Question

Ammonia gas does not evolve from the complex $\text{FeC}{{\text{l}}_{\text{3}}}\text{.4N}{{\text{H}}_{\text{3}}}$ but it gives white precipitate with aqueous solution of $\text{AgN}{{\text{O}}_{3}}$. Coordination number of the central metal ion in the above complex is six. Give the IUPAC name.
A) Ammonium trichloro triammine ferrum (III)
B) Tetraammie ferrum (III) chloride
C) Dicholoro tetraammine ferrate (II) chloride
D) Dicholoro tetraammine ferrum (III) chloride

Answer 1

Knowledge of complex ion chemistry is required to answer this question. Here, in the above complex, the ammonia molecules act as ligands to the ferric ions while the three chloride ions are the counter anions of the ferric-ammonia complex.

Complete step by step solution:
A ligand is a nucleophilic molecule that has a lone pair or an excess or electrons that can be donated to a metal ion that has dearth of electrons. When ammonia molecules are present in a medium along with the tripositive ferric ions, $\text{F}{{\text{e}}^{+3}}$, the lone pair of electrons on the nitrogen atom in the former molecules are donated to the ferrous ions and thus ${{\left[ \text{Fe}{{\left( \text{N}{{\text{H}}_{\text{3}}} \right)}_{\text{4}}} \right]}^{+2}}$is formed.
The ammonia molecules satisfy the secondary valence of the metal ion which is six and there are four ammonia molecules, so the ferric ion can accept electron pairs from two more chloride anions. The chloride ions present in the medium satisfy the positive charge on the complex, and one chloride anion satisfies the primary valance and thus$\left[ \text{Fe}{{\left( \text{N}{{\text{H}}_{\text{3}}} \right)}_{\text{4}}}\text{C}{{\text{l}}_{2}} \right]\text{Cl}$is formed. One chloride ion reacts with silver nitrate to form a white coloured precipitate of Silver (I) Chloride or $\text{AgCl}$. The IUPAC name of is$\left[ \text{Fe}{{\left( \text{N}{{\text{H}}_{\text{3}}} \right)}_{\text{4}}}\text{C}{{\text{l}}_{2}} \right]\text{Cl}$ Dicholoro tetraammine ferrum (III) chloride.

So, the correct answer is option D.

Note:
According to the IUPAC nomenclature, the neutral ligands like ammonia, the common name of the ligand is used, for example water is called “Aqua”, ammonia is called “Ammine”, carbon monoxide is called “carbonyl”, nitrogen and oxygen molecules are called, “dinitrogen” and “oxygen” respectively.
Anionic ligands such as chloride, bromide, iodide, cyanide, etc. end with “o” for example “chloro”, “bromo”, etc.
Greek prefixes, “di, tri, etc.” are used to designate the number of each types of ligand associated with the metal ion.