The Lanthanide Contraction: What is Abnormal and Why?

The lanthanide contraction is a widely known phenomenon in which the ionic radii of the Ln³⁺ ions decrease across the series from La³⁺ to Lu³⁺. As a result, the distance d(Ln-Y), where Y is a ligand donor atom, decreases across the series. As shown previously, the decrease normally has a linear dependence on the number of 4f Ln³⁺ electrons, n, and the net change, Δ', is between 0.15 and 0.20 Å. However, previous reports have shown that Δ' can be much larger than normal, and it is shown here that Δ' can be much smaller than normal. The present study seeks to determine the source and a possible explanation of these anomalies. A definitive conclusion is that the large Δ' is due to large d(Ln-Y) for the early lanthanides. The small Δ' can result from chelate constraints and intramolecular H-bonding. As an outgrowth of this search, it is shown that consistent radii for donor atoms, r(Y), can be obtained if the system satisfies certain conditions. Finally, the impact of the results on the normal and inverse trans influences is discussed.