Metal–Metal Bonding in Bis(alkylthio)hexacarbonyldicobalt Complexes: Open Structures vs. Butterfly and Tetrahedrane Structures

Abstract

AbstractDensity functional calculations on the Co2(CO)6(SR)2 compounds (R = CH3, CF3) predict both open and butterfly structure types of similar energies. The open Co2(CO)6(μ‐SR)2 structures have non‐planar central Co2S2 units with two bridging RS groups and ca. 3.4 Å Co···Co distances indicating a lack of direct metal–metal bonding. The butterfly Co2(CO)5(μ‐SR)(μ‐CO)(SR) structures have direct Co–Co bonds of lengths ca. 2.5 Å forming the “body” of the butterfly, one terminal RS group, one bridging RS group, and one bridging CO group. The lowest energy Co2(CO)6(SCH3)2 structure is an open isomer. However, this open isomer lies only 0.4 kcal/mol below the corresponding butterfly isomer. For the corresponding fluorinated derivative Co2(CO)6(SCF3)2 a butterfly structure with a direct Co–Co bond and a bridging CO group lies at a slightly lower energy than the lowest energy open structures. The relative energy difference between open and butterfly Co2(CO)6(SCF3)2 structures is more than five times higher than for the Co2(CO)6(SCH3)2 structures. The electronegativity of the RS group in the Co2(CO)6(SR)2 structures has little effect on the geometric parameters but exerts a significant influence on the atomic charge distribution. The butterfly structures with a direct metal–metal bond are predicted to be nearly isoenergetic to open structures without a Co–Co bond. Unsuccessful attempts to optimize a previously proposed Co2(CO)6(μ‐η2:η2‐S2R2) structure with a central Co2S2 tetrahedrane unit with one Co–Co bond, one S–S bond, and four Co–S bonds are consistent with the previous reformulation of the originally claimed Co2S2 tetrahedranes Co2(CO)6[μ‐η2:η2‐S2(C6X5)2] (X = F, Cl) as the trinuclear derivatives Co3(μ3‐S)(C6F5)(CO)8.