The Canadian National Crystallographic Committee will be hosting the seventh Dr. Penelope W. Codding Lecture in support of early career crystallographers on Thursday, Aprile 3, 2025 at 12:30 PM Eastern (9:30 AM Pacific). The speaker will be Distinguished Professor Emeritus Dr. Frank Hawthorne, University of Manitoba, Canada.
Biography
NEW ION RADII AND CHARACTERISTIC BONDLENGTHS FOR INORGANIC CRYSTAL STRUCTURES: IMPLICATIONS
Frank C. Hawthorne, Earth Sciences, University of Manitoba, Winnipeg, MB
Ionic radii have played a major role in the interpretation of inorganic crystal structures for the last 100 years. Recently, a comprehensive new set of ion radii and corresponding characteristic bondlengths have been published (Gagné & Hawthorne 2024). There has been a dichotomy between the radii deduced from experimental bondlengths, which assumes a constant (or near-constant) radius for each individual anion, and the quantum-mechanical approach (e.g., QTAIM), which calculates large variations in individual anion (and cation) radii, depending on the identity of the constituent bonded cation and the observed mean bondlength. It may be shown that ion radii derived from observed bondlengths are proxy variables for characteristic bondlengths, they do not represent the radii of ions in crystals, resolving the issue of experimental bondlengths and quantum-mechanical calculations giving different ion radii. The proxy nature of ion radii do not affect their effectiveness when sums of radii are used, but prevent any use of radius ratios, accounting for the ineffectiveness of Pauling’s first rule. The reasoning behind Pauling’s first rule restricts the number of coordination numbers possible for an ion pair to two, whereas most ions violate this constraint and the number of observed coordination numbers for an ion pair can be as large as 12. The Lewis acidity of a cation can be defined as oxidation state / coordination number. The handshaking principle in Graph Theory shows that ion pairs can form a chemical bond when the Lewis acidity of the cation closely matches the Lewis basicity of the anion. Thus the bond-strength matching principle dictates the coordination numbers of the constituent ions in a structure, accounting for the range in coordination numbers observed for each ion. Examples will be shown for simple ions and for polyanions and polycations.
Gagné, O.C. & Hawthorne, F.C. (2024) Acta Crystallographica B80, 326-339.
To join the lecture: https://ubc.zoom.us/j/63584201004?pwd=pYAXu867bLlaG6IbKjkBSvMVSWz7Bn.1
Meeting ID: 635 8420 1004
Passcode: 299673
