Select the complex in which the metal has primary valency $= 2$,secondary valency $= 4$ and shows stereo-isomerism:

Amongst $[Ni(H_{2}O)_{6}]^{2+}$, $[Ni(PPh_{3})_{2}Cl_{2}]$, $[Ni(CO)_{4}]$ and $[Ni(CN)_{4}]^{2-}$, the paramagnetic species are

On treatment of $[Ni(NH_3)_4]^{2+}$ with concentrated $HCl,$ two compounds $I$ and $II$ having the same formula,$Ni(NH_3)_2Cl_2$ are obtained. $I$ can be converted into $II$ by boiling with dilute $HCl.$ $A$ solution of $I$ reacts with oxalic acid to form $[Ni(NH_3)_2(C_2O_4)]$ whereas $II$ does not react. Point out the correct statement of the following:

Which of the following pairs of complexes are coordination isomers?

If the atomic number of an element is $26$, then the magnetic moment of its divalent aqueous ion is . . . . . . $BM$.

Which of the following ligands is able to form linkage isomers?