This is a study of certain aspects of the chemistry of low valency states of the two group VIIa elements, technetium and rhenium. Sections 1 and 2 are on technetium and rhenium, respectively, and Section 3 consists of a comparative review of their chemistry. SECTION 1 Technetium in oxidation states (IV) (III) and (II). New complexes of technetium, with some organic ligands containing group V elements, have been identified. The tetravalent complexes have been prepared by direct reaction between technetium tetrachloride (and tetrabromide) and the ligand. They are all basicall six-coordinate complexes in which technetium has three unpaired electrons. Electronic spectra of these compounds, and the potassium hexahalogenotechnetates (IV), have been studied in detail. The value of Dq for technetium (IV) in a chloride field is ~2400 cm -¹, if the band assignments are correct. The reduction of technetium (IV) to technetium (III) can be brought about be relatively mild reducing agents, e.g. the reaction of technetium tetrachloride (or tetrabromide) with bis(diphenyl-phosphino)ethane, in ethanol, produces the complex [Tc(III)X₂(diphos)₂]X, where X = Cl,Br. This is further reduced by reaction in the cold with sodium borohydride to form [Tc(II)X₂(diphos)₂]. The properties of these compounds are consistent with those expected for six-coordinate d⁴ and d⁵ spin-paired complexes. Several attempts have been made to synthesise technetium trichloride, using a variety of methods; the high stability of the tetrachloride is a dominant factor in the non-formation of the trichloride. SECTION 2 Rhenium(III) “cluster” compounds. Trimeric rhenium trichloride, Re₃Cl₉, has been reacted with several neutral polydentate ligands containing group V elements. From the reaction products (often several), new trimeric complexes have been isolated in which the ligand is attached to one or more metal atoms. For example, in complexes such as [Re₃Cl₈(diarsine)₂]Cl, and [Re₃Cl₈(terp)]Cl, the organic ligand is probably bonded to one rhenium atom; however, in a complex such as Re₃Cl₉(diphos)1.5 (empirical formula), at least some of the ligand groups must be bridged across two metal atoms adjacent trimeric units. The effect of groups attached to the rhenium atom, within the complex anion, has been studied further by the preparation of compounds of types M (x+y-9)[Re₃Clxyy], (where Y is a singly-charged anion e.g. bromide), and M(x+2y-9)[Re₃Clxyy’], (where Y’ is a doubly-charged anion e.g. oxalate); M is a univalent cation and x+y = 10, 11 or 12. A study of the properties of these complexes indicates that the basic unit Re₃Cl₃, is still present in each. The influence of “external” groups in the trimeric anion causes minor shifts in the visible absorption spectra. An interesting compound, which has been formulated as Cs₅Re₄Cl₆Br₁₂, was isolated during the investigations. The physical and chemical properties of this complex indicates that it is a “lattice compound”, consisting of the complex halide, Cs₂Re(IV)Br₆, and the trimeric compound, Cs₃Re₃(III)Cl₆Br₆. The reaction between thionyl chloride and rhenium dioxide (described by R. Colton as a means of preparing Re₃(IV)Cl₁₂) was studied in detail. The main products are usually the monomeric compounds, Re(IV)Cl₃(OH). H₂O, and Re(IV)Cl₃(OH).2H₂O; small amounts of other compounds, e.g. rhenium trichloride, are sometimes formed. The analogous reaction for technetium gives similar results.