The overall complex dissociation equilibrium constant for the complex $[Cu(NH_3)_4]^{2+}$ ion will be (given $\beta_4$ for this complex is $2.1 \times 10^{13}$,where $\beta_4$ is the association constant).

Given below are two statements: one is labelled as "Assertion $A$" and the other is labelled as "Reason $R$".
Assertion $A$: In the complexes $Ni(CO)_4$ and $Fe(CO)_5$,the metals have zero oxidation state.
Reason $R$: Low oxidation states are found when a complex has ligands capable of $\pi$-donor character in addition to the $\sigma$-bonding.
In the light of the above statements,choose the most appropriate answer from the options given below.

$A$ solution containing $2.675 \ g$ of $CoCl_3 \cdot 6 NH_3$ (molar mass $= 267.5 \ g \ mol^{-1}$) is passed through a cation exchanger. The chloride ions obtained in solution were treated with excess of $AgNO_3$ to give $4.78 \ g$ of $AgCl$ (molar mass $= 143.5 \ g \ mol^{-1}$). The formula of the complex is
(At. mass of $Ag = 108 \ u$)

For the reaction given below:
$CoCl_{3} \cdot xNH_{3} + AgNO_{3(aq)} \rightarrow$
If two equivalents of $AgCl$ precipitate out,then the value of $x$ will be $....$

$CuSO_4$ reacts with $KCN$ solution and forms

Which of the following compounds give at least $1$ mole of $AgCl$ precipitate with $100$ mL of a $10$ $M$ solution of $AgNO_3$?