Bond-length distributions for ions bonded to oxygen: results for the transition metals and quantification of the factors underlying bond-length variation in inorganic solids

• Published in: IUCrJ 2020-07, Vol.7 (Pt 4), p.581-629
• Main Authors: Gagné, Olivier Charles, Hawthorne, Frank Christopher
• Format: Article
• Language: English
• Subjects: Analysis; Asymmetry; bond-length variation; bond-topological effects; Configurations; Coordination; Coordination numbers; Crystal structure; Crystallography; Electronic properties; Jahn-Teller effect; Lead; materials design; Mathematical analysis; Metal ions; Optimization; Polyhedra; pseudo jahn–teller effect; Rankings; Topology; Transition metal compounds; Transition metal oxides; vibronic mixing
• ISSN: 2052-2525; 2052-2525
• DOI: 10.1107/s2052252520005928

Bond-length distributions are examined for 63 transition metal ions bonded to O in 147 configurations, for 7522 coordination polyhedra and 41 488 bond distances, providing baseline statistical knowledge of bond lengths for transition metals bonded to O . bond valences are calculated for 140 crystal structures containing 266 coordination polyhedra for 85 transition metal ion configurations with anomalous bond-length distributions. Two new indices, Δ and Δ , are proposed to quantify bond-length variation arising from bond-topological and crystallographic effects in extended solids. Bond-topological mechanisms of bond-length variation are (1) non-local bond-topological asymmetry and (2) multiple-bond formation; crystallographic mechanisms are (3) electronic effects (with an inherent focus on coupled electronic vibrational degeneracy in this work) and (4) crystal-structure effects. The indices Δ and Δ allow one to determine the primary cause(s) of bond-length variation for individual coordination polyhedra and ion configurations, quantify the distorting power of cations via electronic effects (by subtracting the bond-topological contribution to bond-length variation), set expectation limits regarding the extent to which functional properties linked to bond-length variation may be optimized in a given crystal structure (and inform how optimization may be achieved) and more. These indices further provide an equal footing for comparing bond-length variation and the distorting power of ions across ligand types, including resolution for heteroligand polyhedra. The observation of multiple bonds is found to be primarily driven by the bond-topological requirements of crystal structures in solids. However, sometimes multiple bonds are observed to form as a result of electronic effects ( the pseudo Jahn-Teller effect, PJTE); resolution of the origins of multiple-bond formation follows calculation of the Δ and Δ indices on a structure-by-structure basis. Non-local bond-topological asymmetry is the most common cause of bond-length variation in transition metal oxides and oxysalts, followed closely by the PJTE. Non-local bond-topological asymmetry is further suggested to be the most widespread cause of bond-length variation in the solid state, with no limitations with regard to ion identity. Overall, bond-length variations resulting from the PJTE are slightly larger than those resulting from non-local bond-topological asymmetry, comparable with those resulting from the str