Among the following complexes,the one that can exist as facial $(fac)$ and meridional $(mer)$ isomers is

Which of the following will give a pair of enantiomorphs?
$(en = NH_2CH_2CH_2NH_2)$

In which of the following pairs do both the complexes show optical isomerism?

Which of the following is paramagnetic?

$CoSO_{4}Cl \cdot 5NH_{3}$ exists in two isomeric forms '$A$' and '$B$'. Isomer '$A$' reacts with $AgNO_{3}$ to give white precipitate,but does not react with $BaCl_{2}$. Isomer '$B$' gives white precipitate with $BaCl_{2}$ but does not react with $AgNO_{3}$. Answer the following questions.
$(i)$ Identify '$A$' and '$B$' and write their structural formulas.
$(ii)$ Name the type of isomerism involved.
$(iii)$ Give the $IUPAC$ name of '$A$' and '$B$.'

Which of the following compounds has a zero magnetic moment?