Sr₃[Co(CN)₃] and Ba₃[Co(CN)₃]: Crystal Structure, Chemical Bonding, and Conceptional Considerations of Highly Reduced Metalates

Abstract

Überraschende Eigenschaften hält der trigonal‐planare Komplex [Co(CN)3]6− bereit (siehe Strukturbild; Co rot, C grau, N grün, Erdalkalimetall‐Gegenionen gelb). Bindungslängen und Ladungen der CN‐Liganden variieren, und das Co‐Zentrum verfügt über eine geschlossenschalige (d10‐)Konfiguration.

“Guilty” Verdict—Evidence for the Noninnocence of Cyanide

“…However, the CÀN distances and n CN values of the known cyanometalates have proven remarkably invariant. [5,7] Hçhn et al have now reported [8] a completely documented case of reduced cyanide, that is, three CN 1.67À ions stabilized by cobalt(ÀI) in M 3 [Co(CN) 3 ] (M= Sr, Ba). Remarkably, the material was obtained by a high-temperature ( % 1200 K) "solid-state" route and not by a less harsh coordination chemistry procedure.…”

“…The absence of ligand-field effects in a d , the high negative charge of which is offset by three dipositive cations. A low, but not excessively negative oxidation state for cobalt was also suggested by density of states (DOS) calculations and the quantum theory of atoms in molecules (QTAIM) approach, [8] which yielded approximate charges of + 1.4 (Ba), 0.0 (Co), and À1.4 (CN). [8] Figure 1).…”

“…"Noninnocence" as introduced by Jørgensen [10a] and as elaborated by Ward and McCleverty, [10b] as well as Wieghardt and co-workers [10c] pertains to redoxactive ligands that can create an ambi-valence in the electron distribution and thus in the oxidation state formulation of the corresponding coordination compounds. Among the typical noninnocently behaving ligands are often p conjugated organic systems, with [11a] or without bioinorganic relevance, [11] as well as diatomic ligands such as the classical NO and, in analogy, of NOC 2À , [14b,c] the now fully characterized CN 1.67À [8] can be fitted into a corresponding scheme of noninnocent diatomic ligands (Figure 1).…”

“…The compounds could be analyzed by a variety of physical methods: [8] Structure determination by X-ray diffraction of single crystals (M = Ba) or powders revealed trigonal-planar tricyanocobaltate with significantly lengthened C À N bonds of about 1.24 relative to the typical [5,7] 1.15 of coordinated CN À . Consequently, the CN stretching vibrations were found at remarkably lowered frequencies of 1670-1700 cm À1 (IR and Raman), shifted by about 400 cm À1 from the typical 2100 cm À1 [5,7] as found for related cyanometalates such as Na 2 [Cu(CN) 3 ]·3 H 2 O, [7b] or K 3 [Co(CN) 6 ] with low-spin cobalt-(+III).…”

“…[8] Considering the propensity of cyanide to act as a bridge (like in Prussian Blue [1] ) and to adopt several other coordination modes, [5] the now proven potential for noninnocent behavior adds an additional, an electron-transfer dimension, promising exciting developments for the 4th century of cyanometalate chemistry.…”