Which one of the following complexes will exhibit the least paramagnetic behaviour? [Atomic number, $$Cr = 24, Mn = 25, Fe = 26, Co = 27$$]

We need the complex with the least paramagnetic behaviour, i.e., the fewest unpaired electrons. All given complexes are octahedral with $$H_2O$$ (a weak field ligand), so we use high-spin configurations.

$$[Cr(H_2O)_6]^{2+}$$: $$Cr^{2+}$$ is $$d^4$$. High spin: $$t_{2g}^3 e_g^1$$. Unpaired electrons = 4.

$$[Fe(H_2O)_6]^{2+}$$: $$Fe^{2+}$$ is $$d^6$$. High spin: $$t_{2g}^4 e_g^2$$. Unpaired electrons = 4.

$$[Co(H_2O)_6]^{2+}$$: $$Co^{2+}$$ is $$d^7$$. High spin: $$t_{2g}^5 e_g^2$$. Unpaired electrons = 3.

$$[Mn(H_2O)_6]^{2+}$$: $$Mn^{2+}$$ is $$d^5$$. High spin: $$t_{2g}^3 e_g^2$$. Unpaired electrons = 5.

$$[Co(H_2O)_6]^{2+}$$ has the fewest unpaired electrons (3), so it exhibits the least paramagnetic behaviour.

The correct answer is Option (3): $$[Co(H_2O)_6]^{2+}$$.