N-heterocyclic phosphenium, arsenium, and stibenium ions as ligands in transition metal complexes:: A comparative experimental and computational study

Reaction of 2-chloro-1,3.2-diazaarsolenes and -diazaphospholenes with Tl[Co(CO)(4)] gives instable complexes of type [Co(ER2)(CO)(4)] which decarbonylated to yield [Co(ER2)(CO)(3)]. Spectroscopic and X-ray diffraction studies revealed that the tetracarbonyl complexes can be formulated as ion pair for E = P and as covalent metalla-arsine for E = As, and the tricarbonyl complexes as carbene-like species with a formal E=Co double bond. A similar reactivity towards Tl[Co(CO)(4)] was also inferred for 1,3,2-diazastibolenes although the products were not isolable and their constitution remained uncertain. Evaluation of structural and computational data suggests that the weak and polarized Co-As bond in [Co(AsR2)(CO)(4)] can be characterized as an "inverse" M -> L donor-acceptor bond. The computational studies disclosed further eta(2) (EN)-coordination of the EN2C2 heterocycle as an alternative to the formation of a carbene-like structure for [Co(ER2)(CO)(3)]. The eta(2)-complex is less stable for E = P but close in energy for E = As and more stable than the carbene-like complex for E = Sb.