Ni-Cu tetracarbide carbonyls with vacant Ni(CO) fragments as borderline compounds between molecular and quasi-molecular clusters

The reaction of [Ni9C(CO)(17)](2-) with [Cu(CH3CN)(4)][BF4] (1.1-1.5 equiv.) afforded the first Ni-Cu carbide carbonyl cluster, i.e., [H2Ni30C4(CO)(34){Cu(CH3CN)}(2)](4-) ([H21](4-)). This has been crystallised in a pure form with miscellaneous [NR4](+) (R = Me, Et) cations, as well as co-crystallised with [H2Ni29C4(CO)(33){Cu(CH3CN)}(2)](4-) ([H(2)2](4-)) which differs from [H(2)1](4-) by a missing Ni(CO) fragment. By increasing the [Cu(CH3CN)(4)](+)/[Ni9C(CO)(17)](2-) ratio to 1.7-1.8, the closely related [H2Ni30C4(CO)(35){Cu(CH3CN)}(2)](2-) ([H(2)3](2-)), [H2Ni29C4(CO)(34){Cu(CH3CN)}(2)](2-) ([H(2)4](2-)), and [H2Ni29C4(CO)(32)(CH3CN)(2){Cu(CH3CN)}(2)](2-) ([H(2)5](2-)) dianions have been obtained. Replacement of Cu-bonded CH3CN with p-NCC6H4CN afforded, after protonation of the tetra-anion, mixtures of [H3Ni30C4(CO)(34){Cu(NCC6H4CN)}(2)](3-) ([H(3)6](3-)) and [H3Ni29C4(CO)(33){Cu(NCC6H4CN)}(2)](3-) ([H(3)7](3-)). The species 1-7 display a common Ni28C4Cu2 core and differ for the charge, the presence of additional Ni atoms, the number and nature of the ligands, even though they are obtained under similar experimental conditions and often in mixtures. Their nature in solution has been investigated via FT-IR, chemical and electrochemical methods. Electrochemical studies, besides confirming the poly-hydride nature of these clusters, show that they undergo different redox processes with features of chemical reversibility, and this might be taken as proof of the incipient metallisation of their metal cores.