Why are low spin tetrahedral complexes not formed?

Explain the following. Low spin octahedral complexes of nickel are not known.

Why are different colours observed in octahedral and tetrahedral complex for the same metal and same ligands?

Why NCl_5 is not formed?

Why is geometrical isomersim not possible for tetrahedral complexes?

Square planar complexes of MX_(2)L_(2) type with coordination number of 4 exhibit geometrical isomerism whereas tetrahedral complexes with similar composition do not. Why? Justify the formation of a low spin complex and a high spin complex taking the examples of [Fe(CN)_(6)]^(3-) and [FeF_(6)]^(3-) on the bases of "crystal field splitting energy". (Delta_(0))

During which stage of meiosis, synaptonemal complex is formed?