Calculate the oxidation number of cobalt in K[Co(CO)_4]

The correct answer is Option (1) → -1

The calculation for the oxidation number of Cobalt ($\text{Co}$) in the complex $\text{K}[\text{Co}(\text{CO})_4]$ is as follows:

1. Potassium ($\text{K}$): Potassium is an alkali metal and always has an oxidation state of $+1$.
2. Carbonyl ($\text{CO}$): Carbonyl is a neutral ligand, so its oxidation state is $0$.
3. Overall Charge: The complex $\text{K}[\text{Co}(\text{CO})_4]$ is a neutral compound, so the sum of the oxidation states must be zero. (In coordination compounds, CO (carbonyl) is a neutral ligand. The oxidation state of the metal is determined by the overall charge on the complex ion and the charge of ligands. )

Let $x$ be the oxidation number of $\text{Co}$.

$\text{Oxidation State of K} + \text{Oxidation State of } [\text{Co}(\text{CO})_4] = 0$

$+1 + x + 4(0) = 0$

$1 + x = 0$

$x = -1$

The oxidation number of Cobalt in $\text{K}[\text{Co}(\text{CO})_4]$ is $\mathbf{-1}$.