Group 7 element
Group 7 in the periodic table | |||||||||
---|---|---|---|---|---|---|---|---|---|
| |||||||||
↓ Period | |||||||||
4 | Manganese (Mn) 25 Transition metal | ||||||||
5 | Technetium (Tc) 43 Transition metal | ||||||||
6 | Rhenium (Re) 75 Transition metal | ||||||||
7 | Bohrium (Bh) 107 Transition metal | ||||||||
Legend
|
Group 7, numbered by
The group 7 elements tend to have a major group oxidation state (+7), although this trend is markedly less coherent than the previous groups. Like other groups, the members of this family show patterns in their electron configurations, especially the outermost shells resulting in trends in chemical behavior. In nature, manganese is a fairly common element, whereas rhenium is rare, technetium only occurs in trace quantities, and bohrium is entirely synthetic.
Physical properties
The trends in group 7 follow, although less noticeably, those of the other early d-block groups and reflect the addition of a filled f-shell into the core in passing from the fifth to the sixth period. All group 7 elements crystallize in the
The table below is a summary of the key physical properties of the group 7 elements. The question-marked value is predicted.[2]
Name | Mn, manganese | Tc, technetium | Re, rhenium | Bh, bohrium |
---|---|---|---|---|
Melting point | 1519 K (1246 °C) | 2430 K (2157 °C) | 3459 K (3186 °C) | Unknown |
Boiling point | 2334 K (2061 °C) | 4538 K (4265 °C) | 5903 K (5630 °C) | Unknown |
Density | 7.21 g·cm−3 | 11 g·cm−3 | 21.02 g·cm−3 | 26-27 g·cm−3?[3][4] |
Appearance | silvery metallic | silvery-gray | silvery-gray | Unknown |
Atomic radius | 127 pm | 136 pm | 137 pm | 128 pm?[2] |
Chemical properties
Like other groups, the members of this family show patterns in its electron configuration, especially the outermost shells:
Z | Element | No. of electrons/shell |
---|---|---|
25 | manganese | 2, 8, 13, 2 |
43 | technetium | 2, 8, 18, 13, 2 |
75 | rhenium | 2, 8, 18, 32, 13, 2 |
107 | bohrium | 2, 8, 18, 32, 32, 13, 2 |
All the members of the group readily portray their group oxidation state of +7 and the state becomes more stable as the group is descended. Technetium also shows a stable +4 state whilst rhenium exhibits stable +4 and +3 states.
Bohrium may therefore also show these lower states as well. The higher +7 oxidation state is more likely to exist in oxyanions, such as perbohrate, BhO4−, analogous to the lighter permanganate, pertechnetate, and perrhenate. Nevertheless, bohrium(VII) is likely to be unstable in aqueous solution, and would probably be easily reduced to the more stable bohrium(IV).[5]
Compounds
Oxides
Manganese
Manganese forms a variety of oxides:
Technetium
Technetium's main oxides are
Technetium(VII) oxide can be prepared directly by the oxidation of technetium at 450-500 °C.
Rhenium
Rhenium's main oxides are
- 2 NaOH + ReO2 → Na2ReO3 + H2O
Rhenium(VII) oxide can be formed when rhenium or its oxides or sulfides are oxidized a 500-700 °C in air.[24] It dissolves in water to give perrhenic acid. Heating Re2O7 gives rhenium(IV) oxide, signalled by the appearance of the dark blue coloration.[25] In its solid form, Re2O7 consists of alternating octahedral and tetrahedral Re centres. It is the raw material for all rhenium compounds, being the volatile fraction obtained upon roasting the host ore.[26]
Rhenium, in addition to the +4 and +7 oxidation states, also forms a
Halides
Manganese
Manganese can form compounds in the +2, +3 and +4 oxidation states. The manganese(II) compounds are often light pink solids. Like some other metal difluorides, MnF2 crystallizes in the
Technetium
The following binary (containing only two elements) technetium halides are known:
TcCl4 is obtained by chlorination of Tc metal or Tc2O7 Upon heating, TcCl4 gives the corresponding Tc(III) and Tc(II) chlorides.[35]
- TcCl4 → α-TcCl3 + 1/2 Cl2
- TcCl3 → β-TcCl2 + 1/2 Cl2
The structure of TcCl4 is composed of infinite zigzag chains of edge-sharing TcCl6 octahedra. It is isomorphous to transition metal tetrachlorides of zirconium, hafnium, and platinum.[35]
Two polymorphs of
Several anionic technetium halides are known. The binary tetrahalides can be converted to the hexahalides [TcX6]2− (X = F, Cl, Br, I), which adopt octahedral molecular geometry.[37] More reduced halides form anionic clusters with Tc–Tc bonds. The situation is similar for the related elements of Mo, W, Re. These clusters have the nuclearity Tc4, Tc6, Tc8, and Tc13. The more stable Tc6 and Tc8 clusters have prism shapes where vertical pairs of Tc atoms are connected by triple bonds and the planar atoms by single bonds. Every technetium atom makes six bonds, and the remaining valence electrons can be saturated by one axial and two bridging ligand halogen atoms such as chlorine or bromine.[38]
Rhenium
The most common rhenium chlorides are
Like tungsten and molybdenum, with which it shares chemical similarities, rhenium forms a variety of oxyhalides. The oxychlorides are most common, and include ReOCl4, ReOCl3.
Organometallic compounds
Manganese
Organomanganese compounds were first reported in 1937 by Gilman and Bailee who described the reaction of phenyllithium and manganese(II) iodide to form phenylmanganese iodide (PhMnI) and diphenylmanganese (Ph2Mn).[39]
Following this precedent, other organomanganese halides can be obtained by alkylation of
The chemistry of organometallic compounds of Mn(II) are unusual among the
Technetium
Technetium forms a variety of coordination complexes with organic ligands. Many have been well-investigated because of their relevance to nuclear medicine.[41]
Technetium forms a variety of compounds with Tc–C bonds, i.e. organotechnetium complexes. Prominent members of this class are complexes with CO, arene, and cyclopentadienyl ligands.
Rhenium
Dirhenium decacarbonyl is the most common entry to organorhenium chemistry. Its reduction with sodium amalgam gives Na[Re(CO)5] with rhenium in the formal oxidation state −1.[47] Dirhenium decacarbonyl can be oxidised with bromine to bromopentacarbonylrhenium(I):[48]
- Re2(CO)10 + Br2 → 2 Re(CO)5Br
Reduction of this pentacarbonyl with zinc and acetic acid gives pentacarbonylhydridorhenium:[49]
- Re(CO)5Br + Zn + HOAc → Re(CO)5H + ZnBr(OAc)
- Re2O7 + (CH3)4Sn → CH3ReO3 + (CH3)3SnOReO3
Analogous alkyl and aryl derivatives are known. MTO catalyses for the oxidations with
Polyoxometalates
The polyoxotechnetate (polyoxometalate of technetium) contains both Tc(V) and Tc(VII) in ratio 4: 16 and is obtained as the hydronium salt [H7O3]4[Tc20O68]·4H2O by concentrating an HTcO4 solution.[51] The first empirically isolated polyoxorhenate was the white [Re4O15]2− and contained Re(VII) in both octahedral and tetrahedral coordination.[52]
History
Manganese
Manganese dioxide, which is abundant in nature, has long been used as a pigment. The cave paintings in Gargas that are 30,000 to 24,000 years old are made from the mineral form of MnO2 pigments.[53] Manganese compounds were used by Egyptian and Roman glassmakers, either to add to, or remove, color from glass.[54] Use as "glassmakers soap" continued through the Middle Ages until modern times and is evident in 14th-century glass from Venice.[55]
Technetium and rhenium
Rhenium (
Rhenium was rediscovered by
The
Bohrium
Two groups claimed
In 1981, a German research team led by Peter Armbruster and Gottfried Münzenberg at the GSI Helmholtz Centre for Heavy Ion Research (GSI Helmholtzzentrum für Schwerionenforschung) in Darmstadt bombarded a target of bismuth-209 with accelerated nuclei of chromium-54 to produce 5 atoms of the isotope bohrium-262:[70]
This discovery was further substantiated by their detailed measurements of the alpha decay chain of the produced bohrium atoms to previously known isotopes of fermium and californium. The IUPAC/IUPAP Transfermium Working Group (TWG) recognised the GSI collaboration as official discoverers in their 1992 report.[69]
Occurrence and production
Manganese
Manganese comprises about 1000
The most important manganese ore is pyrolusite (
In South Africa, most identified deposits are located near
Manganese is mainly mined in South Africa, Australia, China, Gabon, Brazil, India, Kazakhstan, Ghana, Ukraine and Malaysia.[79]
For the production of ferromanganese, the manganese ore is mixed with iron ore and carbon, and then reduced either in a blast furnace or in an electric arc furnace.[80] The resulting ferromanganese has a manganese content of 30 to 80%.[74] Pure manganese used for the production of iron-free alloys is produced by leaching manganese ore with sulfuric acid and a subsequent electrowinning process.[81]
A more progressive extraction process involves directly reducing (a low grade) manganese ore in a heap leach. This is done by
This process yields approximately 92% recovery of the manganese. For further purification, the manganese can then be sent to an electrowinning facility.[82]
In 1972 the
An abundant resource of manganese in the form of Mn nodules found on the ocean floor.[85][86] These nodules, which are composed of 29% manganese,[87] are located along the ocean floor and the potential impact of mining these nodules is being researched. Physical, chemical, and biological environmental impacts can occur due to this nodule mining disturbing the seafloor and causing sediment plumes to form. This suspension includes metals and inorganic nutrients, which can lead to contamination of the near-bottom waters from dissolved toxic compounds. Mn nodules are also the grazing grounds, living space, and protection for endo- and epifaunal systems. When theses nodules are removed, these systems are directly affected. Overall, this can cause species to leave the area or completely die off.[88] Prior to the commencement of the mining itself, research is being conducted by United Nations affiliated bodies and state-sponsored companies in an attempt to fully understand environmental impacts in the hopes of mitigating these impacts.[89]
Technetium
Technetium was created by bombarding
Rhenium
Rhenium is one of the rarest elements in
Most of the rhenium extracted comes from porphyry molybdenum deposits.[103] These ores typically contain 0.001% to 0.2% rhenium.[6] Roasting the ore volatilizes rhenium oxides.[97] Rhenium(VII) oxide and perrhenic acid readily dissolve in water; they are leached from flue dusts and gasses and extracted by precipitating with potassium or ammonium chloride as the perrhenate salts, and purified by recrystallization.[6] Total world production is between 40 and 50 tons/year; the main producers are in Chile, the United States, Peru, and Poland.[104] Recycling of used Pt-Re catalyst and special alloys allow the recovery of another 10 tons per year. Prices for the metal rose rapidly in early 2008, from $1000–$2000 per kg in 2003–2006 to over $10,000 in February 2008.[105][106] The metal form is prepared by reducing ammonium perrhenate with hydrogen at high temperatures:[25]
- 2 NH4ReO4 + 7 H2 → 2 Re + 8 H2O + 2 NH3
- There are technologies for the associated extraction of rhenium from productive solutions of underground leaching of uranium ores.[107]
Bohrium
Bohrium is a synthetic element that does not occur in nature. Very few atoms have been synthesized, and also due to its radioactivity, only limited research has been conducted. Bohrium is only produced in nuclear reactors and has never been isolated in pure form.
Applications
The facial isomer of both rhenium and manganese 2,2'-bipyridyl tricarbonyl halide complexes have been extensively researched as catalysts for electrochemical carbon dioxide reduction due to their high selectivity and stability. They are commonly abbreviated as M(R-bpy)(CO)3X where M = Mn, Re; R-bpy = 4,4'-disubstituted 2,2'-bipyridine; and X = Cl, Br.
Manganese
The rarity of rhenium has shifted research toward the manganese version of these catalysts as a more sustainable alternative.[108] The first reports of catalytic activity of Mn(R-bpy)(CO)3Br towards CO2 reduction came from Chardon-Noblat and coworkers in 2011.[109] Compared to Re analogs, Mn(R-bpy)(CO)3Br shows catalytic activity at lower overpotentials.[110]
The catalytic mechanism for Mn(R-bpy)(CO)3X is complex and depends on the steric profile of the bipyridine ligand. When R is not bulky, the catalyst dimerizes to form [Mn(R-bpy)(CO)3]2 before forming the active species. When R is bulky, however, the complex forms the active species without dimerizing, reducing the overpotential of CO2 reduction by 200-300 mV. Unlike Re(R-bpy)(CO)3X, Mn(R-bpy)(CO)3X only reduces CO2 in the presence of an acid.[110]
Technetium
The longer-lived isotope, technetium-95m with a half-life of 61 days, is used as a radioactive tracer to study the movement of technetium in the environment and in plant and animal systems.[116]
Technetium-99 decays almost entirely by beta decay, emitting beta particles with consistent low energies and no accompanying gamma rays. Moreover, its long half-life means that this emission decreases very slowly with time. It can also be extracted to a high chemical and isotopic purity from radioactive waste. For these reasons, it is a
Like
When steel is immersed in water, adding a small concentration (55
4 can also inhibit corrosion, it requires a concentration ten times as high. In one experiment, a specimen of carbon steel was kept in an aqueous solution of pertechnetate for 20 years and was still uncorroded.[120] The mechanism by which pertechnetate prevents corrosion is not well understood, but seems to involve the reversible formation of a thin surface layer (passivation). One theory holds that the pertechnetate reacts with the steel surface to form a layer of technetium dioxide which prevents further corrosion; the same effect explains how iron powder can be used to remove pertechnetate from water. The effect disappears rapidly if the concentration of pertechnetate falls below the minimum concentration or if too high a concentration of other ions is added.[122]
As noted, the radioactive nature of technetium (3 MBq/L at the concentrations required) makes this corrosion protection impractical in almost all situations. Nevertheless, corrosion protection by pertechnetate ions was proposed (but never adopted) for use in boiling water reactors.[122]
Rhenium
The catalytic activity of Re(bpy)(CO)3Cl for carbon dioxide reduction was first studied by Lehn et al.[123] and Meyer et al.[124] in 1984 and 1985, respectively. Re(R-bpy)(CO)3X complexes exclusively produce CO from CO2 reduction with Faradaic efficiencies of close to 100% even in solutions with high concentrations of water or Brønsted acids.[108]
The catalytic mechanism of Re(R-bpy)(CO)3X involves reduction of the complex twice and loss of the X ligand to generate a five-coordinate active species which binds CO2. These complexes will reduce CO2 both with and without an additional acid present; however, the presence of an acid increases catalytic activity.[108] The high selectivity of these complexes to CO2 reduction over the competing hydrogen evolution reaction has been shown by density functional theory studies to be related to the faster kinetics of CO2 binding compared to H+ binding.[110]
Bohrium
Bohrium is a synthetic element and is too radioactive to be used in anything.
Toxicity and precautions
Manganese compounds are less toxic than those of other widespread metals, such as nickel and copper.[125] However, exposure to manganese dusts and fumes should not exceed the ceiling value of 5 mg/m3 even for short periods because of its toxicity level.[126] Manganese poisoning has been linked to impaired motor skills and cognitive disorders.[127]
Technetium has low chemical toxicity. For example, no significant change in blood formula, body and organ weights, and food consumption could be detected for rats which ingested up to 15 µg of technetium-99 per gram of food for several weeks.
Very little is known about the toxicity of rhenium and its compounds because they are used in very small amounts. Soluble salts, such as the rhenium halides or perrhenates, could be hazardous due to elements other than rhenium or due to rhenium itself.[131] Only a few compounds of rhenium have been tested for their acute toxicity; two examples are potassium perrhenate and rhenium trichloride, which were injected as a solution into rats. The perrhenate had an LD50 value of 2800 mg/kg after seven days (this is very low toxicity, similar to that of table salt) and the rhenium trichloride showed LD50 of 280 mg/kg.[132]
Biological role
Of the group 7 elements, only manganese has a role in the human body. It is an essential trace nutrient, with the body containing approximately 10
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