Spiro[2.4]hepta-4,6-diene)tricarbonyliron (2) self-converts to the tethered acyl complex (CO) 2 FeC(O)CH 2 CH 2 (η 5 -C 5 H 4 ) (3) when refluxed for 1 h in common organic solvents. The reaction of 2 with an external electrophile (CPh 3 + , H + ) opens the three-membered ring, resulting in the cation (η 5 -C 5 H 4 CH 2 CH 2 R)Fe(CO) 3 + whose structure was determined by spectroscopic methods and confirmed by X-ray analysis of the PPh 3 derivative (η 5 -C 5 H 4 CH 2 -CH 3 )Fe(CO) 2 (PPh 3 ) + . The reaction of 3 with a strong electrophile (CPh 3 + , Me + ) affords the rotationally restricted Fe-(alkoxy)carbene cation (CO) 2 FedC(OR)CH 2 CH 2 (η 5 -C 5 H 4 ) + , which can be converted to the Fe-(anilino)carbene cation (CO) 2 FedC(NHPh)CH 2 CH 2 (η 5 -C 5 H 4 ) + . 146 °C) originating from 4. If brought to reflux for 1 h, compound 2 in solution is fully converted to 3, whose IR ν(CO) stretching bands are shown in Figure 1d. Compound 3 is not soluble in n-hexane or Et 2 O but is soluble in THF and chlorinated solvents.Reactions of 2 with Lewis and Protic Acids. Scheme 1 shows a proposed, intramolecular, ionic rear-