Editing Group 7 element (section)

== Applications ==

[[File:Fac-MbpyCO3X.png|thumb|Structure of the facial isomer of M(R-bpy)(CO)<sub>3</sub>X where M = Mn, Re; X = Cl, Br; R-bpy = 4,4'-disubstituted-2,2'-bipyridine]]
The [[Octahedral molecular geometry|''facial'' isomer]] of both rhenium and manganese 2,2'-bipyridyl tricarbonyl halide complexes have been extensively researched as catalysts for [[Electrochemical reduction of carbon dioxide|electrochemical carbon dioxide reduction]] due to their high selectivity and stability. They are commonly abbreviated as M(R-bpy)(CO)<sub>3</sub>X where M = Mn, Re; R-bpy = 4,4'-disubstituted [[2,2′-Bipyridine|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.<ref name=":2" /> The first reports of catalytic activity of Mn(R-bpy)(CO)<sub>3</sub>Br towards CO<sub>2</sub> reduction came from Chardon-Noblat and coworkers in 2011.<ref>{{Cite journal|last=Bourrez|first=Marc|date=2011|title=[Mn(bipyridyl)(CO)3Br]: an abundant metal carbonyl complex as efficient electrocatalyst for CO2 reduction|journal=Angewandte Chemie International Edition in English |volume=50|issue=42 |pages=9903–9906|doi=10.1002/anie.201103616 |pmid=21922614 }}</ref> Compared to Re analogs, Mn(R-bpy)(CO)<sub>3</sub>Br shows catalytic activity at lower overpotentials.<ref name=":3" />

The catalytic mechanism for Mn(R-bpy)(CO)<sub>3</sub>X 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)<sub>3</sub>]<sub>2</sub> before forming the active species. When R is bulky, however, the complex forms the active species without dimerizing, reducing the overpotential of CO<sub>2</sub> reduction by 200-300 mV. Unlike Re(R-bpy)(CO)<sub>3</sub>X, Mn(R-bpy)(CO)<sub>3</sub>X only reduces CO<sub>2</sub> in the presence of an acid.<ref name=":3" />

=== Technetium ===

[[File:Basedow-vor-nach-RIT.jpg|thumb|upright|Technetium [[Nuclear medicine|scintigraphy]] of a neck of [[Graves' disease]] patient|alt=Upper image: two drop-like features merged at their bottoms; they have a yellow centre and a red rim on a black background. Caption: Graves' Disease Tc-Uptake 16%. Lower image: red dots on black background. Caption: 250 Gy (30mCi) + Prednison.]]

[[Technetium-99m]] ("m" indicates that this is a [[Nuclear isomer#Metastable isomers|metastable]] nuclear isomer) is used in radioactive isotope [[nuclear medicine|medical tests]]. For example, Technetium-99m is a [[radioactive tracer]] that medical imaging equipment tracks in the human body.<ref name="blocks" /><ref name="bbc-20150530">{{cite news |url=https://www.bbc.co.uk/news/magazine-32833599 |title=The element that can make bones glow |author=Laurence Knight |publisher=BBC |date=30 May 2015 |access-date=30 May 2015}}</ref><ref>{{cite journal|display-authors=4|author=Guérin B|author2=Tremblay S|author3=Rodrigue S|author4=Rousseau JA |author5=Dumulon-Perreault V|author6=Lecomte R|author7=van Lier JE|author8=Zyuzin A|author9=van Lier EJ |title=Cyclotron production of <sup>99m</sup>Tc: an approach to the medical isotope crisis|journal=Journal of Nuclear Medicine |date=2010|volume=51|issue=4|pages=13N–6N|pmid=20351346 |url=http://jnm.snmjournals.org/content/51/4/13N.full.pdf}}</ref> It is well suited to the role because it emits readily detectable 140&nbsp;[[Electronvolt|keV]] [[gamma ray]]s, and its half-life is 6.01&nbsp;hours (meaning that about 94% of it decays to technetium-99 in 24&nbsp;hours).<ref name="enc">{{cite book| title=The Encyclopedia of the Chemical Elements| editor=Hampel, C. A.| last=Rimshaw |first=S. J.| location=New York| publisher=Reinhold Book Corporation| date=1968| url=https://archive.org/details/encyclopediaofch00hamp| url-access=registration| pages=[https://archive.org/details/encyclopediaofch00hamp/page/689 689–693]}}</ref> The chemistry of technetium allows it to be bound to a variety of biochemical compounds, each of which determines how it is metabolized and deposited in the body, and this single isotope can be used for a multitude of diagnostic tests. More than 50 common [[radiopharmaceuticals]] are based on technetium-99m for imaging and functional studies of the [[Human brain|brain]], heart muscle, [[thyroid]], [[Human lung|lungs]], [[liver]], [[gall bladder]], [[kidney]]s, [[Human skeleton|skeleton]], [[blood]], and [[tumor]]s.{{sfn|Schwochau|2000|p=414}} Technetium-99m is also used in radioimaging.<ref>{{cite book|first1=Roger |last1=Alberto|first2=Qaisar|last2=Nadeem|title=Metal Ions in Bio-Imaging Techniques|publisher=Springer|year=2021|pages=195–238|chapter=Chapter 7. <sup>99m</sup> Technetium-Based Imaging Agents and Developments in <sup>99</sup>Tc Chemistry|doi=10.1515/9783110685701-013|s2cid=233684677}}</ref>

The longer-lived isotope, technetium-95m with a half-life of 61&nbsp;days, is used as a [[radioactive tracer]] to study the movement of technetium in the environment and in plant and animal systems.{{sfn|Schwochau|2000|pp=12–27}}

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 [[National Institute of Standards and Technology]] (NIST) standard beta emitter, and is used for equipment calibration.{{sfn|Schwochau|2000|p=87}} Technetium-99 has also been proposed for optoelectronic devices and [[nanotechnology|nanoscale]] [[nuclear battery|nuclear batteries]].<ref>{{cite report|date = 2006-11-30|title = University Research Program in Robotics REPORT|publisher = University of Florida|url = http://www.osti.gov/bridge/servlets/purl/895620-n4Nt3U/895620.PDF|access-date = 2007-10-12|doi = 10.2172/895620|author1 = James S. Tulenko|author2 = Dean Schoenfeld |author3 = David Hintenlang|author4 = Carl Crane|author5 = Shannon Ridgeway|author6 = Jose Santiago|author7 = Charles Scheer}}</ref>

Like [[rhenium]] and [[palladium]], technetium can serve as a [[catalyst]]. In processes such as the [[dehydrogenation]] of [[isopropyl alcohol]], it is a far more effective catalyst than either rhenium or palladium. However, its radioactivity is a major problem in safe catalytic applications.{{sfn|Schwochau|2000|pp=87–90}}

When steel is immersed in water, adding a small concentration (55&nbsp;[[parts per notation|ppm]]) of potassium pertechnetate(VII) to the water protects the [[steel]] from corrosion, even if the temperature is raised to {{convert|250|C|K|abbr=on}}.{{sfn|Emsley|2001|p=425}} For this reason, pertechnetate has been used as an anodic [[corrosion]] inhibitor for steel, although technetium's radioactivity poses problems that limit this application to self-contained systems.<ref>{{cite book|chapter=Ch. 14 Separation Techniques |date=July 2004 |title=EPA: 402-b-04-001b-14-final |publisher=US Environmental Protection Agency |chapter-url=https://www.epa.gov/sites/production/files/2015-05/documents/402-b-04-001b-14-final.pdf |archive-url=https://web.archive.org/web/20140308042639/http://www.epa.gov/radiation/docs/marlap/402-b-04-001b-14-final.pdf |archive-date=2014-03-08 |url-status=live |access-date=2008-08-04}}</ref> While (for example) {{chem|CrO|4|2-}} 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&nbsp;years and was still uncorroded.{{sfn|Emsley|2001|p=425}} The mechanism by which pertechnetate prevents corrosion is not well understood, but seems to involve the reversible formation of a thin surface layer ([[Passivation (chemistry)|passivation]]). One theory holds that the pertechnetate reacts with the steel surface to form a layer of technetium [[oxide|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.{{sfn|Schwochau|2000|p=91}}

As noted, the radioactive nature of technetium (3&nbsp;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 reactor]]s.{{sfn|Schwochau|2000|p=91}}

=== Rhenium ===

The catalytic activity of Re(bpy)(CO)<sub>3</sub>Cl for carbon dioxide reduction was first studied by Lehn et al.<ref>{{Cite journal|last=Hawecker|first=Jeannot|date=1984|title=Electrocatalytic Reduction of Carbon Dioxide Mediated by Re(bipy)(CO)3Cl (bipy = 2,2'-bipyridine)|journal=J. Chem. Soc., Chem. Commun.|pages=328–330|doi=10.1039/C39840000328 }}</ref> and Meyer et al.<ref>{{Cite journal|last=Sullivan|first=B. Patrick|date=1985|title=One- and Two-electron Pathways in the Electrocatalytic Reduction of CO2 by fac-Re(bpy)(CO)3Cl (bpy = 2,2'-bipyridine)|journal=J. Chem. Soc., Chem. Commun.|pages=1414–1416|doi=10.1039/C39850001414 }}</ref> in 1984 and 1985, respectively. Re(R-bpy)(CO)<sub>3</sub>X complexes exclusively produce CO from CO<sub>2</sub> reduction with [[Faraday efficiency|Faradaic efficiencies]] of close to 100% even in solutions with high concentrations of water or [[Brønsted–Lowry acid–base theory|Brønsted acids]].<ref name=":2">{{Cite journal|last=Grice|first=Kyle|date=2014|title=Recent Studies of Rhenium and Manganese Bipyridine Carbonyl Catalysts for the Electrochemical Reduction of CO2|journal=Advances in Inorganic Chemistry|volume=66|pages=163–188|doi=10.1016/B978-0-12-420221-4.00005-6 |isbn=9780124202214 }}</ref>

The catalytic mechanism of Re(R-bpy)(CO)<sub>3</sub>X involves reduction of the complex twice and loss of the X ligand to generate a five-coordinate active species which binds CO<sub>2</sub>. These complexes will reduce CO<sub>2</sub> both with and without an additional acid present; however, the presence of an acid increases catalytic activity.<ref name=":2" /> The high selectivity of these complexes to CO<sub>2</sub> reduction over the competing [[Water splitting|hydrogen evolution reaction]] has been shown by [[density functional theory]] studies to be related to the faster kinetics of CO<sub>2</sub> binding compared to H<sup>+</sup> binding.<ref name=":3">{{Cite journal|last=Francke|first=Robert|date=2018|title=Homogeneously Catalyzed Electroreduction of Carbon Dioxide -- Methods, Mechanisms, and Catalysts|journal=Chemical Reviews|volume=118|issue=9 |pages=4631–4701|doi=10.1021/acs.chemrev.7b00459 |pmid=29319300 }}</ref>

=== Bohrium ===

Bohrium is a synthetic element and is too radioactive to be used in anything.