Synthesis, Reactivity, and Structural Characterization of the Nonclassical [MTe₇]^n- Anions (M = Ag, Au, n = 3; M = Hg, n = 2)

Several tellurometalates of the general formula [MTe₇]^n- (n = 2, 3) have been isolated as salts of organic cations by reaction of suitable metal sources with polytelluride solutions in DMF. The [HgTe₇]^2- anion has the same structure in both the NEt₄⁺ and the PPh₄⁺ salts except for a minor change in the ligand conformation. The [AgTe₇]^3- and [HgTe₇]^2- anions contain metal atoms coordinated in trigonal-planar fashion to η³-Te₇^4- ligands. The central Te atom of an η³-Te₇^4- ligand is coordinated to the metal atom and to two Te atoms in a "T"-shaped geometry consistent with a hypervalent 10 e^- center. The planar [AuTe₇]^3- anion may best be described as possessing a square-planar Au^III atom coordinated to an η³-Te₅^4- ligand and to an η¹-Te₂^2- ligand. The reaction of [NEt₄]_n[MTe₇] (M = Hg, n = 2; M = Au, n = 3) with the activated acetylene dimethyl acetylenedicarboxylate (DMAD) has yielded the products [NEt₄]_n[M(Te₂C₂(COOCH ₃)₂)₂] (M = Hg, n = 2; M = Au, n = 1). The metal atoms are coordinated to two Te(COOCH₃)C=C(COOCH₃)Te^2- ligands, for M = Hg in a distorted tetrahedral fashion and for M = Au in a square-planar fashion.