![]() Hexavalent chromium was classified by the International Agency for Research on Cancer (IARC) as a human carcinogen (Group 1) associated with increased lung cancer risk among workers in certain industries and also cancer of the nose and nasal sinuses. Dermal exposure through leather articles and cosmetics, and oral exposure of children through toys have been reported. Other sources of exposure to Cr(VI) need to be considered for general population, including the release of Cr, with Cr(VI) as the predominant species, from orthopedic implants made from stainless steel and cobalt-chromium alloys. For more detailed information on chromium concentrations in these environmental compartments, please consult the HBM4EU Scoping document on cadmium and chromium VI. Mobilisation of Cr occurs mainly through air, water and soil. A report on a critical raw material profile by the European Commision in 2014 revealed that the forecast average annual demand for Cr growth of 3%-4.5% per year ( EC Report, 2014, Report on Critical raw materials for the EU). Within the EU, Finland was the main producer of Cr in 2006, producing over 99% of the total EU Cr production (219,500 tonnes). The main sources for EU imports in 2006 were South Africa (approximately 80%). Due to a lack of internal supply and to demand from the steel industry, the EU has been an importer of Cr ores. Major uses of Cr(VI) compounds include metal plating, manufacture of pigments and dyes, corrosion inhibitors, chemical synthesis, refractory production, leather tanning, and wood preservation ( Blade et al., 2007). Most of Cr(VI) compounds are man-made (products or by-products) and human-caused Cr(VI) contamination is a result of large industrial emissions (mainly from metallurgical, chemical, and refractory brick industries). The occurrence of Cr(VI) is rare naturally. Hexavalent form – Cr(VI) – is more toxic than trivalent form – Cr(III) for its high oxidizing potential – and easily penetrates biological membranes. The +3 and +6 forms are the most important as the +2, +4, and +5 forms are unstable and are rapidly converted to +3, which in turn is oxidized to +6 ( Towill et al. The oxidation state of Cr in Cr(CO)6 is 0.Chromium can exist in oxidation states ranging from -2 to +6, but is most frequently found in the environment in the trivalent (+3) and hexavalent (+6) oxidation states. Therefore, we assume that the oxidation state of Cr is zero, as it is the only possible oxidation state for Cr in this complex. However, this value is not a possible oxidation state for Cr. Oxidation state of Cr + Total charge contributed by six CO molecules = Overall charge of the complex Since the overall charge of the complex is zero, the oxidation state of Cr can be calculated as follows: Therefore, the total charge contributed by six CO molecules is (-2) x 6 = -12. ![]() The oxidation state of carbon in CO is -2, as oxygen is more electronegative than carbon, and electrons are shared unequally in the bond. To find the oxidation state of Cr in Cr(CO)6, we assume that all the CO molecules are neutral and the overall charge of the complex is zero. Since there are six CO molecules, the coordination number of Cr is six. Each CO molecule donates a pair of electrons to the Cr atom, which forms a coordinate covalent bond. In Cr(CO)6, each CO molecule is bonded with Cr through a dative covalent bond. The oxidation state of an atom in a molecule refers to the degree of oxidation of that atom. Oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic. Cr(CO)6 is a coordination compound where Cr is bonded with 6 CO molecules. ![]()
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