The umbrella-shaped trimethylenemethane ligand in binuclear chromium carbonyl complexes

The effect of pairwise replacement of terminal CO groups with the umbrella-shaped trimethylenemethane ligand on the structures and stabilities of binuclear chromium carbonyls has been examined using density functional theory. Such species are likely photolysis products of the known (CH2)3CCr(CO)4. The lowest energy [(CH2)3C]2Cr2(CO)7 structure has two semibridging CO groups similar to the lowest energy Cr2(CO)11 structure. This [(CH2)3C]2Cr2(CO)7 structure is disfavored by ∼14 kcal/mol with respect to dissociation into (CH2)3CCr(CO)4 + (CH2)3CCr(CO)3. The lowest energy [(CH2)3C]2Cr2(CO)6 structure is a singly bridged structure in contrast to the doubly semibridged lowest energy structure of the analogous Cr2(CO)10. However, a doubly semibridged [(CH2)3C]2Cr2(CO)6 structure similar to the Cr2(CO)10 structure is predicted to lie ∼16 kcal/mol above the singly bridged global minimum. The lowest energy [(CH2)3C]2Cr2(CO)5 structure is an unbridged structure in contrast to the triply semibridged lowest energy Cr2(CO)9 structure. However, a related triply semibridged [(CH2)3C]2Cr2(CO)5 structure is predicted to lie only slightly in energy above this global minimum. The [(CH2)3C]2Cr2(CO)5 structures, like the corresponding Cr2(CO)9 structure, have formal CrCr triple bonds of lengths ∼2.3 Å and are predicted to be very stable with respect to dissociation into mononuclear fragments with dissociation energies greater than 35 kcal/mol.

Density functional theory suggests a doubly CO-semibridged structure for [(CH2)3C]2Cr2(CO)7 but this structure is disfavored with respect to dissociation into mononuclear fragments. Singly bridged and doubly semibridged structures are found for [(CH2)3C]2Cr2(CO)6. The lowest energy [(CH2)3C]2Cr2(CO)5 structure is a viable unbridged structure with a CrCr triple bond.Figure optionsDownload as PowerPoint slideHighlights
► The lowest energy [(CH2)3C]2Cr2(CO)7 structure has two semibridging CO groups.
► Dissociation of the [(CH2)3C]2Cr2(CO)7 structure into mononuclear fragments is exothermic.
► The lowest energy [(CH2)3C]2Cr2(CO)6 structure has one bridging CO group.
► Unbridged and triply semibridged low energy [(CH2)3C]2Cr2(CO)5 structures are found.
► The [(CH2)3C]2Cr2(CO)5 structures have CrCr triple bonds and are viable with respect to dissociation.