Question

Which statement about coordination number of a metal ion is true?
(A) Most metal ions exhibit only a single characteristic coordination number
(B) The coordination number is equal to the number of ligands bonded to the metal atom
(C) The coordination number is determined solely by the tendency to surround the metal atom with the same number of electrons as one of the rare gases.
(D) For most cations, the coordination number depends on the size, and charge of the cation.

Answer 1

Hint: The coordination number of metal ions is used to determine the shape of the complex. A single metal ion may exhibit different coordination numbers depending upon the attachment of ligands.

Complete step by step answer:
Metal ions may exhibit more than a one coordination number. Example, depending upon the coordination number of the metal there are octahedral and tetrahedral complexes. Thus, statement A is not true.
The coordination number is equal to the number of ligands bonded to the metal atom is true only for mono dentate ligands. In case of bi or tri dentate ligand the coordination number depends on the denticity of the ligands.
Example, In \[{[Co{(N{H_3})_6}]^{3 + }}\], the number of ligands = 6 and the coordination number = 6 since \[N{H_3}\]is a monodentate ligand.
In \[{\text{[Pt(en}}{{\text{)}}_{\text{2}}}{\text{]C}}{{\text{O}}_{\text{3}}}\], the number of ligands = 2 but the coordination number = 2 x 3 =6 since ethylenediamine (\[{\text{en}}\]) is a bi dentate ligand with denticity = 2.
Thus, statement B is also false.
The coordination number is not solely determined by the tendency to surround the metal atom with the same number of electrons as one of the rare gases. It is one of the methods. Thus, statement C is also false.
The coordination number depends upon charge of the metal ion and the ligand, relative sizes of the metal ion and the ligands.For most cations, the coordination number depends on the size, and charge of the cation. This is true. Thus, statement D is also TRUE.

So, the correct option is D.

Note: In a coordination complex we can easily predict the following:
(i) Oxidation state of the metal ion
(ii) Coordination number of the metal
(iii) Nature of the complex
(iv) Shape of the coordination complex