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Acta Cryst. (2008). E64, m1150    [ doi:10.1107/S1600536808025245 ]

Pentacarbonyl-2[kappa]5C-chlorido-1[kappa]Cl-bis[1([eta]5)-cyclopentadienyl][[mu]-oxido(phenyl)methylene-1:2[kappa]2O:C]hafnium(IV)tungsten(0)

C. Esterhuysen, I. B. J. Nel and S. Cronje

Abstract top

The title compound, [HfW(C5H5)2(C7H5O)Cl(CO)5] or [W(CO)5(C7H5O){Hf(C5H5)2Cl}], contains two metal centres, with a (tungstenpentacarbonyl)oxyphenylcarbene unit coordinated to a hafnocene chloride. The Hf-O-C angle is nearly linear, and the C=O distance is slightly shorter than for equivalent alkoxycarbenes. One of the cyclopentadienyl (Cp) rings undergoes an offset face-to-face [pi]-[pi] interaction [3.495 (7) Å] with the symmetry-related Cp ring of a neighbouring molecule.

Comment top

Anionic Fischer-type carbene ligands, prepared by the standard addition of organolithium compounds to metal carbonyls, act as monodentate ligands towards transition metals like Ti and Zr (Barluenga and Fañanás, 2000). When the second metal unit is a zirconocene moiety, Cp2ZrCl, then such complexes have been proven to catalyse the oligomerization of 1-pentene in the presence of methylaluminoxane, MAO (Brüll et al., 2001). Herein, we report the Hf equivalent of these zirconocene alkoxycarbene complexes.

In the title compound, (I, Fig. 1), the W=Ccarbene and Ccarbene—C distances are similar to those found in alkoxycarbene complexes, whereas the C—O distance is shorter [2.16 (1), 1.50 (3) and 1.47 (2) Å, respectively; Orpen et al., 1989]. The Hf—O distance is also shorter than those in the metallocyclic compounds C26H27HfO5V [2.063 (3) Å; Erker et al., 1991] and C28H29HfO5V [2.066 (3) Å; Berlekamp et al., 1993]. The Hf—O—C angle is nearly linear, with a larger value [171.4 (3)°] than the equivalent Hf—O—C angles of 163.6 (3) and 169.0 (3)° in C26H27HfO5V (Erker et al., 1991) and C28H29HfO5V (Berlekamp et al., 1993), respectively, as well as the Zr—O—C angle of 166.1 (5)° in W(CO)5C(C6H5)OZr(C5H5)2OC6H5 (Erker et al., 1989).

The C21/C22/C23/C24/C25 Cp ring [with centroid Cg(1)] undergoes offset face-to-face π–π interactions with the symmetry related Cp ring on a neighbouring molecule [Cg(1)···Cg(1)i = 3.495 (7) Å; Symmetry code: (i) 1 - x, 2 - y, 1 - z)].

Related literature top

For related literature regarding anionic Fischer-type carbenes, see: Barluenga & Fañanás (2000); Brüll et al. (2001). For comparable structures, see: Berlekamp et al. (1993); Erker et al. (1989, 1991). For comparable bond lengths, see: Orpen et al. (1989).

Experimental top

A solution of LiCH3 (31 ml, 1.6M) in diethylether (50 ml) was added to a well stirred suspension of W(CO)6 (17.802 g) in diethylether (100 ml). After solvent removal, dissolution of the residue in cold water (150 ml) and filtration, a solution of Et4NCl (8.721 g) in cold water (50 ml) was added to the filtrate. Upon further filtration 0.740 g of the product {[W(CO)5C(C6H5)O][NEt4]} was dissolved in dichloromethane (70 ml) and added to a solution of Cp2HfCl2 (0.505 g) in dichloromethane (40 ml). After stirring for 30 min at -40°C AgBF4 (0.261 g) was added. The solvent was removed and the residue extracted in 5 portions of 10 ml toluene. The extract was cooled to -40°C and filtered. The filtrate was dried over anhydrous MgSO4, concentrated to saturation, and kept at -6°C, whereupon red crystals of the title compound suitable for X-ray diffraction analysis were obtained in 19% yield.

Refinement top

H atoms were positioned geometrically with C—H = 0.95 Å and constrained to ride on their parent atoms with Uiso(H) = 1.2Ueq(C). The maximum and minimum residual electron density peaks were located 0.93 and 0.83 Å, respectively from the Hf1 and W1 atoms.

Computing details top

Data collection: COLLECT (Nonius, 1998); cell refinement: DENZO-SMN (Otwinowski & Minor, 1997); data reduction: DENZO-SMN (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: X-SEED (Barbour, 2001; Atwood & Barbour, 2003); software used to prepare material for publication: publCIF (Westrip, 2008).

Figures top
[Figure 1] Fig. 1. The molecular structure of I showing the atomic labelling scheme and displacement ellipsoids drawn at the 50% probability level.
Pentacarbonyl-2κ5C-chlorido-1κCl-bis[1(η5)- cyclopentadienyl][µ-oxido(phenyl)methylene- 1:2κ2O:C]hafnium(IV)tungsten(0) top
Crystal data top
[HfW(C5H5)2(C7H5O)Cl(CO)5]F000 = 1432
Mr = 773.13Dx = 2.291 Mg m−3
Monoclinic, P21/cMo Kα radiation
λ = 0.71073 Å
Hall symbol: -P 2ybcCell parameters from 12410 reflections
a = 8.5422 (2) Åθ = 1.9–27.5º
b = 12.5546 (3) ŵ = 9.91 mm−1
c = 21.0237 (7) ÅT = 173 (2) K
β = 96.152 (1)ºPrism, red
V = 2241.68 (11) Å30.33 × 0.27 × 0.25 mm
Z = 4
Data collection top
Nonius KappaCCD
diffractometer		5106 independent reflections
Radiation source: fine-focus sealed tube4234 reflections with I > 2σ(I)
Monochromator: graphiteRint = 0.049
T = 173(2) Kθmax = 27.5º
φ and ω scans to fill Ewald sphereθmin = 1.9º
Absorption correction: multi-scan
(DENZO-SMN; Otwinowski & Minor, 1997)		h = −11→8
Tmin = 0.056, Tmax = 0.089k = −16→15
12410 measured reflectionsl = −26→27
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.032H-atom parameters constrained
wR(F2) = 0.079  w = 1/[σ2(Fo2) + (0.0436P)2]
where P = (Fo2 + 2Fc2)/3
S = 1.01(Δ/σ)max = 0.002
5106 reflectionsΔρmax = 2.61 e Å−3
280 parametersΔρmin = −1.76 e Å−3
Primary atom site location: heavy-atom methodExtinction correction: none
Crystal data top
[HfW(C5H5)2(C7H5O)Cl(CO)5]V = 2241.68 (11) Å3
Mr = 773.13Z = 4
Monoclinic, P21/cMo Kα
a = 8.5422 (2) ŵ = 9.91 mm−1
b = 12.5546 (3) ÅT = 173 (2) K
c = 21.0237 (7) Å0.33 × 0.27 × 0.25 mm
β = 96.152 (1)º
Data collection top
Nonius KappaCCD
diffractometer		5106 independent reflections
Absorption correction: multi-scan
(DENZO-SMN; Otwinowski & Minor, 1997)		4234 reflections with I > 2σ(I)
Tmin = 0.056, Tmax = 0.089Rint = 0.049
12410 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.032280 parameters
wR(F2) = 0.079H-atom parameters constrained
S = 1.01Δρmax = 2.61 e Å−3
5106 reflectionsΔρmin = −1.76 e Å−3
Special details top

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 (Å2) top
xyzUiso*/Ueq
Hf10.80934 (3)0.779762 (17)0.542399 (9)0.01909 (8)
W10.46096 (3)0.614746 (17)0.676791 (9)0.02237 (8)
Cl11.06982 (17)0.83902 (12)0.58304 (7)0.0346 (3)
O10.3696 (5)0.6436 (4)0.52755 (18)0.0401 (11)
O20.1151 (5)0.5226 (4)0.6774 (2)0.0515 (12)
O30.4999 (6)0.6018 (4)0.82830 (19)0.0530 (13)
O40.3679 (6)0.8575 (4)0.6959 (2)0.0525 (13)
O50.5903 (7)0.3811 (4)0.6553 (2)0.0590 (15)
O60.7432 (5)0.7258 (3)0.62527 (15)0.0235 (8)
C10.4097 (6)0.6334 (4)0.5806 (3)0.0250 (12)
C20.2402 (7)0.5579 (5)0.6782 (3)0.0324 (13)
C30.4897 (7)0.6036 (5)0.7740 (3)0.0328 (14)
C40.3955 (7)0.7690 (5)0.6888 (3)0.0305 (14)
C50.5421 (7)0.4640 (5)0.6637 (3)0.0321 (14)
C60.6979 (7)0.6775 (4)0.6743 (2)0.0228 (11)
C70.8265 (6)0.6792 (4)0.7293 (2)0.0218 (11)
C80.9276 (7)0.7663 (5)0.7384 (3)0.0298 (13)
H80.91380.82590.71050.036*
C91.0477 (8)0.7664 (5)0.7879 (3)0.0405 (16)
H91.11320.82730.79500.049*
C101.0734 (7)0.6781 (6)0.8273 (2)0.0384 (16)
H101.15780.67760.86060.046*
C110.9740 (8)0.5901 (5)0.8175 (3)0.0368 (15)
H110.99180.52890.84390.044*
C120.8507 (7)0.5916 (5)0.7701 (2)0.0299 (13)
H120.78120.53240.76490.036*
C130.7818 (7)0.9777 (4)0.5270 (3)0.0306 (13)
H130.87151.02290.53160.037*
C140.6886 (7)0.9488 (5)0.5760 (3)0.0320 (13)
H140.70680.96810.61990.038*
C150.5638 (7)0.8859 (4)0.5475 (3)0.0308 (13)
H150.47990.85780.56850.037*
C160.5844 (7)0.8717 (5)0.4830 (3)0.0326 (14)
H160.51870.83100.45280.039*
C170.7188 (7)0.9281 (5)0.4710 (3)0.0333 (14)
H170.76050.93200.43090.040*
C180.7079 (8)0.6358 (5)0.4693 (3)0.0393 (16)
H180.59760.63320.45690.047*
C190.8207 (8)0.6914 (5)0.4373 (3)0.0395 (16)
H190.80030.73220.39920.047*
C200.9681 (8)0.6753 (6)0.4722 (3)0.0413 (16)
H201.06530.70370.46180.050*
C210.9484 (9)0.6108 (5)0.5246 (3)0.0459 (18)
H211.02920.58810.55630.055*
C220.7886 (9)0.5853 (5)0.5224 (3)0.0439 (17)
H220.74250.54110.55200.053*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Hf10.01979 (13)0.02021 (13)0.01715 (11)0.00217 (9)0.00150 (8)0.00140 (8)
W10.02045 (13)0.02513 (14)0.02122 (12)−0.00133 (9)0.00071 (9)0.00187 (8)
Cl10.0253 (8)0.0370 (8)0.0403 (7)−0.0042 (7)−0.0014 (6)0.0026 (6)
O10.040 (3)0.053 (3)0.025 (2)−0.007 (2)−0.0049 (18)0.0019 (19)
O20.025 (2)0.066 (3)0.065 (3)−0.013 (2)0.012 (2)−0.007 (3)
O30.053 (3)0.084 (4)0.024 (2)0.002 (3)0.008 (2)0.013 (2)
O40.067 (4)0.036 (3)0.052 (3)0.016 (3)−0.003 (2)−0.011 (2)
O50.074 (4)0.040 (3)0.057 (3)0.024 (3)−0.020 (3)−0.009 (2)
O60.028 (2)0.024 (2)0.0179 (16)−0.0010 (17)0.0020 (15)0.0039 (14)
C10.017 (3)0.023 (3)0.034 (3)−0.007 (2)0.002 (2)0.000 (2)
C20.033 (4)0.033 (3)0.031 (3)0.000 (3)0.006 (2)−0.002 (3)
C30.028 (3)0.040 (4)0.030 (3)−0.007 (3)0.003 (2)0.004 (3)
C40.028 (3)0.038 (4)0.025 (3)0.001 (3)0.001 (2)−0.005 (3)
C50.026 (3)0.039 (4)0.029 (3)0.003 (3)−0.010 (2)−0.001 (3)
C60.031 (3)0.016 (3)0.021 (2)0.003 (2)0.003 (2)0.005 (2)
C70.017 (3)0.027 (3)0.022 (2)0.005 (2)0.002 (2)−0.001 (2)
C80.023 (3)0.034 (3)0.032 (3)−0.007 (3)0.004 (2)0.004 (3)
C90.036 (4)0.051 (4)0.034 (3)−0.013 (3)0.000 (3)−0.005 (3)
C100.027 (3)0.065 (5)0.021 (3)0.011 (3)−0.004 (2)−0.005 (3)
C110.032 (3)0.050 (4)0.027 (3)0.007 (3)0.000 (2)0.005 (3)
C120.035 (3)0.030 (3)0.025 (3)0.001 (3)0.005 (2)0.008 (2)
C130.028 (3)0.014 (3)0.051 (3)−0.002 (2)0.008 (3)0.010 (2)
C140.037 (4)0.026 (3)0.033 (3)0.005 (3)0.005 (3)0.003 (3)
C150.021 (3)0.028 (3)0.045 (3)0.010 (2)0.010 (3)0.014 (3)
C160.029 (3)0.030 (3)0.036 (3)0.006 (3)−0.011 (3)0.005 (2)
C170.029 (3)0.038 (3)0.033 (3)0.006 (3)0.005 (2)0.019 (3)
C180.037 (4)0.047 (4)0.033 (3)−0.009 (3)0.004 (3)−0.021 (3)
C190.047 (4)0.049 (4)0.023 (3)0.004 (3)0.005 (3)−0.006 (3)
C200.030 (4)0.050 (4)0.046 (3)0.010 (3)0.012 (3)−0.011 (3)
C210.055 (5)0.038 (4)0.043 (4)0.018 (4)0.002 (3)−0.007 (3)
C220.071 (5)0.022 (3)0.041 (3)−0.007 (3)0.017 (3)−0.013 (3)
Geometric parameters (Å, °) top
Hf1—O62.006 (3)C8—H80.9500
Hf1—Cl12.4139 (14)C9—C101.387 (9)
Hf1—C162.464 (5)C9—H90.9500
Hf1—C172.465 (5)C10—C111.395 (9)
Hf1—C182.469 (6)C10—H100.9500
Hf1—C212.479 (6)C11—C121.371 (8)
Hf1—C222.480 (6)C11—H110.9500
Hf1—C202.483 (6)C12—H120.9500
Hf1—C192.484 (5)C13—C171.389 (8)
Hf1—C142.494 (6)C13—C141.415 (8)
Hf1—C152.496 (5)C13—H130.9500
Hf1—C132.514 (5)C14—C151.408 (8)
W1—C22.019 (6)C14—H140.9500
W1—C12.037 (6)C15—C161.397 (8)
W1—C32.038 (6)C15—H150.9500
W1—C42.040 (6)C16—C171.395 (8)
W1—C52.044 (6)C16—H160.9500
W1—C62.177 (6)C17—H170.9500
O1—C11.137 (6)C18—C221.401 (9)
O2—C21.156 (7)C18—C191.417 (9)
O3—C31.135 (7)C18—H180.9500
O4—C41.149 (7)C19—C201.403 (9)
O5—C51.139 (7)C19—H190.9500
O6—C61.291 (6)C20—C211.393 (9)
C6—C71.508 (7)C20—H200.9500
C7—C81.394 (8)C21—C221.398 (10)
C7—C121.397 (7)C21—H210.9500
C8—C91.381 (8)C22—H220.9500
O6—Hf1—Cl197.55 (11)O3—C3—W1176.2 (6)
O6—Hf1—C16108.77 (18)O4—C4—W1175.9 (6)
Cl1—Hf1—C16132.40 (15)O5—C5—W1178.1 (6)
O6—Hf1—C17133.27 (18)O6—C6—C7110.4 (5)
Cl1—Hf1—C17101.53 (15)O6—C6—W1123.3 (4)
C16—Hf1—C1732.89 (19)C7—C6—W1126.1 (4)
O6—Hf1—C18100.41 (18)C8—C7—C12119.0 (5)
Cl1—Hf1—C18134.01 (16)C8—C7—C6120.5 (5)
C16—Hf1—C1879.9 (2)C12—C7—C6120.5 (5)
C17—Hf1—C1896.2 (2)C9—C8—C7120.3 (6)
O6—Hf1—C2191.47 (19)C9—C8—H8119.9
Cl1—Hf1—C2183.03 (18)C7—C8—H8119.9
C16—Hf1—C21133.2 (2)C8—C9—C10120.4 (6)
C17—Hf1—C21132.8 (2)C8—C9—H9119.8
C18—Hf1—C2154.7 (2)C10—C9—H9119.8
O6—Hf1—C2278.04 (18)C9—C10—C11119.4 (5)
Cl1—Hf1—C22114.19 (19)C9—C10—H10120.3
C16—Hf1—C22109.7 (2)C11—C10—H10120.3
C17—Hf1—C22129.0 (2)C12—C11—C10120.3 (6)
C18—Hf1—C2232.9 (2)C12—C11—H11119.9
C21—Hf1—C2232.7 (2)C10—C11—H11119.9
O6—Hf1—C20124.08 (19)C11—C12—C7120.6 (6)
Cl1—Hf1—C2080.31 (16)C11—C12—H12119.7
C16—Hf1—C20113.1 (2)C7—C12—H12119.7
C17—Hf1—C20101.1 (2)C17—C13—C14107.8 (5)
C18—Hf1—C2054.6 (2)C17—C13—Hf171.9 (3)
C21—Hf1—C2032.6 (2)C14—C13—Hf172.8 (3)
C22—Hf1—C2054.1 (2)C17—C13—H13126.1
O6—Hf1—C19131.56 (19)C14—C13—H13126.1
Cl1—Hf1—C19109.27 (17)Hf1—C13—H13121.0
C16—Hf1—C1982.0 (2)C15—C14—C13107.0 (5)
C17—Hf1—C1980.5 (2)C15—C14—Hf173.7 (3)
C18—Hf1—C1933.3 (2)C13—C14—Hf174.3 (3)
C21—Hf1—C1954.5 (2)C15—C14—H14126.5
C22—Hf1—C1954.5 (2)C13—C14—H14126.5
C20—Hf1—C1932.8 (2)Hf1—C14—H14117.6
O6—Hf1—C1482.99 (16)C16—C15—C14108.5 (5)
Cl1—Hf1—C1491.77 (15)C16—C15—Hf172.4 (3)
C16—Hf1—C1454.65 (19)C14—C15—Hf173.5 (3)
C17—Hf1—C1454.37 (19)C16—C15—H15125.8
C18—Hf1—C14132.1 (2)C14—C15—H15125.8
C21—Hf1—C14171.9 (2)Hf1—C15—H15120.1
C22—Hf1—C14149.4 (2)C17—C16—C15107.6 (5)
C20—Hf1—C14152.4 (2)C17—C16—Hf173.6 (3)
C19—Hf1—C14133.5 (2)C15—C16—Hf174.9 (3)
O6—Hf1—C1580.05 (17)C17—C16—H16126.2
Cl1—Hf1—C15124.55 (15)C15—C16—H16126.2
C16—Hf1—C1532.70 (19)Hf1—C16—H16117.3
C17—Hf1—C1554.02 (19)C13—C17—C16109.0 (5)
C18—Hf1—C15100.2 (2)C13—C17—Hf175.7 (3)
C21—Hf1—C15151.8 (2)C16—C17—Hf173.5 (3)
C22—Hf1—C15119.2 (2)C13—C17—H17125.5
C20—Hf1—C15145.3 (2)C16—C17—H17125.5
C19—Hf1—C15112.9 (2)Hf1—C17—H17117.1
C14—Hf1—C1532.78 (19)C22—C18—C19107.5 (6)
O6—Hf1—C13114.49 (16)C22—C18—Hf174.0 (3)
Cl1—Hf1—C1379.09 (14)C19—C18—Hf173.9 (3)
C16—Hf1—C1354.2 (2)C22—C18—H18126.3
C17—Hf1—C1332.39 (19)C19—C18—H18126.3
C18—Hf1—C13128.5 (2)Hf1—C18—H18117.8
C21—Hf1—C13150.0 (2)C20—C19—C18107.4 (6)
C22—Hf1—C13161.3 (2)C20—C19—Hf173.6 (3)
C20—Hf1—C13119.7 (2)C18—C19—Hf172.8 (3)
C19—Hf1—C13109.8 (2)C20—C19—H19126.3
C14—Hf1—C1332.82 (18)C18—C19—H19126.3
C15—Hf1—C1353.89 (19)Hf1—C19—H19119.2
C2—W1—C187.3 (2)C21—C20—C19108.7 (6)
C2—W1—C388.5 (2)C21—C20—Hf173.5 (4)
C1—W1—C3173.9 (2)C19—C20—Hf173.6 (3)
C2—W1—C493.7 (2)C21—C20—H20125.7
C1—W1—C488.9 (2)C19—C20—H20125.7
C3—W1—C486.9 (2)Hf1—C20—H20119.0
C2—W1—C590.3 (2)C20—C21—C22107.9 (6)
C1—W1—C590.7 (2)C20—C21—Hf173.9 (4)
C3—W1—C593.8 (2)C22—C21—Hf173.7 (4)
C4—W1—C5175.9 (2)C20—C21—H21126.1
C2—W1—C6179.2 (2)C22—C21—H21126.1
C1—W1—C692.1 (2)Hf1—C21—H21118.4
C3—W1—C692.1 (2)C21—C22—C18108.6 (6)
C4—W1—C685.8 (2)C21—C22—Hf173.6 (4)
C5—W1—C690.1 (2)C18—C22—Hf173.1 (4)
C6—O6—Hf1171.4 (3)C21—C22—H22125.7
O1—C1—W1174.9 (5)C18—C22—H22125.7
O2—C2—W1177.5 (5)Hf1—C22—H22119.4
Acknowledgements top

We thank the NRF and the University of Stellenbosch for financial support.

references
References top

Atwood, J. L. & Barbour, L. J. (2003). Cryst. Growth Des. 3, 3–8.

Barbour, L. J. (2001). J. Supramol. Chem. 1, 189–191.

Barluenga, J. & Fañanás, F. J. (2000). Tetrahedron, 56, 4597–4628.

Berlekamp, M., Erker, G. & Petersen, J. L. (1993). J. Organomet. Chem. 458, 97–103.

Brüll, R., Kgosane, D., Neveling, A., Pasch, H., Raubenheimer, H. G., Sanderson, R. & Wahner, U. M. (2001). Macromol. Symp. 165, 11–18.

Erker, G., Dorf, U., Lecht, R., Ashby, M. T., Aulbach, M., Schlund, R., Krüger, C. & Mynott, R. (1989). Organometallics, 8, 2037–2044.

Erker, G., Pfaff, R., Krüger, C. & Werner, S. (1991). Organometallics, 10, 3559–3568.

Nonius (1998). COLLECT. Nonius BV, Delft, The Netherlands.

Orpen, A. G., Brammer, L., Allen, F. H., Kennard, O., Watson, D. G. & Taylor, R. (1989). J. Chem. Soc. Dalton Trans. pp. S1–83.

Otwinowski, Z. & Minor, W. (1997). Methods in Enzymology, Vol. 276, Macromolecular Crystallography, Part A, edited by C. W. Carter Jr & R. M. Sweet, pp. 307–326. New York: Academic Press.

Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122.

Westrip, S. P. (2008). publCIF. In preparation.