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

Article abstract of DOI:10.1002/zaac.200400538

Reaction of 2-chloro-1,3,2-diazaarsolenes and -diazaphospholenes with Tl[Co(CO)₄] gives instable complexes of type [Co(ER ₂)(CO)₄] which decarbonylated to yield [Co(ER ₂)(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 η²(EN)-coordination of the EN₂C₂ heterocycle as an alternative to the formation of a carbene-like structure for [Co(ER₂)(CO)₃]. The η2-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.