Question

why are low spin tetrahedral complesus rarely observed

Answer 1

Low spin tetrahedral complexes are rarely observed because for them, the crystal field stabilisation energy is less and always lower than pairing energy. And there is less energy gap B/W it's two energy levels {e and t2}

Answer 2

It is rare for the Δt of tetrahedral complexes to exceed the pairing energy. Usually, electrons will move up to the higher energy orbitals rather than pair. Because of this, most tetrahedral complexes are high spin.