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

The reaction of Ni(9)C(CO)(17) with [Cu(CH(3)CN)(4)][BF(4)] (1.1-1.5 equiv.) afforded the first Ni-Cu carbide carbonyl cluster, i.e., H(2)Ni(30)C(4)(CO)(34){Cu(CH(3)CN)}(2) (H(2)1). This has been crystallised in a pure form with miscellaneous NR(4) (R = Me, Et) cations, as well as co-crystallised with H(2)Ni(29)C(4)(CO)(33){Cu(CH(3)CN)}(2) (H(2)2) which differs from H(2)1 by a missing Ni(CO) fragment. By increasing the Cu(CH(3)CN)(4)/Ni(9)C(CO)(17) ratio to 1.7-1.8, the closely related H(2)Ni(30)C(4)(CO)(35){Cu(CH(3)CN)}(2) (H(2)3), H(2)Ni(29)C(4)(CO)(34){Cu(CH(3)CN)}(2) (H(2)4), and H(2)Ni(29)C(4)(CO)(32)(CH(3)CN)(2){Cu(CH(3)CN)}(2) (H(2)5) dianions have been obtained. Replacement of Cu-bonded CH(3)CN with p-NCC(6)H(4)CN afforded, after protonation of the tetra-anion, mixtures of H(3)Ni(30)C(4)(CO)(34){Cu(NCC(6)H(4)CN)}(2) (H(3)6) and H(3)Ni(29)C(4)(CO)(33){Cu(NCC(6)H(4)CN)}(2) (H(3)7). The species 1-7 display a common Ni(28)C(4)Cu(2) 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.