Reactions of η 5-C 5H 5Fe(CO) 2CH 2CCR (R CH 3, C 6H 5, and CH 2Fe(CO) 2(η 5-C 5H 5)) with HBF 4 in acetic anhydride yield [η 5-C 5H 5Fe(CO) 2(η 2CH 2CCHR)] +BF − 4. The resultant cationic iron-η 2-allene complexes react with a wide range of nucleophiles (Nu) to give the following types of behavior: ( a) addition of Nu to carbon-1 of the η 2-allene fragment (with NaBH 4, (C 2H 5) 2NH, and P(C 6H 5) 3, inter alia), ( b) addition of Nu to carbon-2 of the η 2-allene fragment (with NaOCH 3), ( c) addition of Nu to the carbonyl carbon (with NaOC 2H 5), ( d) deprotonation of the iron-η 2-allene cation to the parent propargylic complex (with N(C 2H 5) 3), and ( e) nonselective reactions to yield a mixture of products (with CH 3Li). Of these, the most common is behavior ( a); together with the protonation of η 5-C 5H 5Fe(CO) 2CH 2CCR it stimulates the two-step (3 + 2) cycloaddition reactions between electrophilic molecules and these iron-propargyl complexes.