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, Fe 4 (CS) 4 (CO) n ( n = 12, 11, 10, 9), which are not found in their homoleptic analogues, Fe 4 (CO) n +4 . Thus the lowest energy Fe 4 (CS) 4 (CO) 12 structures have a central Fe 3 triangle with an exotriangular iron atom joined to the Fe 3 triangle through a four‐electron donor CS bridge. This contrasts with the structure of Os 4 (CO) 16 and the predicted structure of Fe 4 (CO) 16 , which consist of an M 4 rhombus and two‐electron donor carbonyl groups. An even more remarkable new structural motif for the Fe 4 (CS) 4 (CO) n derivatives is the irregular Fe 4 “rhombus” (actually a trapezium) bridged by a six‐electron donor η 2 ‐μ 4 ‐CS group. This type of structure is found in the lowest energy structures of both Fe 4 (CS) 4 (CO) 10 and Fe 4 (CS) 4 (CO) 9 and makes Fe 4 (CS) 4 (CO) 10 viable enough to be a promising synthetic objective. On the contrary, Fe 4 (CS) 4 (CO) 11 is found to be thermochemically unfavorable both with respect to CO dissociation and disproportionation into Fe 4 (CS) 4 (CO) 12 and Fe 4 (CS) 4 (CO) 10 .