Question

tetrahedral complexes do not show geometrical isomers why?

Answer 1

tetrahedral compounds do not show geometrical isomerism because of their labile or fast changing bonds i.e.the compounds rotate clockwise or anticlockwise in the solution very fast due to which they do not show geometrical isomerism

Answer 2

A and B are both unidentate. The best thing to do is to stick matchsticks into a piece of play dough, or "links" into a tetrahedral "atom" of a molecular modelling kit, and play with it. It is difficult to see what is happening in two dimensions.

If the book meant to refer to bidentate ligands, it would talk about A-A and B-B, but there would still be only one isomer.

For a square planar complex, such as those of Pt(II), [MA2B2], you have two possibilities, cis and trans. For [M(A-A)(B-B)], unless the ligands have a very large bite, you will only be able to make cis.