Manganese Group
Manganese, Techicium and Rhenium comprises the group VII of the periodic table.
| Properties | Mn | Tc | Re |
|---|---|---|---|
| Atomic Number | 25 | 43 | 75 |
| Electronic Configuration | [Ar] 3d5 4s2 | [Kr] 4d5 5s2 | [Xe] 4f14 5d5 6s2 |
| Metallic radius (nm) | 0.126 | 0.136 | 0.137 |
| Number of natural isotopes | 1 | - | 2 |
| I1 (kJ mol-1) | 653 | 692 | 770 |
| Melting point (K) | 1517 | 2600 | 3450 |
All these elements have the d5s2 configuration. Therefore, the +VII state is their group
valency. This state is observed in MnO4–, TcO4– and ReO4– ions.
Tc and Re solids have hcp lattices; Mn exists in four allotropes; the α-form is the stable allotrope at room temperature; it has a bcc structure.
These three are typical d-block metals with high densities and melting points.
In group 7, the first ionization energy of the second element (Tc) is less than that of the first element, Mn. In this respect, the group is different from group 6 but is similar to the iron and cobalt groups.
Mn is a better-reducing agent than its neighbours, Cr and Fe.
Mn occurs in abundance in nature. Tc was the first synthetic element prepared. Re is a rare element.
Oxidation States
The oxidation states of Mn. Tc and Re in some of their compounds are as follows:
| Oxidation States | Mn | Tc | Re |
|---|---|---|---|
| -I | Mn(CO)5- | - | Re(Co)5- |
| 0 | Mn2(CO)10 | Tc2(CO)10 | Re2(CO)10 |
| +I | Mn(CO)5Cl | K5Tc(CN)6 | Re(CO)5Cl |
| +II | MnX2 | (π-C5H5)2Tc | ReCl2 |
| +III | MnF3 | - | (ReCl3)3 |
| +IV | MnO2 | TcCl4 | K2ReCl6 |
| +V | K3MnO4 | TcOBr3 | ReOCl4- |
| +VI | MnO42- | TcF6 | ReO3 |
| +VII | MnO4- | Tc2O7 | ReO3Cl |
These three elements show a wide range of oxidation states. The higher number of d electrons available for bonding and also the possibility of back donation of electrons from metal to ligand facilitates the formation of multiple oxidation states.
Re has a greater tendency to form compounds with high coordination numbers than Mn and Tc.
All three metals are toxic to humans. Manganism (manganese poisoning) can be caused by long-term exposure to manganese dust.
The group valency +VII is common. Te(VII) and Re(VII) are more stable than Mn(VII), which is also an efficient oxidizing agent. It is more powerful than Cr(VI);
its 3d electrons are more tightly held by its atomic nucleus. TcO, and ReO,
weak oxidizing agents.
The +VI and +V states are less stable than the other states.
The +II state is the most stable oxidation state for Mn due to its d5 configuration.
Te and Re are less reactive than Mn, and they resemble each other closely. They resist oxidation and are only tarnished slowly in moist air.
Mn is known to exist in negative oxidation states; examples:
Mn³–: Mn(NO)3CO
Mn²–: [Mn(phthalocyanine)]²-
Mn¹–: [Mn(CO)5]–
Chemical Nature of Mn, Tc and Re
(i) Finely divided Mn can be burnt in the air to form Mn3O4.
(ii) On heating, Mn reacts with N₂, F2 and Cl₂.
(iii) Mn directly combines with B, C, Si, and P. As and S. Te and Re, when heated in O₂, form volatile heptoxides.
(Te or Re)
(iv) With F₂ they give TcF5 + TcF6 and ReF6 + ReF7.
(v) On heating with S, they form MS2 (TcS2 and ReS2).
(vi) Tc and Re are insoluble in hydrochloric acid and hydrofluoric acid; however, the dissolve in oxidising acids such as concentrated HNO3 and concentrated H2SO4 to form pertechnic acid and perrhenic acid (HMO4).
Oxides
Mn2O7 is a green oil and is formed by the action of con. H₂SO4 on a manganate(VII) salt. It detonates at 95°C. It slowly loses O₂ on standing and forms MnO₂. It oxidises organic compounds explosively. Te2O7 and Re2O7 are yellow solids formed on burning metals in excess of O2. Only Re forms a stable trioxide.
Its blue pentoxide disproportionates to the +VII and +IV species. All three elements form the stable + IV oxides. MnO₂ is the most important of the MO2 compounds.
MnO₂ decomposes to Mn2O3 above 350°C. Hot con. H₂SO4 and HCl reduce it to Mn(II).
The latter reaction is the basis for the manufacture of chlorine. MnO₂ occurs in nature as pyrolusite.
MnO₂ is inert to most of the acids except when heated. It does not dissolve in a hot acid to give Mn(IV) solution but oxidises the acid.
This reaction is used for small-scale generation of Cl₂ in the laboratory. On reaction with H₂SO4 at 110°C, O₂ is evolved, and Mn(III) acid sulphate is formed. MnO₂ is used in dry cell batteries, ferrite production and as a catalyst for the oxidation of alcohols.
Uses
KMnO4 is used in the production of saccharin and benzoic acid. It is a disinfectant used for
purifying water, unlike Cl2, does not impart any taste to water and the MnO2 produced acts as a coagulant for colloidal impurities.
Manganin is a nonferrous alloy (84% Cu, 12% Mn and 4 % Ni) used in electrical instruments because its temperature coefficient of resistivity is almost zero.
As Tc is a synthetic element obtained from nuclear power stations, it has no specific use Tc is used in detecting breast cancer.
Re is used in making mass spectrometer filters and thermocouples and as a catalyst in hydrogenation and dehydrogenation reactions. It is used as a Pt/Re bimetallic catalyst for producing low-lead high-octane petrol.
