New Structural Features in Tetranuclear Iron Carbonyl Thiocarbonyls: Exotriangular Iron Atoms and Six-Electron-Donating Thiocarbonyl Groups Bridging Four Iron Atoms

DFT led to the discovery of two new structural motifs in tetranuclear iron carbonyl thiocarbonyls, Fe4(CS)4(CO)n (n = 12, 11, 10, 9), which are not found in their homoleptic analogues, Fe4(CO)n+4. Thus the lowest energy Fe4(CS)4(CO)12 structures have a central Fe3 triangle with an exotriangular iron atom joined to the Fe3 triangle through a four‐electron donor CS bridge. This contrasts with the structure of Os4(CO)16 and the predicted structure of Fe4(CO)16, which consist of an M4 rhombus and two‐electron donor carbonyl groups. An even more remarkable new structural motif for the Fe4(CS)4(CO)n derivatives is the irregular Fe4 “rhombus” (actually a trapezium) bridged by a six‐electron donor η2‐μ4‐CS group. This type of structure is found in the lowest energy structures of both Fe4(CS)4(CO)10 and Fe4(CS)4(CO)9 and makes Fe4(CS)4(CO)10 viable enough to be a promising synthetic objective. On the contrary, Fe4(CS)4(CO)11 is found to be thermochemically unfavorable both with respect to CO dissociation and disproportionation into Fe4(CS)4(CO)12 and Fe4(CS)4(CO)10. The lowest energy Fe4(CS)4(CO)12 structures have a central Fe3 triangle with an exotriangular iron atom joined to the Fe3 triangle by a four‐electron donor “end‐on” CS bridge. An Fe4 rhombus bridged by a six‐electron donor η2‐μ4‐CS group is found in the lowest energy structures of Fe4(CS)4(CO)10 and Fe4(CS)4(CO)9 and makes Fe4(CS)4(CO)10 viable enough to be a promising synthetic objective.