From spiropentane to butterfly and tetrahedral structures in tetranuclear iron carbonyl carbide chemistry†
Abstract
Oxidative degradation of the octahedral dianion [Fe6C(CO)16]2− with an interstitial carbon atom leads eventually to the neutral Fe4C(CO)13 cluster with a butterfly-shaped central Fe4C unit. The complete series of related Fe4C(CO)n (n = 16, 15, 14, 13, 12, 11) derivatives have now been investigated using density functional theory. For the lowest energy Fe4C(CO)n (n = 16, 15, 14, 13) structures the geometries obey the n + f = 18 rule where f is the number of Fe–Fe bonds. This leads to a spiropentane geometry with two Fe–Fe bonds for Fe4C(CO)16, a central bent Fe–Fe–Fe–Fe chain for Fe4C(CO)15, a distorted trigonal pyramidal structure with four Fe–Fe bonds for Fe4C(CO)14, and the experimentally observed butterfly structure with five Fe–Fe bonds for Fe4C(CO)13. A symmetrical higher energy centered tetrahedral structure for Fe4C(CO)12 with six Fe–Fe bonds also follows the n + f = 18 rule. However, the lowest energy Fe4C(CO)n (n = 12, 11) structures are derived from the lowest energy Fe4C(CO)13 structure by removal of CO groups with retention of the central Fe4C butterfly unit.