The addition of various carbanionic nucleophiles Nu1 to the cationic complex [(η5-Cp)Fe(η6-Cot)]+ (1) exclusively yields the neutral complex [(η5-Cp)Fe(1,2,3,4,5-η-C8H8Nu1)] (2) with Nu1 in exo position with respect to the metal center: Nu1 = CH(CO2Me)2 (2a), CEt(CO2Me)2 (2b), CPh2CN (2c), CH(CN)2 (2d), C(CO2Et)2(CH2)2CO2Et (2e), C(CO2Et)2(CH2)3CO2Et (2f), C(CO2Me)2(CH2)2CN (2g), CH(COMe)CO2Et (2h). Protonation of 2 by HBF4 reveals the complexes [(η5-Cp)Fe(η6-C8H9Nu1)]BF4 (3BF4) with a 1,2,3,4,5,6-η coordination mode of the cyclo-C8 ligand. The cationic complexes 3 are suitable for a second nucleophilic addition affording the exo-6,8-disubstituted cyclooctadienyl complex [(η5-Cp)Fe(1,2,3,4,5-η-C8H9-6-Nu1-8-Nu2)] (4): Nu1/Nu2 = CH(CO2Me)2/CH(CO2Me)2 (4a), CEt(CO2Me)2/CH(CO2Me)2 (4b), CPh2CN/CPh2CN (4c). It can be shown that the nucleophilic addition occurs not only with the carbanionic nucleophiles, which must be prepared separately by deprotonation reactions with NaH, but also in-situ, when the cationic complex, the C,H acidic substrate, and the strong base tetramethyl guanidine (TMG) are present. The capability of facile deprotonation reaction by TMG enables a one-pot procedure of the synthesis of 4a without isolation of the intermediates 2 and 3. Attempts of intramolecular nucleophilic additions in 3dBF4 (Nu1 = C(CO2Et)2(CH2)2CO2Et) and 3eBF4 (Nu1 = C(CO2Et)2(CH2)3CO2Et) by application of TMG failed in the formation of cyclo-C8-based bicycles but rather result in addition of TMG to the cyclo-C8 ligand forming 4d and 4e with tetramethyl guanidinyl as Nu2. The protonation of 4a−c in acetonitrile by addition of CF3CO2H splits off the cyclo-C8 ligand as a cis-5,7-disubstituted cycloocta-1,3-diene (6): Nu1/Nu2 = CH(CO2Me)2/CH(CO2Me)2 (6a), CEt(CO2Me)2/CH(CO2Me)2 (6b), CPh2CN/CPh2CN (6c). The protonation of 4d and 4e cleaves the guanidinyl substituent and recovers the starting complexes 3d and 3e. When the steric demand of Nu1 and Nu2 is different, as in 4b, the isomer that bears the sterically most demanding nucleophile proximal to the endocyclic carbon−carbon double bond is preferentially formed.