Question

why are low spin tetrahedral complexes not formed

Answer 1

 Octahedral Complex Splitting: In an octahedral complex, when Δ is large (strong field ligand), the electrons will first fill the lower energy d orbitals before any electrons are placed on the higher energy d orbitals. It is then classified as low spin because there is a minimal amount of unpaired electrons.

Answer 2

For tetrahedral complexes, the crystal field stabilisation energy is lower than pairing energy, so they are not formed in low spin state.