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Extensive chemical, spectrophotometric, and x-ray structural studies have shown that trivalent rhenium is strongly homophilic-that is, it tends to form bonds to other Re(111) atoms-and it forms at least three different series of ReX(4)(n)(n-) complexes. The mononuclear, square complex, ReBr(4)(-), adds two water molecules to give trans-ReBr(4)(H(2)O)(2)(-). The binuclear complexes Re(2)Cl(8)(2-) and Re(2)Br(8)(2-) have strong Re-Re bonds, unsupported by halide bridges. The trinuclear species, Re(3)X(12)(3-) or Re(3)X(9)L(3), contain the triangular Re(3)X(9) clusters. Use of ReCI(3) appears always to lead directly to products containing Re(3)Cl(9); this unit exists in ReCl(3) itself and does not appear to be kinetically labile. The Re(2)X(8)(2-) ions are obtained by reduction of ReO(4)(-) in aqueous HCl or HBr. Salts of ReBr(4)(H(2)O)(2)(-) can be obtained directly from solutions of ReBr(3) in HBr along with numerous other compounds, some containing trinuclear clusters.
Cotton et al. (Fri,) studied this question.