Donor-substituted allenylidene(carbonyl)(XR3)chromium complexes (X=P, As, Sb) – synthesis and properties

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 Cγ 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.