Reaction of $[Co(H_2O)_6]^{2+}$ with excess ammonia and in the presence of oxygen results into a diamagnetic product. Number of electrons present in $t_{2g}$-orbitals of the product is $.....$ .

Which one of the following is an inner orbital complex as well as diamagnetic in behaviour? (Atomic number: $Zn = 30$,$Cr = 24$,$Co = 27$,$Ni = 28$)

Addition of excess aqueous ammonia to a pink coloured aqueous solution of $MCl_2 \cdot 6H_2O (X)$ and $NH_4Cl$ gives an octahedral complex $Y$ in the presence of air. In aqueous solution,complex $Y$ behaves as a $1:3$ electrolyte. The reaction of $X$ with excess $HCl$ at room temperature results in the formation of a blue coloured complex $Z$. The calculated spin-only magnetic moment of $X$ and $Z$ is $3.87 \ B.M.$,whereas it is zero for complex $Y$.
Among the following options,which statement$(s)$ is(are) correct?
$[A]$ The hybridization of the central metal ion in $Y$ is $d^2sp^3$
$[B]$ $Z$ is a tetrahedral complex
$[C]$ Addition of silver nitrate to $Y$ gives only two equivalents of silver chloride
$[D]$ When $X$ and $Z$ are in equilibrium at $0^{\circ}C$,the colour of the solution is pink

Which one is an example of an octahedral complex?

The number of species from the following that are involved in $sp^3 d^2$ hybridization is $[Co(NH_3)_6]^{3+}, SF_6, [CrF_6]^{3-}, [CoF_6]^{3-}, [Mn(CN)_6]^{3-}$ and $[MnCl_6]^{3-}$.

The magnetic moment of $[NiX_4]^{2-}$ ion is found to be zero. Then the ion is : ($X =$ monodentate anionic ligand)