Question

Which among the following will be named as dibromidobis(ethylenediamine)chromium(III) bromide?

• A. $\left[Cr\right(en{)}_3]B{r}_3$
• B. $\left[Cr\right(en{)}_{2}B{r}_2]Br$
• C. $\left[Cr\right(en)B{r}_4{]}^{-}$
• D. $\left[Cr\right(en\left)B{r}_2\right]Br$

Answer 1

The correct option is B $\left[Cr\right(en{)}_{2}B{r}_2]Br$

Rules to write the formula of coordination from its IUPAC name:
1. Central atom should be listed first
2. Both the monodentate and polydentate ligand are listed in alphabetical orders. In case of abbreviated ligand, the first letter​
of the abbreviation is used to determine the position of the ligand in the alphabetical order
3. When ligands are polyatomic, their formulas are enclosed in parentheses​
4. There should be no space​ between the ligands and​ the metal within a​ coordination sphere.
5. Formula for the entire​ coordination entity​ should be enclosed in square bracket.
6. When the formula of a charged coordination entity is to be written without that of the counter ion,​ the charge is indicated outside the square brackets as a right superscript with the number before the sign. ​
7. The charge of the cation(s) is balanced by the charge of the anion(s).​

Here, $Cr$ is central metal ion and its oxidation state is +3.
Also, two negative $Br$ ligand is present and two neutral bidentane ethylenediamine ligand is present which is represented by the word "Bis(en)".
The charge on coordination entity is balanced by a bromide ion.

Hence, formula of given coordination compound is
$\left[Cr\right(en{)}_{2}B{r}_2]Br$