Question

In the EPR part of the book 'Physical methods in Inorganic Chemistry' by S. Drago i found that relative magnitudes of crystal field and spin-orbit coupling affect the g values. I understand that the chemical environment of a metal complex partly removes the orbital degeneracy, but i don't find the way to imagine a sustaining effect which implies that if an electron has an orbital angular momentum this is manteined by coupling to the spin angular momentum. I have read that consequently a net orbital magnetic moment results, giving a g value different from the free electron. I also can't imagine how when the crystal field increases the LS coupling is broken down completely.

Answer 1

Crystal field theory describes the effect of ligands on d orbitals, causing crystal field splitting energy (Δ), while spin-orbit coupling links an electron's spin to its orbital momentum, affecting the magnetic moment. The relative strengths of crystal field splitting and spin-orbit coupling determine the g values observed in EPR spectroscopy, with a high crystal field potentially 'quenching' the orbital contribution.

Step-by-step explanation:

Crystal field theory and spin-orbit coupling are essential concepts in understanding the magnetic properties of transition metal complexes. The former describes how ligands affect the energy levels of metal d orbitals, resulting in splitting of these orbitals into different energy levels, known as crystal field splitting energy (Δ).

A greater splitting, often caused by strong-field ligands, tends to produce low-spin complexes, leading to smaller magnetic moments and often alterations in the observed g values in electron paramagnetic resonance (EPR) spectroscopy. On the other hand, the spin-orbit coupling describes the linked behavior of an electron's spin with its orbital angular momentum, affecting both the magnitude and direction of the electron's total magnetic moment.

When the crystal field splitting is significant, it can overpower the spin-orbit coupling, leading to a reduction in the total magnetic moment and moving the g values closer to that of a free electron. In contrast, when spin-orbit coupling is significant, unquenched orbital angular momentum can contribute to the magnetic moment, resulting in g values that depart from the free electron value.

This is why the relative magnitudes of crystal field splitting and spin-orbit coupling are crucial in understanding magnetic properties of metal complexes, and how variations in the crystal field can effectively 'quench' or reduce the influence of orbital angular momentum.