question_answer

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)  \[+\,3,\text{ }3d\]and 4    

B) \[~+\,3,\text{ }4d\] and 6

C)  \[+\,3,\text{ }3d\]and 6     

D) \[~+\,2,\text{ }3d\] and 6

Correct Answer: C

Solution :

 Let the oxidation number of Cr is \[x.\] \[cis[Cr{{(en)}_{2}}C{{l}_{2}}]Cl\] \[x+(0)\times 2+(-1)\times 2-1=0\] \[x-3=0\] \[x=+\,3\] In \[[Cr{{(en)}_{2}}C{{l}_{2}}]Cl,\] the Cr atom is present as \[C{{r}^{3+}}.\] \[C{{r}^{3+}}=[Ar]\,3{{d}^{3}}4{{s}^{0}}\] \[cis[Cr{{(en)}_{2}}C{{l}_{2}}]Cl=[Ar]\] (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 \[=2+\underbrace{2\times 2}_{(due\,to\,en)}=6\]