Question

The coordination number and oxidation number of \(\mathrm{Cr}\) in \(\mathrm{K}\left[\mathrm{Cr}\left(\mathrm{H}_2\mathrm{O}\right)_2\left(\mathrm{C}_2\mathrm{O}_4\right)_2\right]\) are:

• A. 4 and \(+3\)
• B. 3 and \(+3\)
• C. 6 and \(+2\)
• D. 6 and \(+3\)

Hint:

For coordination compounds:
- Coordination number = number of ligand binding sites.
- Oxidation number: Sum charges of ligands and complex.
- Bidentate ligands (e.g., oxalate) occupy two sites.

Answer 1

The Correct Option is D

Step 1: Coordination number
The complex is \(\left[\mathrm{Cr}\left(\mathrm{H}_2\mathrm{O}\right)_2\left(\mathrm{C}_2\mathrm{O}_4\right)_2\right]^-\). - \(\mathrm{H}_2\mathrm{O}\): Monodentate, occupies 1 coordination site each (\(2 \times 1 = 2\)). - \(\mathrm{C}_2\mathrm{O}_4^{2-}\) (oxalate): Bidentate, occupies 2 sites each (\(2 \times 2 = 4\)). Total coordination number = \(2 + 4 = 6\). The complex is octahedral.
Step 2: Oxidation number
The complex ion is \(\left[\mathrm{Cr}\left(\mathrm{H}_2\mathrm{O}\right)_2\left(\mathrm{C}_2\mathrm{O}_4\right)_2\right]^-\), with charge \(-1\). - \(\mathrm{H}_2\mathrm{O}\): Neutral, charge 0. - \(\mathrm{C}_2\mathrm{O}_4^{2-}\): Each oxalate has \(-2\), so \(2 \times (-2) = -4\). - Cr: Let oxidation number be \(x\). Charge of complex: \[ x + 0 + (-4) = -1 \quad \Rightarrow \quad x - 4 = -1 \quad \Rightarrow \quad x = +3 \] Outside, \(\mathrm{K}^+\) balances the \(-1\) charge of the complex.
Step 3: Conclusion Coordination number is 6, oxidation number is \(+3\), matching option (D).