Triphenyltelluronium(IV) bromide acetone hemisolvate

The asymmetric unit of the title compound, 2C18H15Te+·2Br−·C3H6O or Ph3TeBr·0.5Me2CO, contains two crystallographically independent triphenyltelluronium cations, two bromide anions, and one disordered [site-occupancy ratio = 0.581 (7):0.419 (7)] solvent molecule. Interionic Te⋯Br interactions connect the cations and anions into a tetrameric step-like structure. The primary coordination spheres of both Te atoms are TeC3 trigonal pyramids: three short secondary tellurium–bromine interactions expand the coordination geometry of one of the Te atoms to an octahedron. While the other Te atom shows only two Te⋯Br secondary bonding interactions, it is also six-coordinated due to a Te⋯π interaction [3.769 (2) Å] with one of the phenyl rings of the adjacent cation.

The asymmetric unit of the title compound, 2C 18 H 15 Te + Á-2Br À ÁC 3 H 6 O or Ph 3 TeBrÁ0.5Me 2 CO, contains two crystallographically independent triphenyltelluronium cations, two bromide anions, and one disordered [site-occupancy ratio = 0.581 (7):0.419 (7)] solvent molecule. Interionic TeÁ Á ÁBr interactions connect the cations and anions into a tetrameric steplike structure. The primary coordination spheres of both Te atoms are TeC 3 trigonal pyramids: three short secondary tellurium-bromine interactions expand the coordination geometry of one of the Te atoms to an octahedron. While the other Te atom shows only two TeÁ Á ÁBr secondary bonding interactions, it is also six-coordinated due to a TeÁ Á Á interaction [3.769 (2) Å ] with one of the phenyl rings of the adjacent cation.

Related literature
For the structures of unsolvated triphenyltelluronium chloride and Ph 3 TeCl . 0.5CHCl 3 , see: Ziolo & Extine (1980) and Collins et al. (1988) Financial support from the Academy of Finland is gratefully acknowledged.
The structural features of organotellurium salts R 3 TeX are governed by weak tellurium-anion secondary bonding interactions, which expand the AX 3 E trigonal pyramidal geometry around the tellurium atom generally into a sixcoordinate environment. The two crystallographically independent tellurium atoms in the title compound, however, show different coordination environments. The trigonal pyramidal geometry around the Te1 atom is expanded into an octahedron by three Te···Br contacts, whereas Te2 shows two Te···Br contacts. Te2 becomes six-coordinated through the Te···π interaction with one of the phenyl rings of an adjacent cation. These interactions expand the coordination of the tellurium atoms into AX 3 Y 3 E and AX 3 Y 2 ZE environments, respectively. The Te···Br distances range from 3.3529 (5) Å to 3.4483 (4) Å.
The interionic Te···Br contacts build a tetrameric step-like unit in the lattice. This kind of polymeric structures are common in tellurium-halogen compounds, especially with heavier halogens. Ph 3 TeCl . 1/2 CHCl 3 , for example, shows a similar tetrameric structure as the title compound (Collins et al., 1988). In fact, Ph 3 TeBr . 1/2 Me 2 CO is isomorphic with Ph 3 TeCl . 1/2 CHCl 3 . The unsolvated Ph 3 TeCl consists of dimeric structural units (Ziolo et al., 1980). The (Ph 3 TeBr) 4 tetramers form two-dimensional network in the crystal as shown in Fig. 2. The planes are linked together by the solvent molecules.

Experimental
A few colorless crystals of Ph 3 TeBr . 1/2 Me 2 CO were obtained by slow evaporation of the solvent from the acetone solution of the precipitate isolated from the reaction of [(Ph 3 PO) 2 H] 2 [Te 2 Br 10 ] (94.5 mg; 0.04 mmol) and Ph 3 TeCl (56.3 mg; 0.14 mmol) in CH 2 Cl 2 .

Refinement
The solvent molecule was found to be disordered and refined in two positions. Since the site occupation factors and thermal parameters of the disordered atoms correlate with each other, the thermal parameters of the corresponding pairs of atoms were restrained to be equal. The site occupation factors were 58 (1):42 (1) after the final refinement. H atoms were positioned geometrically and refined using a riding model with 0.98 Å and U iso (H) = 1.5 U eq (C) and 0.95 Å and U iso (H) = 1.2 U eq (C) for the methyl and aromatic H atoms, respectively.

Figure 1
The asymmetric unit of triphenyltellurium bromide indicating the interionic Te···Br contacts. The thermal ellipsoids have been drawn at 50% probability.  The two-dimensional planar packing of the tetrameric (Ph 3 TeBr) 4 units. The solvent molecules and the hydrogen atoms have been omitted for clarity.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.