Redetermination of tantalum pentabromide, (TaBr5)2

Crystals of di-μ-bromido-bis[tetrabromidotantalum(V)], (TaBr5)2, were obtained by recrystallization at 773 K. A first crystal structure study of (TaBr5)2 was reported by Rolsten [J. Am. Chem. Soc. (1958) ▶, 80, 2952–2953], who analysed the powder diffraction pattern and came to the conclusion that it crystallizes isotypically with (NbBr5)2 in a primitive orthorhombic cell. These findings are not in agreement with our current results of a monoclinic C-centred structure. (TaBr5)2 is isotypic with α-(NbCl5)2. The crystal structure contains [TaBr6] octahedra sharing common edges forming [TaBr5]2 dimers. Two crystallographically independent dimers with symmetries m and 2/m and Ta⋯Ta distances of 4.1574 (11) and 4.1551 (15) Å, respectively, are present in the structure.

Financial support from the State of Nordrhein-Westfalen and Universitä t zu Kö ln is greatly appreciated. Comment (TaBr 5 ) 2 was first described by Rolsten (Rolsten, 1958a). He compared the powder diffraction pattern of (TaBr 5 ) 2 with orthorhombic NbBr 5 (space group Pbam) and came to the conclusion that the structures are alike. The structure of (NbBr 5 ) 2 was already known from single-crystal structure analysis (Rolsten, 1958b). However, the apparent isotypism between (TaBr 5 ) 2 and (NbBr 5 ) 2 is not in agreement with our result that shows (TaBr 5 ) 2 to crystallize in the monoclinic space goup C2/m.

Structure Reports Online
The crystal structures of all pentahalides besides the fluorides contain [MX 6 ] octahedra (M = Nb, Ta; X = Cl, B, I) sharing common edges forming [MX 5 ] 2 dimers. These double octahedra can be stacked in different ways, resulting in different structure types. In the title compound, the stacking of the (TaBr 5 ) 2 layers along b can be described as A 1 B 1 A 1 B 1 ··· . Within one layer the molecules are stacked in a "2 1" stacking scheme (Fig. 1). A summary of all possible stacking possibilities has been given by Müller (1978). The [TaBr 6 ] octahedra are distorted due to the repulsive forces of the highly charged metal atoms centering the octahedra with d(Ta-Ta) = 4.1574 (11) Å for the (Ta1Br 5 ) 2 dimer (m symmetry) and 4.1551 (15) Å for the (Ta2Br 5 ) 2 dimer (2/m symmetry).

Experimental
TaBr 5 was synthesized according to an experimental procedure reported in Brauer's handbook (Brauer, 1962). Orange polyhedric crystals were obtained by recrystallization of TaBr 5 in a silica ampoule. The ampoule was heated with 50 K per hour to 773 K and annealed for 12 h, after which it was slowly cooled to room temperature with 2 K per hour. Single crystals of TaBr 5 were selected under a microscope in an argon-filled glove box.

Refinement
PLATON (Spek, 2009) indicates higher (pseudo)-symmetry and suggests a change of the crystal system from monoclinic C to orthorhombic C, but the experimentally determined unit cell angles differ with 0.72° considerably from orthogonality.
The maximum residual density lies 1.29Å from Ta1 and the density minimum lies at the Br4 atom.
supplementary materials sup-2 Figures   Fig. 1. Unit cell of TaBr 5 in a view along [001]. Double octahedral molecules are coloured in orange or blue for depth perception. Thermal ellipsoids are given on the 99% probability level.

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 Rfactors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq