How Strong is a Reverse Dative Bond? Compliance Constants as Unique Bond Strength Descriptors

The concepts of bond strength and bond order are at the very heart of chemistry. Nevertheless, the study of local mechanical bond strengths in large polyatomic molecules is a nontrivial task. In a recent publication, Peters and Lehnert et al. [Inorg. Chem. 2020, 59 (20), 14967-14982] found a quite strong (reverse) dative Fe···B bond for a synthesized low-spin {FeNO}¹⁰ complex supported by an ambiphilic trisphosphineborane ligand. Their conclusion was, among other criteria, based on a surprisingly high Fe-B force constant of 1.56 mdyn/Å. Nevertheless, this high value might be based on numerical ambiguities of (rigid) real-space force constants, a flaw, that may affect many other combined spectroscopic and electronic structure studies.