Question

The primary and secondary valencies of chromium in the complex ion, \[ [\mathrm{CrCl_2(C_2O_4)_2}]^{3-} \] are respectively \(x\) and \(y\). The sum of \(x\) and \(y\) is

• A. \(7\)
• B. \(8\)
• C. \(9\)
• D. \(6\)

Hint:

For coordination compounds, \[ \boxed{\text{Primary valency}=\text{Oxidation state}} \] and \[ \boxed{\text{Secondary valency}=\text{Coordination number}.} \] A bidentate ligand contributes two coordination sites.

Answer 1

The Correct Option is C

Step 1: Find the primary valency of chromium. Let the oxidation state of chromium be \(x\). Since, \[ 2(\mathrm{Cl^-})=-2, \] and \[ 2(\mathrm{C_2O_4^{2-}})=-4, \] the overall charge is \[ -3. \] Therefore, \[ x-2-4=-3, \] \[ x=+3. \] Hence, the primary valency is \[ \boxed{3.} \]

Step 2: Find the secondary valency. Secondary valency is the coordination number. Here, \[ 2\ \mathrm{Cl^-} \] contribute \[ 2 \] coordination sites and each oxalate ligand is bidentate. Thus, \[ 2+2\times2=6. \] Hence, the secondary valency is \[ \boxed{6.} \]

Step 3: Calculate \(x+y\). \[ x+y=3+6=9. \] Hence, \[ \boxed{9}. \] Thus, \[ \boxed{(C)} \] is the correct answer.