Question

The oxidation number, $d$-orbital occupation and coordination number of $Cr$ in the complex $cis[Cr(en{)}_{2}C{l}_2]Cl$ are respectively

• A. +3, 3d and 4
• B. +3, 4d and 6
• C. +3, 3d and 6
• D. +2, 3d and 6

Answer 1

Answer: (C)

Let the oxidation number of $Cr$ is $x$.
cis $\left[Cr(\text{ en }{)}_{2}C{l}_2\right]Cl$
$x+(0)\times 2+(-1)\times 2-1=0$
$x-3=0x=+3$
In $\left[Cr(en{)}_{2}C{l}_2\right]Cl$,
the $Cr$ atom is present as $C{r}^{3+}$.
$C{r}^{3+}=[Ar]3d^{3}4s^0$
$cis\left[Cr(en{)}_{2}C{l}_2\right]Cl=[Ar]$
(en being a strong field ligand, can cause pairing.) Thus,3d orbitals are occupied.
Since, two $Cl$ atoms (monodentate) and two ettiylene diammine, ie, en molecules
(didentate) are linked two the central atom, the coordination number of central atom $=2+\underset{(\text{due to en })}{\underbrace{2\times 2}}=6$