Question

Which of the following compounds is paramagnetic?
A.Tetracyanonickelate $\left( {{\text{II}}} \right)$ ion
B.Tetraamminezinc $\left( {{\text{II}}} \right)$ ion
C.Hexaamine chromium $\left( {{\text{III}}} \right)$ ion
D.Diammine silver $\left( {\text{I}} \right)$ ion

Answer 1

Paramagnetic complexes are high spin complexes, that is, they have at least one unpaired electron and weak ligands favours the formation of paramagnetic compounds. The one with 3 electrons in d orbital will be paramagnetic in nature.

Complete step by step solution:
According to valence bond theory intermixing of orbital occurs. Intermixing orbitals are vacant orbitals of the central metal atom and the filled orbital of the ligand. The strength of ligands is determined with the help of spectrochemical series. It is an experimentally determined series. Strength of ligand determines whether the electrons will be paid or not. The strong field ligands such as cyanide, carbonyl, ammonia and EDTA pair up the electrons whereas weak ligands such as iodide, bromide, fluoride and chloride have no ability to pair the electrons.
The first complex has coordination number four because of the presence of four cyanide ligands. Cyanide is a strong field ligand so it will pair up the electrons and hence the complex will be diamagnetic.
Similarly in the option B the complex will be diamagnetic in nature because of the presence of amine. But in case of Hexaamminechromium $\left( {{\text{III}}} \right)$ ion, it has only three electrons because the oxidation state of chromium is 3. The oxidation state of chromium is 3 as given to us. We know in neutral chromium 6 valence electrons are there. So that makes the number of electron in chromium $\left( {{\text{III}}} \right)$ ions as $6 - 3 = 3$
This complex will be paramagnetic in nature because 3 electrons cannot be paired completely. This paramagnetism occurs due to the higher oxidation state of chromium that is plus 3.

Hence, the correct option is C.

Note:
The paramagnetic complexes has magnetic moment 0 and the magnetic moment for the paramagnetic substances can be calculated using the formula $\mu = \sqrt {{\text{n}}\left( {{\text{n}} + 2} \right)}$ . The Paramagnetic substances are those substances which are attracted when placed in a magnetic field.