Why are low spin tetrahedral complexes not formed ?

(a) What type of isomerism is shown by the complex [Co(NH_(3))_(5)(SCN)]^(2+) ? (b) why is [NiCl_(4)]^(2-) paramagnetic while [Ni(CN)_(4)]^(2-) is diamagnetic ? (Atomic number of Ni=28) (c) Why are low spin tetrahedral complexes rarely observed?

(a) What type of isomerism is shown by the complex [Co(NH_(3))_(5)(SCN)]^(2+) ? (b) why is [NiCl_(4)]^(2-) paramagnetic while [Ni(CN)_(4)]^(2-) is diamagnetic ? (Atomic number of Ni=28) (c) Why are low spin tetrahedral complexes rarely observed?

(a) What type of isomerism is shown by the complex [Co(NH_(3))_(5)(SCN)]^(2+) ? (b) why is [NiCl_(4)]^(2-) paramagnetic while [Ni(CN)_(4)]^(2-) is diamagnetic ? (Atomic number of Ni=28) (c) Why are low spin tetrahedral complexes rarely observed?

Assertion (A) : Low spin tetrahedral complexes are rarely observed. Reason (R) : Crystal field splitting is less than pairing energy for tetrahedral complexes.

Why do metal ions never form low-spin tetrahedral complex?