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Step-by-Step Explanation
Step 1: Recognize the Context
In the laboratory, while analyzing second group cations, copper(II) ions (Cu2+) may be made to react with potassium cyanide (KCN). This leads to the formation of certain cyanide complexes of copper.
Step 2: Possible Complexes with Cyanide Ions
Copper can form complexes such as [Cu(CN)2]n−. To determine the stability of the complex, it is important to check the oxidation state of copper within that complex.
Step 3: Determination of Oxidation State of Copper
Let us consider K3[Cu(CN)2] as a possible complex. The sum of oxidation numbers in a neutral molecule must be zero. Potassium (K) has an oxidation state of +1, and each cyanide ion (CN−) has -1 for the entire CN group. Hence:
$3(\,+1\,) \;+\; x \;+\; 2(\,-1\,) \;=\; 0$
$3 \;+\; x \;-\; 2 \;=\; 0$
$x \;=\; -1
This implies the oxidation state of copper in this complex would be -1.
Step 4: Assessing the Stability
Having copper in a negative oxidation state (−1) in the cyanide complex makes the complex noticeably unstable. Generally, copper prefers positive oxidation states like +1 or +2, so a −1 state is uncommon and results in instability of K2[Cu(CN)2]. Therefore, K2[Cu(CN)2] is less stable.
Conclusion
Among the given choices, the correct statement is that K2[Cu(CN)2] is less stable, which matches the observation that the copper center would be forced into a −1 oxidation state, leading to an unstable complex.
