Abstract
Density functional calculations on the Co~2~(CO)~6~(SR)~2~ compounds (R = CH~3~, CF~3~) predict both open and butterfly structure types of similar energies. The open Co~2~(CO)~6~(μ‐SR)~2~ structures have non‐planar central Co~2~S~2~ units with two bridging RS groups and ca. 3.4 Å Co···Co distances indicating a lack of direct metal–metal bonding. The butterfly Co~2~(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 Co~2~(CO)~6~(SCH~3~)~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 Co~2~(CO)~6~(SCF~3~)~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 Co~2~(CO)~6~(SCF~3~)~2~ structures is more than five times higher than for the Co~2~(CO)~6~(SCH~3~)~2~ structures. The electronegativity of the RS group in the Co~2~(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 Co~2~(CO)~6~(μ‐η^2^:η^2^‐S~2~R~2~) structure with a central Co~2~S~2~ 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 Co~2~S~2~ tetrahedranes Co~2~(CO)~6~[μ‐η^2^:η^2^‐S~2~(C~6~X~5~)~2~] (X = F, Cl) as the trinuclear derivatives Co~3~(μ~3~‐S)(C~6~F~5~)(CO)~8~.