Group 6 of the periodic table consists of Chromium, Molybdenum and Tungsten. Tungsten has d4s2 electronic configuration, while Chromium and Molybdenum have d5s1 as the ground state configuration.
| Properties | Cr | Mo | W |
|---|---|---|---|
| Atomic Number | 24 | 42 | 74 |
| Electronic Configuration | [Ar] 3d5 4s1 | [Kr] 4d5 5s1 | [[Xe] 4f14 5d4 6s2 |
| Metallic radius (nm) | 0.127 | 0.139 | 0.146 |
| M3+ radius (nm) | 0.064 | - | - |
| M4+ radius (nm) | 0.055 | 0.068 | 0.068 |
| I1 (kJ mol-1) | 653 | 692 | 770 |
Because of lanthanide contraction, the metallic radii of Mo and W are the same.
Chromium has an expectedly low I1.
They are silvery-white and soft. They have melting points. Tungsten has the highest melting point among them; hence, it is used as an electric light filament.
Chromium is a strong reducing agent. Because of the formation of a protective oxide coating around the metal surface of chromium, its reactivity is reduced, so it is used as a plating on steel to prevent its corrosion.
Oxidation States
|
Oxidation States |
Config. | Cr | Mo | W |
|---|---|---|---|---|
| -II | d8 | Cr(CO)52- | Mo(CO)52- | - |
| -I | d7 | Cr2(CO)52- | - | - |
| 0 | d6 | Cr(C6H6)2 | Mo(CO)3py3 | W(CO)6 |
| +I | d5 | Cr(dipy)3+ | (C6H6)2Mo+ | - |
| +II | d4 | CrCl2 | (MoCl2)6 | (WCl2)6 |
| +III | d3 | Cr2O3 | MoCl3 | WBr3 |
| +IV | d2 | Ba2CrO4 | MoS2 | WO2 |
| +V | d1 | CrF5 | MoF5 | WCl5 |
| +VI | d0 | K2CrO4 | MoF6 | WBr6 |
Chromium is a powerful oxidant and has the highest oxidation state of +VI. However, for Molybdenum and Tungsten, the +VI state is stable to reduction.
Since Cr2+ is a powerful reducing agent, it doesn’t exist in an aqueous medium.
The π-accepter CO stabilizes the lowest oxidation states.
Halides
Chromium forms four halides in both the +II and +III oxidation states. Its +IV, +V and +VI oxidation states are found only in chloride. Molybdenum(II) and tungsten(II) halides are polymeric, and both chlorides and fluorides are known in their +IV state. CrF6, MoF6, WF6, WBr, and WCl6 are the well-known hexahalides. Heating chromium with chlorine produces red violet CrCl3, which varies its intensity with varying temperatures and chloride ion concentration. In the cold, dilute solution, the colour of Cr(H2O)6 is violet, and if the temperature is raised and the concentration of Cl– ion is increased, the colour changes to green because of the following reaction:
Three isomers of Cr(H2O)6Cl3 can be crystallized depending on the conditions.
Caution
Hexavalent Chromium is toxic and carcinogenic; hence, it should be handled with care in laboratories. We should avoid spillage; if it spilt, then careful cleaning and disposal are required. Chromium is an environmental hazard in areas where leather tanners are operated.
Biological Uses
Molybdenum is an essential constituent of enzymes in a biological system that oxidizes aldehydes, xanthines and other purines and reduces nitrate and molecular nitrogen. Molybdenum is helpful in biological activity as it provides facile electron transfer pathways, exists in different oxidation states, forms bonds with Nitrogen, Oxygen and Sulphur donors and forms labile complexes with easy exchange of ligands.
Chromium is an essential trace nutrient required for regular sugar and fat metabolism. It is helpful for the normal action of insulin and is produced in the liver, kidney, spleen and bone. Chromium deficiency in humans causes impaired insulin function, obesity and tiredness.
