Amongst $\left[NiCl_{4}\right]^{2-}, \left[Ni\left(H_{2}O\right)_{6}\right]^{2+},\left[Ni\left(PPh_{3}\right)_{2}Cl_{2}\right], \left[Ni\left(CO\right)_{4}\right]$ and $\left[Ni\left(CN\right)_{4}\right]^{2-}$. the paramagnetic species are
Species having unpaired electrons are paramagnetic $Ni ^{2+}=[ Ar ] 3 d ^{8} $
(i) In $\left[ NiCl _{4}\right]^{2-}$ and $\left.N i \left( H _{2} O \right)_{6}\right]^{2+}$ ligands $Cl ^{-}$and $H _{2} O$ are weak ligands, therefore no pairing will be possible. Thus, there are two unpained electrons. (ii) In $\left[ Ni \left( Ph _{3}\right)_{2} Cl _{2}\right\}$ although $PPh _{3}$ has d-acceptance nature but presence of $Cl$, makes electrons unpaired.
A coordination compound holds a central metal atom or ion surrounded by various oppositely charged ions or neutral molecules. These molecules or ions are re-bonded to the metal atom or ion by a coordinate bond.
Coordination entity:
A coordination entity composes of a central metal atom or ion bonded to a fixed number of ions or molecules.
Ligands:
A molecule, ion, or group which is bonded to the metal atom or ion in a complex or coordination compound by a coordinate bond is commonly called a ligand. It may be either neutral, positively, or negatively charged.
