Question

Give reasons for: (a) \(\mathrm{La}(\mathrm{OH})_{3}\) is most basic among \(\mathrm{Ln}(\mathrm{OH})_{3}\). (b) Lanthanides have similar properties (c) Most of the lanthanide ions are coloured.

Short answer

(a) La(OH)3 is most basic due to the 'double effect' of ionic size and hydration enthalpy. (b) Lanthanides have similar properties because of their similar electronic configurations and the effect of lanthanide contraction. (c) Most of the lanthanide ions are coloured due to the f->f transitions.

Step-by-step solution

Most Basic

(a) The basic strength of hydroxides of lanthanides increase from La to Lu. This is due to the fact that the size of lanthanide ions decrease from La to Lu (lanthanide contraction). Thus, the smaller the size of the ion, the greater is its polarizing power, causing greater degree of ionization of water molecules and hence increasing the basic strength of hydroxides. However, La(OH)3 is considered to be most basic among Ln(OH)3, despite being the first in the series, due to what's known as the 'double effect' of the ionic size and hydration enthalpy. Since La3+ is largest in size, it is less polarised, leading to less covalent and more ionic character of La(OH)3 and hence it is most basic.

Similar Properties

(b) The properties of lanthanides are similar due to the similar electronic configurations of their atoms. The 'lanthanide contraction', a steady decrease in atomic and ionic size of elements with increasing atomic number, as one proceeds in the period from left to right, causes a shielding of the nuclear charge by 4f electrons so that outer electrons feel similar effective nuclear charges. Thus, their chemical and physical properties are very much similar to each other.

Coloured Ions

(c) Most of the lanthanide ions are coloured due to the f -> f transitions. The ions of lanthanides with partially filled inner 4f levels give colours to their solutions and salts because of the absorption of radiant energy corresponding to the visible region to excite an electron from one 4f orbital to another. The absorption of radiant energy corresponding to the visible region results in the promotion of an electron from a lower to higher energy level.

Key concepts

Lanthanide Contraction

The lanthanide contraction is a fascinating phenomenon observed within the lanthanide series. As we move from lanthanum (La) to lutetium (Lu) across the periodic table, there's a gradual decrease in ionic and atomic radii, despite the increasing atomic number. This decrease is not immediately obvious, but it occurs due to the poor shielding effect of the 4f-electrons. These electrons do not shield the nuclear charge effectively, resulting in a stronger pull of the nucleus on the outer electrons. This increased pull causes the atoms to contract, reducing their size.
One critical consequence of lanthanide contraction is its impact on the chemistry of the elements. Despite having consecutive atomic numbers, lanthanides display very similar physical and chemical properties. This contraction also affects the subsequent elements in the periodic table, influencing properties like ionic radii and electronegativity. It even impacts the separation processes for lanthanides, making it challenging since their chemistry is quite similar.

f-f transitions

f-f transitions are a key reason why lanthanides often display vibrant colors. Unlike d-d transitions in transition metals, f-f transitions involve the inner 4f orbitals. Because these orbitals are well shielded by the outer electrons, the energy required for transitions between them falls within the visible light spectrum. When lanthanide ions absorb light energy, it causes an electron to jump from a lower energy 4f orbital to a higher one.
These transitions result in the characteristic colors observed in lanthanide compounds. The specific colors depend on various factors, including the particular lanthanide ion, its oxidation state, and the nature of the ligands attached to it. It's important to note that not all lanthanide ions display color this way—only those with partially filled 4f orbitals can undergo these transitions.

Lanthanide Ionic Size

The ionic size of lanthanides changes systematically across the series, primarily due to the concepts of lanthanide contraction. As mentioned earlier, the contraction is the gradual decrease in size as we move from La to Lu. This phenomenon strongly influences many properties, including solubility and the formation of complex compounds.
Understanding the size of lanthanide ions helps explain their reactivity and how they interact with other elements and compounds. The larger ionic size at the beginning of the series means that early lanthanides like La have lower effective nuclear charge affecting their ionic bonds and interactions. This has a direct impact on their basicity. For example, larger ions are less polarizing, so La(OH) _3 is particularly basic, as it forms more ionic and less covalent bonds compared to other lanthanide hydroxides.

Basicity of Lanthanide Hydroxides

Basicity among lanthanide hydroxides increases from lutetium hydroxide to lanthanum hydroxide. This trend is directly related to the ionic size of the lanthanides caused by lanthanide contraction. Basicity is influenced by the ionic character of the hydroxide bond. The larger the ion, the more ionic character it has, enhancing its basicity.
La(OH)_3 stands out as the most basic among lanthanide hydroxides. The large ionic size of La^3+ means it has a lower polarizing power and forms with hydroxide ions a more ionic bond compared to its smaller counterparts towards the end of the series. This characteristic makes it release hydroxide ions more readily in water, increasing its basicity. Furthermore, factors like hydration enthalpy also play a role, making the basic strength of lanthanide hydroxides an intricate play of size, charge, and solution chemistry.