The size of actinoid(III) ions – structural analysis vs. common misinterpretations

• Ionic radii for nine-coordinate actinoid(III) ions are reported.
• Suitable lanthanoid(III) ion substitutes for actinoid(III) ions are given.
• The incline of the lanthanoid contraction is steeper than the actinoid contraction.

A fundamental property of ions is their size, a known fact since before the acceptance of the modern atom model. The common way to describe the size of an ion is to determine its radius, defined as one of a pair of radii adding up to the bond distance between the centers of two nuclei. There are numerous factors that influence the ionic radius of a metal ion, where both valence and coordination number are essential when explaining reactivity, complexation, and chemical behavior. The similarity in ionic radii and chemical behavior between the elements in the lanthanoid and actinoid series is well-known and frequently used, making members of the former safe substitutes to avoid hazardous experiments with the radioactive actinoids. This review establishes reliable ionic radii for the nine-coordinate actinoid(III) ions, based on reported structural data, shedding light upon common misconceptions and clarifying the relationship between the ionic radii in the lantanoid and actinoid series.

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