Paramagnetism in Coordination Compounds

Paramagnetism is a property of coordination compounds that arises due to the presence of unpaired electrons. When an external magnetic field is applied, the unpaired electrons in the compound interact with the field and become aligned with it, resulting in a net magnetic moment. This causes the compound to be attracted to the magnetic field and is known as paramagnetism.

Determining the Paramagnetic Complex

To determine whether a coordination compound is paramagnetic, we need to know the number of unpaired electrons in the metal ion. This can be done by using the crystal field theory or the molecular orbital theory.

In crystal field theory, the ligands in the complex create a crystal field that splits the d orbitals of the metal ion into two sets: the lower energy set (t2g) and the higher energy set (eg). The number of electrons in the t2g set determines the paramagnetic or diamagnetic nature of the complex.

In molecular orbital theory, the d orbitals of the metal ion and the ligands combine to form molecular orbitals. The number of electrons in the bonding and antibonding orbitals determines the paramagnetic or diamagnetic nature of the complex.

Answer Explanation

a) [Ni(HO)]2 - This complex has an unpaired electron in the d orbital of the nickel ion, making it a paramagnetic complex.

b) [Ni(CO)] - This complex has all the electrons paired in the d orbital of the nickel ion, making it a diamagnetic complex.

c) [Zn(NH)]2 - This complex has no d electrons in the zinc ion, making it a diamagnetic complex.

d) [Co(NH)]3 - This complex has four unpaired electrons in the d orbital of the cobalt ion, making it a paramagnetic complex.

Therefore, the correct answer is option 'A' - [Ni(HO)]2.