Question

Aluminium reacts with ${\text{NaOH}}$ and forms a compound \[{\text{X}}\]. If the coordination number of \[{\text{X}}\] is \[6\], the correct formula of \[{\text{X}}\] is:
A. \[\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_4}{{\left. {{{\left( {{\text{OH}}} \right)}_2}} \right]}^ + }} \right.\]
B. \[\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_3}{{\left( {{\text{OH}}} \right)}_3}} \right]\]
C. \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_2}{{\left( {{\text{OH}}} \right)}_4}} \right]^ - }\]
D. \[\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}{{\left( {{\text{OH}}} \right)}_3}} \right]\]

Answer 1

Hint: Coordination number is the number of neighboring atoms to the central atom. The product formed is an anionic complex entity. It has a negative charge outside the coordination sphere.

Given data:
The coordination number of the compound \[{\text{X}}\] is \[6\].

Complete step by step solution:
Coordination compounds are the compounds having coordinate bonds. Coordinate bond is the bond in which the sharing of both electrons is done by the same metal atom. It is also called dative bond. It can also be a complex compound. Most of the coordination compounds are using transition metals. This is because they have small ionic sizes and high positive charge density. Therefore this makes it easier for accepting lone pairs of electrons from neighboring molecules.
Aluminium is commonly found in \[ + 3\] oxidation state. Oxidation number or oxidation state is the number of electrons an atom gains or loses. All the options given have 6 neighboring atoms.
When aluminium reacts with ${\text{NaOH}}$, sodium aluminate is formed initially. It is not soluble in water. So it gets changed to \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_2}{{\left( {{\text{OH}}} \right)}_4}} \right]^ - }\]. The reaction is given below:
${\text{Al}} + {\text{NaOH}} + {{\text{H}}_2}{\text{O}} \to {\text{NaAl}}{{\text{O}}_2} + {{\text{H}}_2}$
                                       Sodium aluminate
Here, aluminium exists in \[ + 3\] oxidation state.

Additional information:
Central metal atom with the neighboring molecules which are kept in a square bracket is called a coordination sphere. The neighboring molecules are called ligands. Based on the number of donor atoms, they are classified into four-unidentate, bidentate, polydentate, chelating ligands.

Note: There are plenty of applications for coordination compounds. They are used in the medicinal industry, analytical chemistry, and electroplating. They are used for estimation of hardness of water.