Zero- and mono-coordinate transition metals in crystal structures: A box full of surprises

Coordinating Ability of Anions, Solvents, Amino Acids, and Gases towards Alkaline and Alkaline‐Earth Elements, Transition Metals, and Lanthanides

“…We noticed, however,t hat it is frequent to find complexes in which ac ounterion or as olvation molecule is weakly bound to am etal atom, in what is often called "secondary coordination", "incipient bonding", "contact", or "semi-coordinating interaction", [14] including Jahn-Te ller elongated bonds in copper-(II) complexes and also undisclosed bonds to metal atoms, for instancei ns tructures in which am etal is described as zero-or mono-coordinated. [17] We refer to those purportedly nonbondedg roups that form contacts to at ransition metal at a distance shorter than the van der Waals radii sum as "semi-coordinated", and consider semi-coordination also as an indication of the coordinatinga bility of ag roup. We have therefore defined ac oordinating ability index of ag roup toward transition metals, a TM ,a si nE quation 1, in which c, s and u are the number of structuresw ith the group coordinated, semi-coordinated and uncoordinated, respectively,t oany transition metal.…”

“…The strategy proposed earlier by us to calibrate the coordinating ability of a given species toward Lewis acids, consists in comparing the number of crystal structures in which it appears coordinated with the number of structures in which it co‐crystallizes with the Lewis acid of interest without being coordinated. We noticed, however, that it is frequent to find complexes in which a counterion or a solvation molecule is weakly bound to a metal atom, in what is often called “secondary coordination”, “incipient bonding”, “contact”, or “semi‐coordinating interaction”, including Jahn–Teller elongated bonds in copper(II) complexes and also undisclosed bonds to metal atoms, for instance in structures in which a metal is described as zero‐ or mono‐coordinated . We refer to those purportedly non‐bonded groups that form contacts to a transition metal at a distance shorter than the van der Waals radii sum as “semi‐coordinated”, and consider semi‐coordination also as an indication of the coordinating ability of a group.…”

Continuous Shape Measures Study of the Coordination Spheres of Actinide Complexes – Part 1: Low Coordination Numbers

“…It has already been mentioned that monocoordinated metal atoms are unlikely in the solid state and in solution. [19] Consistently, the only two structures retrieved in a structural search for monocoordinated actinide atoms correspond to U atoms in U=N = U and CSc 2 U units encapsulated within a C 80 fullerene, whose rings are actually π-coordinated to uranium [20] and should not be considered as monocoordinated. In contrast, in the rarefied environment of the gas phase, a monocoordinated metal can retain its coordinative unsaturation thanks to the absence of nearby donor species.…”

“…Compounds with coordination numbers zero or one have also been disregarded, since in the solid state they are unlikely and usually have higher undisclosed coordination numbers. [19] A total of 7,122 structures with atomic coordinates were retained, containing a total of 10,022 actinide-containing fragments with coordination numbers between 2 and 14. The abundance of each coordination number among the actinides and among the uranium compounds only is represented in Figure 3, together with the distribution found for transition metals and lanthanides.…”