Which statement is not correct with respect to transition metals?

Most transition metals should, on thermodynamic considerations, liberate hydrogen from acids

The correct answer is option 2. Most transition metals should, on thermodynamic considerations, liberate hydrogen from acids.

Let us delve into each statement regarding transition metals to clarify their accuracy:

1. The colour of the hydrated ions is due to transitions of electrons from different ‘d’ orbitals of the same principal quantum number:

Transition metals are known for their vibrant colors in aqueous solutions of their hydrated ions. These colors arise primarily due to electronic transitions between different d orbitals within the same principal quantum number \((n)\).  For instance, when transition metal ions form complexes with water molecules (hydrated ions), the d electrons absorb specific wavelengths of visible light corresponding to energy differences between these d orbitals. The absorbed light appears complementary to the absorbed wavelengths, giving the complex its characteristic color.

2. Most transition metals should, on thermodynamic considerations, liberate hydrogen from acids:

This statement is not correct. Transition metals vary significantly in their reactivity towards acids. While some transition metals, particularly those in lower oxidation states and with more active chemistry (such as magnesium or zinc), can react with acids to liberate hydrogen gas, this is not universally true for all transition metals. Metals like magnesium \((Mg)\) and zinc \((Zn)\), which are not considered typical transition metals but can exhibit similar behavior in some cases, can react with acids such as hydrochloric acid \((HCl)\) to produce hydrogen gas:

\(Mg + 2HCl \rightarrow MgCl_2 + H_2 \)

However, many transition metals in higher oxidation states or with stable electronic configurations do not readily react with acids to liberate hydrogen due to their lower reactivity under normal conditions.

3. The complex formation of transition metal ions is often accompanied by a change of colour and sometimes by a change in the intensity of the colour:

This statement is correct. Transition metal ions can form complexes with ligands, such as water molecules or other neutral or charged species. The coordination of these ligands around the metal ion alters the electronic environment and the splitting of d orbitals (ligand field theory), which affects the absorption of visible light. For example, the hydrated ion \([Cu(H_2O)_6]^{2+}\) appears blue due to the absorption of certain wavelengths of light that are complementary to the absorbed wavelengths. When a ligand such as ammonia \((NH_3)\) replaces water molecules in the coordination sphere of the copper ion, the absorption spectrum shifts, resulting in a change in the color of the complex (in this case, from blue to deep blue or purple).

4. The compounds of the elements in low oxidation states are generally ionic

This statement is correct. Transition metals in lower oxidation states, such as +1 or +2, tend to form ionic compounds rather than covalent compounds. This is because these metals typically lose electrons to form cations, which are stabilized by anions or neutral ligands through ionic interactions.

Copper(I) oxide \((Cu_2O)\) is an example of a compound where copper is in the +1 oxidation state and forms an ionic lattice with oxide ions \((O^{2-})\). In this compound, copper(I) exists as \(Cu^+\) cations and oxide ions exist as \(O^{2-}\) anions, held together by ionic bonds.

Summary:

The correct understanding of these statements helps clarify the unique properties and behaviors of transition metals, particularly in relation to their colors, reactivity with acids, formation of complexes, and tendencies to form ionic compounds in certain oxidation states.