A paramagnetic complex contains at least one unpaired electron. In the case of [Fe(H2O)6]²⁺, the complex is paramagnetic because water (H2O) is a weak field ligand. As a result, the energy gap between the t₂g and eₖ orbitals is small, making it energetically favourable for electrons to occupy the higher energy eₖ orbitals rather than pairing up in the t₂g orbitals. This leads to a high-spin configuration, with three electrons in the t₂g orbitals and two in the eₖ orbitals, resulting in five unpaired electrons. Therefore, [Fe(H2O)6]²⁺ is paramagnetic.

In contrast, [Fe(CN)6]²⁻ is diamagnetic because cyanide (CN⁻) is a strong field ligand, which creates a large energy gap between the t₂g and eₖ orbitals. This large gap forces the complex to adopt a low-spin configuration, filling all the t₂g orbitals with six electrons and leaving no unpaired electrons. As a result, [Fe(CN)6]²⁻ is diamagnetic.

There are two main factors that explain why it becomes easier to remove an electron as you move down a group: increasing atomic size and increasing shielding effect.

Increasing Atomic Size: As you move down a group, the atomic size increases, meaning the outermost electron is farther from the nucleus. This increased distance weakens the attraction between the positive nucleus and the outer electron, requiring less energy to remove the electron.

Increasing Shielding Effect: Electrons in the inner energy levels act as a shield for those in the outer levels. This shielding reduces the effective nuclear charge felt by the outer electron, further decreasing the attraction between the nucleus and the electron. As a result, it becomes easier to remove the outer electron.