Which of the following complex ions does not absorb visible light?

Which of the following complexes will not show optical isomerism?

The spin magnetic moment of cobalt in $Hg[Co(SCN)_4]$ is

An octahedral complex of $M^{+3}$ with $NH_3$ and $NO_2^{-}$ ligands has four $M-N$ linkages and two $M-O$ linkages. Which type of isomerism will such a complex exhibit?

Consider the complexes:
$I$. $[Pd(NH_3)_2 ClBr]$
$II$. $[Pd(NH_3)_2 Cl_2]$
$III$. $[Pd(en) Cl_2]$
$IV$. $[Pd(en) ClBr]$
$V$. $[Pd(en)_2 Cl_2]$
(en = ethylenediamine)
The total number of geometrical isomers of $(I)$ is the same as the total number of geometrical isomers of:

Give an example of solvate isomerism.