When solver or gold is leached as follows:

\(4M + 8CN^- + 2H_2O + O_2 \longrightarrow _{-------}\), the product is:

(M = Ag or Au)

\([M(CN)_2]^-\)

The correct answer is option 1. \([M(CN)_2]^-\).

Cyanide Leaching Process (MacArthur-Forrest Process):

This process is used to extract precious metals like gold (Au) and silver (Ag) from their ores using cyanide ions \((CN^-)\). It involves the formation of metal-cyanide complexes that are soluble in water, allowing the metals to be separated from their ores.

Leaching Reaction:

The general reaction for the leaching of silver or gold is:

\(4M + 8CN^- + 2H_2O + O_2 \longrightarrow 4[M(CN)_2]^- + 4OH^-\)

Where:

\(M\) represents either gold \((Au)\) or silver \((Ag)\).

Cyanide ions \((CN^-)\) act as ligands, forming complexes with the metal.

Water and oxygen are also involved in the reaction to oxidize the metal.

Step-by-step Process:

Oxidation of the Metal: The metal \(M\) (either gold or silver) is oxidized by the oxygen in the reaction. This step allows the metal to enter into a solution by reacting with cyanide.

Formation of the Metal-Cyanide Complex: The cyanide ion \((CN^-)\) coordinates with the metal ion, forming a soluble complex. For silver or gold, the primary product is the dicyano complex \([M(CN)_2]^-\), where the metal is in its +1 oxidation state.

Hydroxide Formation: As a byproduct, hydroxide ions \((OH^-)\) are formed.

Product Formation:

The metal (silver or gold) reacts with cyanide ions to form the soluble complex \([M(CN)_2]^-\). This is a dicyano complex, where the metal is coordinated to two cyanide ions. This complex is stable and soluble in water, allowing the metal to be extracted from its ore in a dissolved form.

Why \([M(CN)_2]^-\) is the Correct Answer:

The formation of the complex ion \([M(CN)_2]^-\) is the hallmark of the cyanide leaching process. It allows the metal to dissolve in the aqueous solution, which is critical for the extraction of gold and silver.

Other Options:

\(M_2O\) and \(M_2O_3\) (options 2 and 4) are oxides of the metal, but oxides are not formed in this process.

\([M(CN)_4]^{3-}\) (option 3) is not relevant here, as the dicyano complex \([M(CN)_2]^-\) is the actual product in the leaching of gold and silver.

Conclusion:

The cyanide leaching process results in the formation of the dicyano complex \([M(CN)_2]^-\), where the metal is extracted in a soluble form. Thus, the correct answer is option 1: \([M(CN)_2]^-\).