Metalloboranes as Unusual Superhalogens

Electron affinity plays a major role in evaluating the chemical reactivity of clusters. The electron affinity of halogens is highest among all elements. Among halogens, Cl has largest electron affinity (3.62 eV). A class called “superhalogens” known in chemistry from the past three decades has even greater electron affinity that Cl. These investigated superhalogens follow either the 8-electron rule or the 18-electron rule. In this chapter we will discuss a class of metalloboranes developed by doping closo-boranes (e.g., B₁₂H₁₂) with selected metal atoms such as Zn and Al as well as by replacing a B atom with Be or C, in which the unusual behaviour of these superhalogens are predicted by Density functional theory (DFT), Natural bond orbital analysis (NBO), Bader’s Quantum theory of atoms in the molecule (QTAIM) approach, C-squared population analysis (CSPA) and Ab initio molecular dynamics (AIMD) simulation methods. Remarkably, these metalloboranes defy the 8-electron rule or the 18-electron rule. The unexpected behaviour is due to the residing of added electron on the doped metal atom (positively charged) due to electron transfer. These unconventional superhalogens have now opened a new dimension to design and synthesize novel materials with tailored properties for wider applications in cluster chemistry.