A strategically different synthetic scheme leads to an "inside-out" mercuracarborand in which the mercury atoms are bonded to the boron vertices and the carbon vertices of the meta-carborane are directed away from the internal cavity. This use of the boron vertices of the carboranes for the skeletal linkage within the mercuracarborand array allows the peripheral carbon vertices to be derivatized. Although structurally similar to the carbon-linked mercuracarborand trimer, the electronic properties of the "inside-out" trimer are very different. The boron vertices attached to the mercury atoms are electron-rich and the mercuracarborand does not coordinate guest anions.
          As mentioned above, the enhancement of hydrocarbon solubility of mercuracarborands was attained by the attachment of hydrophobic organic substituents to the boron vertices of the component carborane cages. This solubility in weakly coordinating lipophlic solvents was desirable for the study of electrophilic catalysis by mercuracarborands or the coordination of weakly-bound electron donors with the same acceptors. An equally important challenge is finding a means of solubilizing mercuracarborands and their complexes in aqueous media by introducing hydrophilic substituents. The purpose of moving this chemistry toward aqueous media is several-fold. First, biomolecules such as amino acids, peptides, proteins, nucleic acids, DNA and RNA may be examined as their mercuracarborand complexes using ¹⁹⁹Hg and perhaps ¹¹B NMR studies coupled, where possible, to crystallographic structural determinations. Second, it should be possible to quantitatively evaluate ionic and related equilibria involving the formation of a broad range of mercuracarborand complexes and thereby achieve a quantitative understanding of guest-binding selectivities and quantitative activity-structure relationships; the formation constant, K_a, for the [12]-mercuracarborand-4^.Cl^- complex shown in Figure 1 is estimated to be 10⁷. Third, aqueous organic reactions which are subject to electrophilic catalysis may be assisted with mercuracarborand catalysts. Recently, we reported mercuracarborand-catalyzed Diels-Alder reactions of a thioester with cyclopentadiene in methylene chloride (see Figure 8).