Question

Find IUPAC name of the hydrate isomer of \[CrC{l_3}.6{H_2}O\], which is having lowest electrical conductivity.
A. \[{\rm{Hexaaquachromium}}\,{\rm{(III)}}\,{\rm{chloride}}\]
B. \[{\rm{Tetraaquadichloridochromium}}\,{\rm{(III)}}\,{\rm{chloridedihydrate}}\]
C. \[{\rm{Pentaaquachloridochromium}}\,{\rm{(III)}}\,{\rm{chloridemonohydrate}}\]
D. \[{\rm{Triaquatrichloridochromium}}\,{\rm{(III)}}\,{\rm{trihydrate}}\]

Answer 1

Hint: Coordination complex: It is a compound in which ligands that act as Lewis bases form coordinate bonds with a central metal that act as Lewis acid to form a complex known as coordination complex. The ions of these compounds do not dissociate on dilution.

Complete answer:
IUPAC naming of a coordination compound follows some rules which are as follows:
The name of action is always written before the name of anion.
Naming of ligands should be done alphabetically if more than one ligand is present in the coordination sphere.
Proper suffix should be added to the name of ligand if it is anionic in nature.
In the case of neutral ligands, common names are written.
The name of the central metal atom comes after the names of ligands.
It is mandatory to mention the oxidation state of the central metal atom as roman numerals in parenthesis.
If there is any number present in the name of ligand, then bis, tris, tetrakis is used for indicating presence of more than one ligand.
If the coordination complex is anionic in nature, then \[ - ate\] suffix is added to the name central metal atom.
For the given compound i.e., \[CrC{l_3}.6{H_2}O\], the isomer which will have lowest electrical conductivity is the compound which has no water molecule in its primary valency. Hence the required isomer is \[\left[ {Cr{{({H_2}O)}_6}} \right]C{l_3}\].
For the given coordination compound, the coordination complex is the cationic part and surrounding atom i.e., chlorine is an anionic part. There are 6 water molecules present in the complex which is a neutral ligand so, \[ - hexa\]prefix will be added to the name of ligand.
Assume the oxidation state of chromium be x.
Therefore,
\[\begin{array}{l}x - 0 = + 3\\x = + 3\end{array}\]
Hence the IUPAC name of the coordination compound is \[{\rm{Hexaaquachromium}}\,{\rm{(III)}}\,{\rm{chloride}}\].

So, option (A) is the correct answer.

Note:
The water molecule as a ligand in coordination complex is named as aqua.
Hydrate isomer: It is a specific type of ionization isomer, in which compounds have similar composition but differ in the presence of water molecules in the primary valency as well as ligands.