The novel [4,4]-sigmahaptotropic rearrangement II → IV, in which a σ-bonded group and a Fe(CO)3 unit exchange bonding sites antarafacially across a four carbon skeleton, has been shown by kinetic data to involve a concerted, non-synchronous, one step process. First order rate constants for the rearrangement at 23°C are k 1.25 × 10-6 s-1 (acetone) and k 2.2 × 10-5 s-1 (methanol), with activation parameters ΔH# 21 kcal mol-1 and ΔS# -15 e.u. (acetone). The moderate value of the ratio of k in methanol to that in acetone, viz. 18, indicates that although a minor charge separation develops upon activation, no intermediate is formed. The least motion pathway mechanism is shown to involve a Berry pseudorotation about the metal, which maintains the bonding interaction between the metal and the organic fragment orbitals during the rearrangement. Topologically this rearrangement corresponds to a [σ2a + (σ2s + π2a)] thermally allowed pericyclic reaction. The structure of the rearranged complex IV was determined by single-crystal X-ray diffraction.