Correct Answer - Option 4 : none of these
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 (Li, Na, K, etc.) always have +1, and alkaline earth metals (Be, Mg, 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:
|Cl-, Br-, I-, NO3-, N3-, S2- etc
Ethylenediamine(en), water, ammonia, CO, PPh3, etc
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.