TcO4]− converts to the octahedral form TcO3(OH)(H2O)2, the conjugate base of the hypothetical triaquo complex [TcO3(H2O)3]+. Other chalcogenide derivatives Technetium forms a dioxide, disulfide, diselenide, and ditelluride. An ill-defined Tc2S7 forms upon treating pertechnate with hydrogen sulfide. It thermally decomposes into disulfide and elemental sulfur. Similarly the dioxide can be produced by reduction of the Tc2O7. Unlike the case for rhenium, a trioxide has not been isolated for technetium. However, TcO3 has been identified in the gas phase using mass spectrometry. Simple hydride and halide complexes Technetium forms the simple complex . The potassium salt is isostructural with . The following binary (containing only two elements) technetium halides are