Low spin octahedral complexes are not formed by nickel . Why?
Nickel usually forms octahedral complexes and exits in +2 oxidation state. The electronic configuration of Ni is [Ar] 3d8 4s2. So the electronic configuration of Ni+2 ion will be [Ar] 3d8. We know that in the presence of ligands, the 5 degenerate d-orbitals split into 2 set of orbitals - the t2g set and the eg set. Strong ligands cause pairing of electrons and result in low spin complexes. On the other hand, weak ligands do not cause the pairing of electrons and result in high spin complexes. As there are 8 electrons in d-orbitals of Ni+2 ion, therefore for both strong field and weak field ligands, the electronic configuration will be (t2g)2(eg)2. Hence, Ni2+ ion will always contain 2 unpaired electrons in the eg set and will therefore form high spin complexes, irrespective of the strength of ligand.