Question

Iron carbonyl $Fe{{(CO)}_{5}}$ is ____
(A) trinuclear
(B) tetranuclear
(C) dinuclear
(D) mononuclear

Answer 1

Hint: Neutral molecules or anions bond to a central metal atom or ion in a Lewis acid-base reaction, the product formed in this reaction is coordination complex. The bond which is involved in the formation of a coordination complex is a coordinate covalent bond.

Complete step by step solution:
Given complex compound, $[Fe{{(CO)}_{5}}]$
The formula shown above which is enclosed in brackets is known as the coordination sphere. This coordination sphere of the complex compound consists of a central metal atom attached to ligands. The central metal atom in the coordination sphere is Fe and attaching to 5 CO which is a ligand.
According to Lewis acid-base theory, the central metal atom/ion in a coordination compound is Lewis acid which accepts at least one electron pair from ligand and ligand is Lewis base which donates at least one electron pair to the central metal atom/ion. Compounds that contain a coordination complex are coordination compounds.
The type of covalent bond in which one atom supplies both electrons in coordination compounds is a coordination covalent bond. This bond is different from normal covalent bonds in which each atom shares two electrons.
Based on the nature of ligands, the coordination compounds are nomenclature changes. That is a ligand which donates a number of electrons to the central metal atom/ion.
 If a ligand donates one electron pair to a central metal atom, then the ligand is a monodentate ligand and the complex compound is a mononuclear compound.

Given complex compound $[Fe{{(CO)}_{5}}]$is a mononuclear complex, where one Fe atom is surrounded by 5 CO ligands.

Note: Complex compounds and double salts are differing in the property of dissociation when dissolved in water. Both are formed by the combination of two or more stable compounds in stoichiometric ratio. Double salts like Mohr’s salt, potash alum dissociate completely into ions when dissolved in water. But complex compounds do not dissociate into complex ions.