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.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](Ta-Br) = 0.002 Å
• R factor = 0.042
• wR factor = 0.109
• Data-to-parameter ratio = 29.6

checkCIF/PLATON results

No syntax errors found



Alert level B
PLAT112_ALERT_2_B ADDSYM Detects Additional (Pseudo) Symm. Elem...          2
PLAT112_ALERT_2_B ADDSYM Detects Additional (Pseudo) Symm. Elem...          c
PLAT112_ALERT_2_B ADDSYM Detects Additional (Pseudo) Symm. Elem...          2
PLAT112_ALERT_2_B ADDSYM Detects Additional (Pseudo) Symm. Elem...          c

Alert level C
RINTA01_ALERT_3_C  The value of Rint is greater than 0.12
            Rint given   0.124
PLAT971_ALERT_2_C Large Calcd. Non-Metal Positive Residual Density       1.51 eA-3
PLAT912_ALERT_4_C Missing # of FCF Reflections Above STh/L=  0.600         21

Alert level G
PLAT199_ALERT_1_G Check the Reported _cell_measurement_temperature        293 K
PLAT200_ALERT_1_G Check the Reported   _diffrn_ambient_temperature        293 K

   0 ALERT level A = In general: serious problem
   4 ALERT level B = Potentially serious problem
   3 ALERT level C = Check and explain
   2 ALERT level G = General alerts; check

   2 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   5 ALERT type 2 Indicator that the structure model may be wrong or deficient
   1 ALERT type 3 Indicator that the structure quality may be low
   1 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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S1. Comment

(TaBr₅)₂ was first described by Rolsten (Rolsten, 1958a). He compared the powder diffraction pattern of (TaBr₅)₂ with orthorhombic NbBr₅ (space group Pbam) and came to the conclusion that the structures are alike. The structure of (NbBr₅)₂ was already known from single-crystal structure analysis (Rolsten, 1958b). However, the apparent isotypism between (TaBr₅)₂ and (NbBr₅)₂ is not in agreement with our result that shows (TaBr₅)₂ to crystallize in the monoclinic space goup C2/m.

The crystal structures of all pentahalides besides the fluorides contain [MX₆] octahedra (M = Nb, Ta; X = Cl, B, I) sharing common edges forming [MX₅]₂ dimers. These double octahedra can be stacked in different ways, resulting in different structure types. In the title compound, the stacking of the (TaBr₅)₂ layers along b can be described as A₁B₁A₁B₁··· . 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 [TaBr₆] 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 (Ta1Br₅)₂ dimer (m symmetry) and 4.1551 (15) Å for the (Ta2Br₅)₂ dimer (2/m symmetry).

(TaBr₅)₂ is isotypic with α-(NbCl₅)₂ (Zalkin & Sands, 1958; Hoenle & von Schnering, 1990).

Related literature

For a previous study of (TaBr₅)₂, see: Rolsten (1958a), who also reported the crystal structure of (NbBr₅)₂ (Rolsten, 1958b). (TaBr₅)₂ is isotypic with α-(NbCl₅)₂, the structure of which was first described by Zalkin & Sands (1958) and was redetermined by Hoenle & von Schnering (1990). For a summary of all possible stackings of double octahedral molecules in pentahalides of Nb and Ta, see: Müller (1978). Experimental details can be found in Brauer's handbook (Brauer, 1962). For data analysis, see: Spek (2009).

S2. Experimental

TaBr₅ was synthesized according to an experimental procedure reported in Brauer's handbook (Brauer, 1962). Orange polyhedric crystals were obtained by recrystallization of TaBr₅ 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 TaBr₅ were selected under a microscope in an argon-filled glove box.

S3. 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.

Computing details

Data collection: X-AREA (Stoe & Cie, 2002); cell refinement: X-RED (Stoe & Cie, 2002); data reduction: X-RED (Stoe & Cie, 2002); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2005); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

Fig. 1. Unit cell of TaBr5 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.

di-µ-bromido-bis[tetrabromidotantalum(V)]

Crystal data

Ta2Br10	F(000) = 2976
Mr = 1161.00	none
Monoclinic, C2/m	Dx = 5.111 Mg m^−3
Hall symbol: -C 2y	Mo Kα radiation, λ = 0.71073 Å
a = 19.433 (3) Å	Cell parameters from 7929 reflections
b = 18.775 (2) Å	θ = 2.1–27.1°
c = 6.2034 (10) Å	µ = 40.93 mm^−1
β = 90.716 (13)°	T = 293 K
V = 2263.2 (6) Å^3	Polyhedron, orange
Z = 6	0.14 × 0.09 × 0.04 mm

Data collection

Stoe IPDS 2 diffractometer	2605 independent reflections
Radiation source: fine-focus sealed tube	1678 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.125
Detector resolution: 0 pixels mm^-1	θmax = 27.3°, θmin = 1.5°
oscillation scans	h = −24→24
Absorption correction: numerical (X-SHAPE; Stoe & Cie, 1999)	k = −24→24
Tmin = 0.014, Tmax = 0.089	l = −7→7
17962 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.042	w = 1/[σ^2(Fo^2) + (0.0563P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.109	(Δ/σ)max = 0.001
S = 0.95	Δρmax = 1.71 e Å^−3
2605 reflections	Δρmin = −1.53 e Å^−3
88 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.00064 (4)
0 constraints

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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² 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 (Ų)

	x	y	z	Uiso*/Ueq
Ta1	1.0000	0.38934 (4)	0.5000	0.0296 (2)
Ta2	0.66677 (3)	0.38928 (3)	0.97909 (7)	0.02938 (18)
Br1	0.61422 (10)	0.5000	1.2034 (3)	0.0326 (4)
Br2	0.61078 (9)	0.30802 (8)	1.2210 (2)	0.0430 (4)
Br3	0.56469 (8)	0.40167 (8)	0.7436 (2)	0.0406 (3)
Br4	0.72279 (9)	0.30797 (8)	0.7378 (2)	0.0430 (4)
Br5	0.71903 (11)	0.5000	0.7544 (3)	0.0321 (4)
Br6	0.89789 (8)	0.40192 (8)	0.7271 (2)	0.0421 (3)
Br7	0.76900 (8)	0.40188 (8)	1.2143 (2)	0.0415 (3)
Br8	1.05614 (9)	0.30768 (8)	0.7446 (2)	0.0451 (4)
Br9	1.05245 (11)	0.5000	0.7294 (3)	0.0323 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ta1	0.0289 (4)	0.0278 (4)	0.0320 (4)	0.000	0.0032 (3)	0.000
Ta2	0.0292 (3)	0.0280 (3)	0.0310 (3)	0.0002 (2)	0.00055 (19)	−0.00014 (18)
Br1	0.0353 (11)	0.0314 (9)	0.0312 (8)	0.000	0.0070 (7)	0.000
Br2	0.0464 (10)	0.0408 (8)	0.0419 (7)	−0.0076 (6)	0.0083 (6)	0.0062 (5)
Br3	0.0338 (8)	0.0460 (8)	0.0419 (7)	−0.0012 (6)	−0.0074 (5)	−0.0021 (5)
Br4	0.0465 (10)	0.0396 (7)	0.0431 (7)	0.0060 (7)	0.0079 (6)	−0.0087 (6)
Br5	0.0343 (11)	0.0316 (9)	0.0305 (8)	0.000	0.0067 (7)	0.000
Br6	0.0353 (8)	0.0457 (8)	0.0455 (7)	−0.0017 (6)	0.0125 (6)	0.0023 (6)
Br7	0.0349 (8)	0.0453 (8)	0.0440 (7)	0.0008 (6)	−0.0079 (5)	0.0031 (6)
Br8	0.0464 (10)	0.0400 (7)	0.0487 (8)	0.0075 (7)	−0.0038 (6)	0.0096 (6)
Br9	0.0360 (11)	0.0305 (8)	0.0303 (8)	0.000	−0.0039 (7)	0.000

Geometric parameters (Å, º)

Ta1—Br8^i	2.4087 (15)	Ta2—Br3	2.4598 (16)
Ta1—Br8	2.4087 (15)	Ta2—Br7	2.4613 (17)
Ta1—Br6	2.4591 (14)	Ta2—Br5	2.7075 (12)
Ta1—Br6^i	2.4591 (14)	Ta2—Br1	2.7084 (12)
Ta1—Br9	2.7100 (13)	Br1—Ta2^iii	2.7084 (12)
Ta1—Br9^ii	2.7100 (13)	Br5—Ta2^iii	2.7075 (12)
Ta2—Br4	2.4075 (14)	Br9—Ta1^ii	2.7100 (13)
Ta2—Br2	2.4092 (15)
Br8^i—Ta1—Br8	100.93 (9)	Br2—Ta2—Br3	93.59 (6)
Br8^i—Ta1—Br6	93.41 (6)	Br4—Ta2—Br7	93.53 (6)
Br8—Ta1—Br6	93.60 (6)	Br2—Ta2—Br7	93.39 (6)
Br8^i—Ta1—Br6^i	93.60 (6)	Br3—Ta2—Br7	169.06 (5)
Br8—Ta1—Br6^i	93.41 (6)	Br4—Ta2—Br5	89.51 (5)
Br6—Ta1—Br6^i	168.98 (8)	Br2—Ta2—Br5	169.14 (5)
Br8^i—Ta1—Br9	169.48 (6)	Br3—Ta2—Br5	85.79 (6)
Br8—Ta1—Br9	89.59 (5)	Br7—Ta2—Br5	85.77 (6)
Br6—Ta1—Br9	85.78 (6)	Br4—Ta2—Br1	169.22 (5)
Br6^i—Ta1—Br9	85.77 (6)	Br2—Ta2—Br1	89.43 (5)
Br8^i—Ta1—Br9^ii	89.59 (5)	Br3—Ta2—Br1	85.76 (6)
Br8—Ta1—Br9^ii	169.48 (6)	Br7—Ta2—Br1	85.90 (6)
Br6—Ta1—Br9^ii	85.77 (6)	Br5—Ta2—Br1	79.71 (4)
Br6^i—Ta1—Br9^ii	85.78 (6)	Ta2—Br1—Ta2^iii	100.26 (6)
Br9—Ta1—Br9^ii	79.90 (6)	Ta2—Br5—Ta2^iii	100.31 (6)
Br4—Ta2—Br2	101.35 (6)	Ta1—Br9—Ta1^ii	100.10 (6)
Br4—Ta2—Br3	93.34 (6)

Symmetry codes: (i) −x+2, y, −z+1; (ii) −x+2, −y+1, −z+1; (iii) x, −y+1, z.