Which type of complexes are generally formed with ligands high in the spectrochemical series?

Complexes formed with ligands that are high in the spectrochemical series tend to be low spin complexes. Ligands that are strong field, such as CN⁻ and CO, generate a significant splitting of the d-orbitals in transition metals. The larger energy gap between the split d-orbitals encourages the preferential pairing of electrons in the lower energy d-orbitals rather than occupying the higher energy ones, resulting in fewer unpaired electrons and a low spin state.

In contrast, weak field ligands produce smaller splitting and typically lead to high spin complexes, where electrons remain unpaired in higher energy d-orbitals to minimize repulsion. Since strong field ligands create a low spin environment, complexes that associate with such ligands exhibit distinctive electronic configurations characterized by minimized spin state due to electron pairing.

This principle is foundational in crystal field theory, which elucidates how variations in ligand strength directly affect the electronic characteristics and magnetic properties of the metal complexes. In summary, strong field ligands lead to a tendency to form low spin complexes, as observed with ligands high on the spectrochemical series.