Compounds of Manganese, Technetium and Rhenium

Oxidation of the decacarbonyldimetal(O) compounds of manganese, technetium, and rhenium by halogens gives the corresponding halopentacarbonyhnetal(I) complexes (see equation 34). 

The decacarbonyls of manganese, technetium, and rhenium, of formula M2(CO)io, have terminal carbonyl groups and a metal-metal bond. The molecular symmetry is D d with the two M(CO)s fragments in a staggered conformation. The heterodinuclear decacarbonyl MnRe(CO)io is also known as obtained by the redox reaction of a rhenium pentacarbonyl halide with the pentacarbonylmanganate(-I) anion at room temperature (at 160 °C or higher, the heterodinuclear carbonyl tends to form the homodinuclear compounds with an equilibrium constant close to the statistical value ). The X-ray diffraction stndy of MnRe(CO)io has shown the Mn-Re distance of 2.909 A to be shorter than the sum of the covalent radii obtained from the homodinuclear compounds (Table 3). 

Technetium and rhenium differ markedly from manganese, but they are very similar to each other. They have little cationic chemistry, few compounds in the oxidation state II, more extensive chemistry in the IV and V states. The metals resemble Pt in their appearance (usually, however, they are in the form of a grey powder) they tarnish slowly in moist air, do not react with water. Metal dust is a fire and explosion hazard. 

Many compounds of technetium and rhenium are of analogous composition and of corresponding physical and chemical properties. Because of the very similar ionic radii, isotypic crystal structure formation of analogous compounds could often be observed. Technetium remarkably differs from manganese by the high stability of pertechnetate compared with permanganate. Moreover, divalent technetium does not exist as a hydrated ion but only as a stabilized complex. 

The synthesis of pentacarbonyl rhenium(I) halides, Re(CO)5X, succeeded from simple and complex rhenium halides below 200 atm of CO at 200° C. The compounds are extraordinarily stable and form easily, often quantitatively, from carbon monoxide and rhenium metal in the presence of other heavy metal halides or halogen sources such as CC14. Later we prepared the corresponding carbonyl halides of manganese (67) and technetium (68) from their respective carbonyls. It was found that the corresponding binuclear tetracarbonyl halides [M(CO)4X]2 (M = Mn, Re) could be made by heating the mononuclear M(CO)5X complexes (15, 69), as well as by other methods. 

Tetrahedral [Mn(NCSe)4] and octahedral [Mn(NCSe)g] anions are known (162, 295, 6i7, 655, 656), and [Re2(NCSe)8] has been tentatively identified (377). There seem to be no reports of similar Tc compounds, nor indeed of any mixed-ligand selenocyanate complexes of either technetium or rhenium. Some mixed-ligand complexes of manganese(II) selenocyanate have been prepared, and in every case so far described the selenocyanate is N-bonded. Analogous to the thio-... 

Under the influence of water, cationic manganese compounds decompose immediately, as in organic donor solvents such as acetone and tetrahydrofuran. In contrast to isoelectronic compounds of the chromium group, diolefin technetium and rhenium complexes have cis structures. In aqueous solution, the cation [Re(CO)4 ( 2114)2] is stable the Re —C2H4 bond in [Re(CO)5 ( 2114)] is also stable. There is no exchange between free ethylene and the ethylene in the complex. 

This facile approach to the carbene chemistry of rhenium has not yet been investigated with technetium. Further reactions with the technetium cluster 44a have been performed in C6H6/HC1 to yield the compound [( 6H6)Tc(CO)3]+ (66) which previously had only been described for manganese and rhenium [81]. "Tc-NMR of the latter compound exhibits a resonance at -1983 ppm (relative to [Tc04]- ), and it therefore fits very well into the range proposed for Tc(I) complexes. 

These metals form chalcogenolate complexes in several oxidation states, and from the application-oriented point of view manganese compounds have been synthesized as models for hydrodesulfurization processes and rhenium and technetium derivatives as models for radiopharmaceuticals. 

Manganese is a reactive metal that has several oxidation states (2, 3, 4, 6, and 7) that are responsible for its varied chemical compounds. The chemical and physical properties of manganese are similar to the properties of its companions in group 7—technetium ( jTc) and rhenium ( jRe). 

Reports on metallacyclopentane compounds of the group 7 transition metals are restricted to those of manganese and rhenium. To our knowledge, no metallacyclopentane compounds have been reported for technetium. 

The congeners of manganese are very rare. The middle element of the group, technetium (Z = 43), has not been isolated from mineral sources, and there is good reason to suppose that detectable quantities do not occur naturally (Chap. 27, Exercise 7). Weighable amounts of this element and its compounds have been made from molybdenum by nuclear displacement and from uranium by nuclear fission (p. 474). Rhenium (Z =5 75) occurs naturally but in only tiny amounts. It has been estimated that there is one atom of rhenium present in the earth s crust for each two billion (2 X 109) atoms of silicon. Rhenium was discovered in 1925, technetium in 1937. 

Only one compound of technetium, an oxide, has been investigated during the period covered by this Report. Interest in manganese and rhenium has been focused mainly on various halide, oxo-, and carbonyl complexes. 

Three of these elements were soon discovered (they were named scandium, gallium, and germanium by their discoverers), and it was found that their properties and the properties of their compounds are very close to those predicted by Mendelyeev for eka-boron, eka-aluminum, and eka-silicon, respectively. Since then the elements technetium, rhenium, and protactinium have been discovered or made artificially, and have been found to have properties similar to those predicted for eka-manganese, dvi-manganese, and eka-tantalum. A comparison of the properties predicted by Mendelyeev for eka-silicon and those determined experimentally for germanium is given below. 

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