Reaction of the [Ni9C(CO)17]2− dianion with CdCl2⋅2.5 H2O in THF affords the novel bimetallic NiCd carbide carbonyl clusters [H6−nNi30C4(CO)34(μ5-CdCl)2]n− (n=3–6), which undergo several protonation–deprotonation equilibria in solution depending on the basicity of the solvent or upon addition of acids or bases. Although the occurrence in solution of these equilibria complicates the pertinent electrochemical studies on their electron-transfer activity, they clearly indicate that the clusters [H6−nNi30C4(CO)34(μ5-CdCl)2]n− (n=3–6), as well as the structurally related [H6−nNi34C4(CO)38]n− (n=4–6), undergo reversible or partially reversible redox processes and provide circumstantial and unambiguous evidence for the presence of hydrides for n=3, 4 and 5. Three of the [H6−nNi30C4(CO)34(μ5-CdCl)2]n− anions (n=4–6) have been structurally characterized in their [NMe3(CH2Ph)]4[H2Ni30C4(CO)34(CdCl)2]⋅2 COMe2, [NEt4]5[HNi30C4(CO)34(CdCl)2]⋅2 MeCN and [NMe4]6[Ni30C4(CO)34(CdCl)2]⋅6 MeCN salts, respectively. All three anions display almost identical geometries and bonding parameters, probably because charge effects are minimized by delocalization over such a large metal carbonyl anion. Moreover, the Ni30C4 core in these NiCd carbide clusters is identical within experimental error to those present in the [HNi34C4(CO)38]5− and [Ni35C4(CO)39]6− species, suggesting that the stepwise assembly of their nickel carbide cores may represent a general pathway of growth of nickel polycarbide clusters. The fact that the [H6−nNi30C4(CO)34(μ5-CdCl)2]n−(n=4–6) anions display two valence electrons more than the structurally related [H6−nNi34C4(CO)38]n− (n=4–6) species has been rationalized by extended Hückel molecular orbital (EHMO) analysis.