\(Ni^{2+}\) in \([NiCl_2(H_2O)_4]\) and \(Fe^{3+}\) in \([Fe(CN)_6]^{3-}\) are called:

Lewis acid

The correct answer is option 1. Lewis acid.

Let us delve into the roles of \(Ni^{2+}\) and \(Fe^{3+}\) in their respective complexes and how they fit the definition of a Lewis acid:

Lewis Acids and Bases

Lewis Acid: A Lewis acid is defined as an electron pair acceptor. It can accept a pair of electrons from a Lewis base to form a coordinate covalent bond. Typically, Lewis acids are electron-deficient species or positively charged ions.

Lewis Base: A Lewis base is defined as an electron pair donor. It donates a pair of electrons to a Lewis acid.

Analysis of the Complexes

\(Ni^{2+}\) in \([NiCl_2(H_2O)_4]\):

Complex: \([NiCl_2(H_2O)_4]\) is a coordination complex where \(Ni^{2+}\) is the central metal ion.

Role of \(Ni^{2+}\): In this complex, \(Ni^{2+}\) has an electron-deficient nature due to its positive charge. It is surrounded by ligands (chloride ions and water molecules) that donate electron pairs to it. The \(Ni^{2+}\) ion accepts electron pairs from these ligands to form the complex. Therefore, \(Ni^{2+}\) is acting as a Lewis acid because it is accepting electron pairs from the ligands.

\(Fe^{3+}\) in \([Fe(CN)_6]^{3-}\):

Complex: \([Fe(CN)_6]^{3-}\) is a coordination complex where \(Fe^{3+}\) is the central metal ion.

Role of \(Fe^{3+}\):

In this complex, \(Fe^{3+}\) is also positively charged and electron-deficient. It is surrounded by cyanide ions (\(CN^-\)), which are ligands that donate electron pairs to the \(Fe^{3+}\) ion. The \(Fe^{3+}\) ion accepts electron pairs from the cyanide ligands to form the complex. Thus, \(Fe^{3+}\) acts as a Lewis acid for the same reason as \(Ni^{2+}\): it is accepting electron pairs from the ligands.

Why They Are Lewis Acids

Both \(Ni^{2+}\) and \(Fe^{3+}\) are metal cations with positive charges. Their positive charges create an electron-deficient environment, which makes them capable of accepting electron pairs from ligands. The ligands in these complexes are Lewis bases because they donate electron pairs to the metal ions (the Lewis acids).

Summary

In coordination chemistry, metal ions like \(Ni^{2+}\) and \(Fe^{3+}\) often act as Lewis acids because they accept electron pairs from ligands to form stable coordination complexes. They are not Lewis bases or amphoteric in this context. Thus, the correct classification for \(Ni^{2+}\) in \([NiCl_2(H_2O)_4]\) and \(Fe^{3+}\) in \([Fe(CN)_6]^{3-}\) is: 1. Lewis acid.