Question

Why are tetrahedral complexes generally high spin?

Answer 1

Tetrahedral complexes, with `2//3` as many ligands binding, and all of them off-axis (reducing repulsive interactions), generally have small d-orbital splitting energies `Delta_t`, where `Delta_t ~~ 4/9 Delta_o`.

As a result, even with strong-field ligands, the splitting energy is generally smaller than the electron pairing energy.

When electron pairing energy is large, electron pairing is unfavorable. What that implies is that generally, high spin is favored.