The synthesis and characterization of the chloride and iodide ion complexes of a set of isomeric hydrophobic tetraphenyl [12]mercuracarborand-4 hosts having sterically encumbered cavities are described. These compounds [(3-C6H5-1,2-C2B10H9Hg)(4)]. XLi (3 . XLi, X = CL(-) and I-) were prepared by the reaction of the corresponding mercuric halides with closo-3-Ph-1,2-Li-2-1,2C2B10H9. The presence of phenyl substituents in this system requires the existence of isomers in which the phenyl substituents are arrayed about the periphery of the host cavity. Three of four possible stereoisomers of the chloride ion complex were detected by HPLC analysis of the reaction mixtures, separated by recrystallization, identified as the desired species by the negative ion FAB mass spectrum, and structurally characterized by crystallography, X-ray diffraction data are as follows : 3a . ClLi (1,2-alternate structure with phenyl substituents in an "up", "up", "down", "down" fashion) : monoclinic, space group P2/a, a = 24.080(1) Angstrom, b = 13.4509(8) Angstrom, and c = 26.054(2) Angstrom, beta = 99.488(2)degrees, V = 8323 Angstrom(3), Z = 4, rho(calcd) = 1.61 g cm(-3), R = 0.056, R(w) 0.067. 3b . ClLi (1,3-alternate structure; phenyl substituents "up", "down", "up", "down") : triclinic, space group P (1) over bar, a = 12.8692(9) Angstrom, b = 18.126(1) Angstrom, and c = 18.309(1) Angstrom, a = 114.593(2)degrees, beta 100.016(2)degrees, and gamma 97.259(2)degrees, V = 3729 Angstrom(3), Z = 2, R = 0.076, R(w) = 0.097. 3c . ClLi (partial cone structure; phenyl substituents "up", "up", "up", "down") : monoclinic, space group Cm, a 16.405(2) Angstrom, b = 18.928(3) Angstrom, and c = 10.409(1) Angstrom, beta = 112.610(4)degrees, V = 2984 Angstrom(3), Z = 2, rho(calcd), and 1.91 g cm(-3), R = 0.032, R(w) = 0.045. This molecule has a mirror plane. In contrast to these results, only one of four possible stereoisomers of the iodide ion complex was detected by HPLC analysis of the reaction mixtures.