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

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 .