Question

Why do compounds having similar geometry have different magnetic moment?

Answer 1

It is due to the presence of weak and strong field ligands in complexes. If CFSE is high, the complex will show low value of magnetic and vice-versa, e.g. $[Co{F}_6{]}^{3-}$ and $\left[CO\right(N{H}_3{)}_6{]}^{3+},$ the former is paramagnetic, and the latter is diamagnetic because $F^{-}$ is a weak field ligand and $N{H}_3$ is a strong field ligand while both have similar geometry.