What gives rise to paramagnetism in transition metal ions, and why is the contribution of orbital angular momentum effectively quenched in compounds of the first series of transition metals?

Paramagnetism in transition metal ions arises from unpaired electrons, leading to magnetic moments. In the first series of transition metals, the contribution of orbital angular momentum is effectively quenched due to strong spin-orbit coupling. In these elements, the energy difference between the orbitals with different angular momentum becomes comparable to the electron-electron repulsion energy. Consequently, electrons redistribute among orbitals to minimize repulsion, resulting in the quenching of orbital angular momentum. This phenomenon diminishes the orbital contribution to magnetic moments in compounds of the first series of transition metals, emphasizing the dominance of spin magnetic moments.