Question

Which among the following will be named as dibromobis (ethylenediamine) chromium (lll) bromide?
(A) $\left[ {Cr{{(en)}_3}} \right]B{r_3}$
(B) $\left[ {Cr{{(en)}_2}B{r_2}} \right]Br$
(C) ${\left[ {Cr(en)B{r_4}} \right]^ - }$
(D) $\left[ {Cr(en)B{r_2}} \right]Br$

Answer 1

While naming a coordination complex, the cation is named first and then the anion. Also when the cation is a complex we first name the ligands in an alphabetical sequence and then the central metal ion.

Complete answer:
In this what we basically have to do is identify the coordination complex with this particular given name. So, first let us see in short what a coordination complex is.
-A coordination complex is basically a complex molecule which is made up of a central atom or ion surrounded by ligands or complexing agents. The metal ion or say the central atom is the coordination centre of the complex.
-Now let us take a short look at how these coordination compounds are named.
The naming can be done in the following manner:

1. In the complex the ligands are named before the metal ion.
2. The naming of the ligands is done according to their alphabetical order and the sequence is not affected by the numerical prefixes.
3. The cation is named before the anion.
4. When the cation is complex, we first name the ligand alphabetically and then the central atom. The oxidation state of the central atom is written alongside in roman numeral in common brackets.
   -So, now let us try and deduce the formula of the coordination complex named.
   The name of the coordination complex is: dibromobis (ethylenediamine) chromium (lll) bromide.
   So, from this we can tell that the name of the cationic part is: dibromobis (ethylenediamine) chromium (lll) and that of the anionic part is: bromide. This tells us that the anionic part contains just 1 Br atom.
   Now we will bring out the formula of the cation. Since in the cation we first name the ligands and then the metal ion with its oxidation state in brackets, we can tell that the central atom is Cr (lll) and the name of the ligands is: dibromobis (ethylenediamine). For ligands: dibromide means 2 atoms of Br (which is a monodentate ligand) and bis(ethylenediamine) means 2 attached ethylenediamine (which is a polydentate ligand).
   We now have all the constituents of the complex so let us put together the formula. The molecular formula would be: $\left[ {Cr{{(en)}_2}B{r_2}} \right]Br$

Hence the correct option will be: (B) $\left[ {Cr{{(en)}_2}B{r_2}} \right]Br$ .

Note:
A monodentate ligand is one which has the ability to attach to the metal ion via one position only and when they are present in multiple numbers in a complex, the prefix used is: di-, tri-, tetra-, penta-, etc. Example of such ligands are: $B{r^ - }$, $C{l^ - }$, ${I^ - }$, ${F^ - }$, $N{H_3}$, $O{H^ - }$, CO, etc.
A polydentate ligand is one which has the ability to attach to the metal ion from more than one position and when they are present in multiple numbers in a complex, the prefix used is: bis-, tris-, tetrakis-, etc. Examples of such ligands are: ethylenediamine (en), oxalate (ox), etc.