Question

why are low spin tetrahedral complexes are formed?

Answer 1

Now the low spin complexes are formed when a strong field ligands forms a bond with the metal or metal ion. The strong field ligands invariably cause pairing of electron and thus it makes some in most cases the last d-orbital empty and thus tetrahedral is not formed.

Answer 2

Hey !!

The  low spin tetrahedral complexes are formed because of very low CFSE which is not able to pair up the electrons.

DETAILED EXPLANATION

Low spin tetrahedral and complexes are rarely observed, because for the same metal and same ligand. It is observed that,

                Δt = 4/9 Δ₀

where,          Δt = crystal field splitting energy in Tetrahedral complex

                     Δ₀ = crystal field splitting energy in Octahedral complex

the Δt rarely exceeds the pairing energy, so mainly high spin tetrahedral complexes are formed.

Good luck !!