Synthesis, molecular structure and properties of the [H6-n,Ni30C4(CO)(34)(CdCl)(2)](n-) (n=3-6) bimetallic carbide carbonyl cluster: A model for the growth of noncompact interstitial metal carbides

摘要

Reaction of the [Ni9C(CO)(17)](2-) dianion with CdCl2 center dot 2.5H(2)O in THF affords the novel bimetallic Ni-Cd carbide carbonyl clusters [H6-nNi30C4(CO)(34)(mu(5)- 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. thev clearly indicate that the clusters [H6-nNi30C4(CO)(34)(mu(5)-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)- (mu(5)-CdCl)(2)](n-) anions (n=4-6) have been structurally characterized in their [NMe3(CH2Ph)](4)[H2Ni30C4(CO)(34)- (CdCl)(2)]center dot 2 COMe2, [NEt4](5)[HNi30C4-(CO)(34)(CdCl)(2)]center dot 2MeCN and [NMe4](6)- [Ni30C4(CO)(34)(CdCl)(2)]center dot 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 Ni-Cd 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) (mu(5)-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 H ickel molecular orbital (EHMO) analysis.