Q. The oxidation number, $d$-orbital occupation and coordination number of $Cr$ in the complex $ cis[Cr(en)_{2}Cl_{2}]Cl$ are respectively
J & K CETJ & K CET 2010Coordination Compounds
Solution:
Let the oxidation number of $Cr$ is $x$.
cis $\left[ Cr (\text { en })_{2} Cl _{2}\right] Cl$
$x+(0) \times 2+(-1) \times 2-1=0$
$x-3=0 x=+3$
In $\left[ Cr ( en )_{2} Cl _{2}\right] Cl$,
the $Cr$ atom is present as $Cr ^{3+}$.
$C r^{3+}=[A r] 3 d^{3} 4 s^{0}$
$cis\left[ Cr ( en )_{2} Cl _{2}\right] Cl =[ Ar ]$
(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 $=2+\underbrace{2 \times 2}_{(\text {due to en })}=6$
