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The oxidation state of chromium in Cr(CO)5 is:
1. 4
2. 2
3. 10
4. none of these

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Correct Answer - Option 4 : none of these

Concept: 

  • The oxidation number/ oxidation state of an element in a compound is the number of charges (positive or negative) assigned to an atom in a molecule or ion according to a set of some arbitrary rules.

Rules for assigning Oxidation number:

  • The oxidation number of all elements in the free state is zero.  
  • The most electronegative element, Fluorine has a -1 oxidation number in all compounds.
  • The oxidation number of monoatomic ions is the same as its charge. For example:
  • ​Oxidation state of Li+ , Ca2+ and Al3+ are +1+2 and +3 respectively.
  • The oxidation state of  Cl and SO42-  are -1 and -2 respectively.
  • Hydrogen has a +1 oxidation state in all compounds except ionic hydrides (-1 oxidation state).
  • The oxidation state of oxygen is -2 in a compound except in peroxides which are -1.
  • Alkali metals (LiNaK, etc.) always have +1, and alkaline earth metals (BeMg, etc.) have +2 oxidation states respectively. 
  • In transition metal complexes, the charge of ligands is taken into consideration.
  • The ligands can be categorized into anionic, cationic, and neutral. The neutral ligands do not carry any charge.
  • Some examples of ligands are:
Ligands Category
Cl-, Br-, I-, NO3-, N3-, S2- etc Anionic ligands
Ethylenediamine(en), water, ammonia, CO, PPh3, etc Neutral ligands
NO+, NO2+ Positive ligands
 

Explanation:

  • Cr(CO)5 is an organometallic complex. Organometallic compounds are complexes that have a Carbon or organic part attached to a metal atom.
  • The metal atom Chromium is attached to the carbonyl ligand.
  • The carbonyl ligand is a Neutral ligand and has no charge.
  • In the complex Cr(CO)5, the central metal atom thus has no charge or zero oxidation state because the ligand is neutral and the complex has no overall charge.
  • The complex is trigonal pyramidal in shape.
  • Hence, the oxidation state of chromium in Cr(CO)5 is zero.

The correct option is none of the above.

  • In the complex, the contribution from chromium is 6 electrons.
  • Contribution from 5 carbonyl ligands is 5 × 2 = 10.
  • Hence, the total number of outermost electrons is 10 + 6 = 16.
  • The complex does not obey 18 electron rule.

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