Donor-substituted allenylidene(carbonyl)(XR 3)chromium complexes (X=P, As, Sb) – synthesis and properties

Abstract

Photolysis of the allenylidene pentacarbonyl chromium complexes [(CO)₅Cr=C=C=C(R¹)R²] (R¹ = NMe₂, NPh₂; R² = NMe₂, OMe, Ph) in THF in the presence of equimolar amounts of XR₃(XR₃ = various phosphanes, P(OMe)₃, AsPh₃, SbPh₃) affords cis-allenylidene tetracarbonyl XR₃ complexes, cis-[(CO)₄(XR₃)Cr=C=C=C(R¹)R²]. When in the photolysis of [(CO)₅Cr=C=C=C(NMe₂)Ph], the phosphanes PR₃ (R=C₆H₄F-p, C₆H₄Cl-p, OMe) are used in excess (three equivalents) two carbonyl ligands are displaced and the mer-tricarbonyl complexes mer-[(CO)₃(PR₃)₂Cr=C=C=C(NMe₂)Ph] are formed both PR3 ligands being mutually trans. The structure of the new complexes is established by IR, NMR, and UV–Vis spectroscopy, that of cis-[(CO)4(PPh3)Cr=C=C=C(NMe2)Ph] additionally by an X-ray structural analysis. As indicated by the spectroscopic data of the compounds, these complexes are best described as hybrids of allenylidene and zwitterionic alkynyl complexes with delocalization of the electron pair at nitrogen bonded to the C_γ atom of the allenylidene ligand towards the metal center. The relative contribution of the allenylidene and zwitterionic alkynyl resonance forms is influenced by XR₃. Increasing the donor properties of XR₃ favors the allenylidene resonance form.