Question

Which of the following complexes can act as an oxidizing agent as well as reducing agent?
A. \[Ti{{\left( CO \right)}_{6}}\]
B. \[Mn{{\left( CO \right)}_{3}}\]
C. \[Mn{{\left( CO \right)}_{6}}\]
D. None of the above

Answer 1

Hint: The oxidation agent is a substance which is used to oxidize another compound (loss electron from itself or increase oxidation number) and reduce itself (means gain electron from another compound or reduce oxidation number of itself). Whereas the reducing agent is the opposite of oxidizing agent. The reducing agent is a substance which is used to reduce other compounds and tend to oxidize itself. The compound which is an oxidizing agent and which is a reducing agent can be determined by the formula of effective atom in number (EAN) which is equal to the atomic number of central metal (Z) – oxidation charge on central metal atom (a) + number of electron donated by ligands (number of legends, n + electron donated by one ligand, b) such as
\[E.A.N\text{ }=\text{ }\left( Z-a \right)+\left( n\times b \right)\]

Complete Step by Step Answer:
In the given option we are provided with complex compounds (central metal and ligands). In the first option, titanium is the central atom and CO is a ligand (lone pair donor) as there are 6 CO ligands present so we can say 12 electrons will get donated by 6 CO (6 lone pairs). Also, there is no charge for titanium. Now the atomic number of titanium is 22 which accepts 12 electrons from 6 CO ligands in the empty orbital. So the total number of electron titanium having is 34 which is an effective atomic number (22 + 12) but the number of electrons which stable in the atom is 36 so it has a tendency to accept electrons so it tends to act as an oxidizing agent (tends to gain itself and increase oxidation number of another compound) such as
\[E.A.N\text{ }=\text{ }\left( Z-a \right)+\left( n\times b \right)\]
\[E.A.N\text{ }=\text{ }\left( 22-0 \right)+\left( 6\times 2 \right)\]
\[E.A.N\text{ }=\text{ 22+12}\]
\[E.A.N\text{ }=\text{ 34}\]

In the second option, Manganese is the central atom and 3 CO are donating three lone pairs (ligands) also there is no charge on central atom so the effective atomic number of the central atom is given as an atomic number of Mn + number of electron donated by legends so 25 + 6 = 31 so aging it is less than 36 so it tends to act as the oxidizing agent such as
\[E.A.N\text{ }=\text{ }\left( Z-a \right)+\left( n\times b \right)\]
\[E.A.N\text{ }=\text{ }\left( 25-0 \right)+\left( 3\times 2 \right)\]
\[E.A.N\text{ }=\text{ 25+6}\]
\[E.A.N\text{ }=\text{ 31}\]
In the third compound, Mn is the central atom and 6 Co ligands are given so an effective atomic number is given at 25 + 12 (atomic number of Mn + number of electrons donated by ligands) which is equal to 37. As 37 is treated more than 36 so it tends to give one electron to get stable so it will act as reducing when (tends to reduce other and increase oxidation number of itself) such as
\[E.A.N\text{ }=\text{ }\left( Z-a \right)+\left( n\times b \right)\]
\[E.A.N\text{ }=\text{ }\left( 25-0 \right)+\left( 6\times 2 \right)\]
\[E.A.N\text{ }=\text{ 25+12}\]
\[E.A.N\text{ }=\text{ 37}\]
Thus, the correct option is D as there is no substance which acts as a reducing and as well as an oxidizing agent.

Note: The effective atomic number determines the total number of electrons which is surrounded by the nucleus of the central metal atom. Now as per the atomic number of noble gases the atom with atomic numbers 2, 10, 18, 36, and so on. In the given question, the number of electrons which are surrounded by the central metal nucleus in an atom is around 30 so we compared all EAN with atomic number 36 for the stable metal atom.