Reaction of [(η5-C5H4Me)4Fe4(HCCH)(HCCBr)](PF6) ([2](PF6)) with AgPF6 in acetonitrile gave [(η5-C5H4Me)4Fe4(HCCH)(HCCNCMe)](PF6)2 ([3](PF6)2). The X-ray diffraction study revealed that the cationic [CCH] subunit is stabilized through coordination of the acetonitrile molecule to the cationic carbon atom. As a synthon for a donor-free [(η5-C5H4Me)4Fe4(HCCH)(HCC)]2+, [(η5-C5H4Me)4Fe4(HCCH)(HCCL)](PF6)2 ([6](PF6)2, L = pyrazine) was synthesized by the reaction of [2](PF6) with AgPF6 in the presence of pyrazine. Treatment of [6](PF6)2 with tertiary amines in acetonitrile led to deprotonation of acetonitrile to form [(η5-C5H4Me)4Fe4(HCCH)(HCCCH2CN)](PF6) ([11](PF6)). Treatment of [6](PF6)2 with maleimide in the presence of tertiary amine in acetonitrile allowed the functionalization of the coordinated acetonitrile through the nucleophilic attack of maleimide at the nitrile carbon atom.

A 3:1 molar ratio mixture of [(η5-C5H4Me)4Fe4(HCCBr)2](PF6) (1a) and [(η5-C5H4Me)4Fe4(HCCH)(BrCCBr)](PF6) (1b) was converted to [(η5-C5H4Me)4Fe4(μ3-CH)2(μ3-CNPh)2](PF6)2 (2) upon treatment with aniline, followed by NiPr2Et and finally [Cp2Fe](PF6). The X-ray diffraction analysis revealed that 2 can be described as a cubane-type tetrairon cluster possessing two μ3-CH and two μ3-isonitrile ligands. Treatment of 2 with 2.5 equiv of [Cp2Co] gave the neutral form 3, formulated as [(η5-C5H4Me)4Fe4(HCCH)2(μ3-CNPh)2]. The redox reactions were chemically reversible; treatment of 3 with [Cp2Fe](PF6) reproduced 2 quantitatively. The structure of 3 was determined by X-ray diffraction analysis. The molecule exhibits a butterfly geometry resulting from the scission of one of the iron–iron bonds of the tetrahedron in 2. In accordance with the conversion of the core structure from tetrahedron to butterfly, the coupling of two μ3-CH ligands occurs to form an acetylene ligand. Further treatment of 3 with LiAlH4 followed by air-oxidation resulted in reductive coupling of two isonitrile ligands to give a bis(acetylene) cluster, [(η5-C5H4Me)4Fe4(HCCH)2](PF6) (6).

Reaction of [(η5-C5H4Me)4Fe4(HCCH)(HCC–Br)](PF6) with tBuNH2 forms [(η5-C5H4Me)4Fe4(HCCH)(HCC–NHtBu)](PF6), of which one electron oxidation leads to carbon–carbon bond cleavage to give [(η5-C5H4Me)4Fe4(HCCH)(μ3-CH)(μ3-CNHtBu)](PF6)2, and further treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) affords [(η5-C5H4Me)4Fe4(HCCH)(μ3-CH)(μ3-η1-CNtBu)](PF6). Thus, the HCCBr group is converted to tBuNC and CH groups on the redox-responsive tetrairon core.

Reaction of [(η5-C5H4Me)4Fe4(HCCH)(HCCBr)](PF6) (1) with HPPh2 in the presence of NEt3, followed by treatment of [Cp2Co], afforded [(η5-C5H4Me)4Fe4(HCCH)(HCC−PPh2)] (2). The electron-rich [4Fe−4C] core substituent leads to the extremely electron-releasing character of the phosphine part, estimated by the JPSe coupling constant of the corresponding selenide [(η5-C5H4Me)4Fe4(HCCH)(HCC−P(Se)Ph2)] (3). Reaction of 2 with [AuCl(SMe2)] gave [(η5-C5H4Me)4Fe4(HCCH)(HCC−P(AuCl)Ph2)] (4). The cyclic voltammogram of 4 shows two reversible one-electron oxidation waves, indicating the existence of one- and two-electron oxidized forms.