Why are tetrahedral complexes generally high spin?

Tetrahedral complexes are generally associated with high spin configurations due to the nature of their electronic structure and the ligand field they experience. To understand this, let's consider the key factors that influence spin state in coordination complexes.

Ligand Field: The ligand field is the arrangement of ligands around the central metal ion. In tetrahedral complexes, the ligands are located at the vertices of a tetrahedron, resulting in a distorted octahedral field. This ligand field causes splitting of the d orbitals into two sets of energy levels: t2g (lower energy) and eg (higher energy).

Hund's Rule: Hund's rule states that for degenerate orbitals (orbitals with the same energy), electrons will occupy separate orbitals before pairing up. This rule leads to the maximization of the total spin in the system.

Based on these factors, when considering tetrahedral complexes, the following observations can be made:

In tetrahedral complexes, the t2g and eg orbitals are not degenerate due to the ligand field splitting. The energy difference between the t2g and eg orbitals is relatively small.
Since the energy difference is small, it becomes favorable for electrons to occupy the higher energy eg orbitals instead of pairing up in the lower energy t2g orbitals.
This leads to the promotion of electrons from the t2g to the eg orbitals, resulting in unpaired electrons in the higher energy orbitals. Consequently, the complex adopts a high spin state.
It's important to note that while tetrahedral complexes are generally associated with high spin configurations, there can be exceptions depending on the specific ligands and the metal ion involved. Factors such as ligand strength and the electronic configuration of the metal ion can influence the spin state and result in low spin tetrahedral complexes.