As water is a weak field ligand and nickel in +2 oxidation state the electronic configuration is 4s0 3d8 and two electrons are unpaired due to weak field ligand..

Introduction:
In order to determine the number of unpaired electrons present in the complex ion [Ni(H2O)6]2+, we need to consider the electronic configuration and the coordination number of the central metal ion (Ni) in the complex. The coordination number refers to the number of ligands (in this case, water molecules) directly bonded to the central metal ion.

Electronic configuration of Nickel:
The atomic number of Nickel is 28, and its electronic configuration is [Ar]3d84s2. In the case of the complex ion [Ni(H2O)6]2+, the Nickel ion (Ni2+) has lost two electrons from its outermost 4s orbital, leaving behind the electronic configuration [Ar]3d8.

Octahedral complex:
The complex ion [Ni(H2O)6]2+ has a coordination number of 6, indicating that it forms an octahedral complex. In an octahedral complex, the central metal ion is surrounded by six ligands, which occupy the six coordination positions.

Explanation:
To determine the number of unpaired electrons present in the complex, we need to consider the electronic configuration and the ligand field splitting of the d orbitals. In an octahedral complex, the d orbitals split into two sets of orbitals: the lower energy set (t2g) and the higher energy set (eg).

Ligand field splitting:
Due to the presence of the ligands, the d orbitals experience a ligand field splitting, where the energy of the eg orbitals is raised while the energy of the t2g orbitals is lowered. The magnitude of this splitting depends on the nature and strength of the ligands.

Counting unpaired electrons:
In the case of [Ni(H2O)6]2+, the water molecules are weak-field ligands. This means that the ligand field splitting is relatively small, and the energy separation between the t2g and eg orbitals is less significant. As a result, the electrons occupy the t2g orbitals before pairing up in the eg orbitals.

Since the electronic configuration of Nickel in the complex is [Ar]3d8, it implies that there are two unpaired electrons in the t2g orbitals. These unpaired electrons can participate in bonding and chemical reactions, giving the complex its characteristic properties.

Conclusion:
Therefore, the number of unpaired electrons present in the complex ion [Ni(H2O)6]2+ is two. This is because the electronic configuration of Nickel in the complex is [Ar]3d8, and the weak-field ligands cause the electrons to occupy the t2g orbitals before pairing up in the eg orbitals.