trans-Bis[1,2-bis(dimethylphosphino)ethane]bromidonitrosyltungsten(0)

The crystal structure of the title compound, [WBr(NO)(C6H16P2)2], reveals a distorted octahedral geometry around the W centre. The W atom lies on a special position at an inversion centre (the Br and NO ligands are equally disordered). The bis(dimethylphosphino)ethane ligand is also severely disordered (site occupancy factors 0.52 and 0.48). This is the first structure of a tungsten species with nitrosyl and bromide ligands.


S2. Experimental
[W(Br) 2 (NO)(dmpe) 2 ]Br was prepared from the complex [W(Br) 3 (NO)(CH 3 CN) 2 ], which is easily synthesized by the reaction of W(Br) 5 with gaseous NO in dibromomethane in the presence of acetonitrile at room temperature according to the literature procedure (Johnson, 1967;Berg & Dehnicke, 1985). 3.00 g (5.14 mmol) of W(Br) 5 and 0.54 ml (10.28 mmol) of acetonitrile were dissolved in 100 ml of dibromomethane in a 250 ml three-necked flask. Nitric oxide was passed through the solution, which was stirred at room temperature until the black colour of the solution turned to a light green precipitate after ca 1 h. The solution was concentrated to one third of its original volume and the addition of pentane (10 ml) afforded a green-yellow voluminous precipitate, which was filtered off, washed with pentane and dried in vacuum. Then [W(Br) 2 (NO)(dmpe) 2 ]Br (0.188 g, 0.25 mmol) was added to a stirred suspension of 1% sodium amalgam (0.026 g of Na, 1.12 mmol) in 20 ml of tetrahydrofurane. The mixture was then stirred overnight at room temperature. The solution was filtered off, separated from the mercury-containing residue, and the solvent was removed under vacuum. The residue was washed with pentane (10 ml x 2) and extracted with tetrahydrofurane (20 ml), concentrated and cooled to -30°C overnight yielding the title compound in the form of yellow crystals.

S3. Refinement
The initial refinement of the structure produced very large thermal parameters for bromine, nitrogen, oxygen and all carbon atoms, especially for C2, C5 and C6 (> 0.175), and inadequate geometry of the dmpe ligand with coplanar P1, C1, C2 and P2 atoms. The location of the highest residual peaks showed unambigously that each of the Br, N, O and C atoms are distributed over two sites. The introduction of the disordered model, with respect to NO/Br and all carbon atoms of dmpe, yielded significantly lower discrepancy factors and ensured reasonable geometry of the dmpe ligand. Nevertheless, the highly disordered refinement model prompted us to consider possibility of alternative space groups (P2/c, P2 and Pc).
However, none of them allowed to carry out reasonable refinement of the structure. Therefore we ended up with the P2 1 /c refinement with significantly disordered model.
All hydrogen atoms were included at calculated positions and treated as riding atoms with C-H distances of 0.96-0.97 Å and U iso (H) = 1.3U eq (C). Positional disorders were refined with an occupancy factor of 0.5 for the trans NO and Br ligands since the metal atom occupies a special position in the inversion centre; the occupancy factors for two components of the dmpe disorder were deterimined by the refinement. The temperature factors of the C atoms of the dmpe ligand were refined with the SIMU, DELU and EADP restraints (Sheldrick, 1997). The largest positive and negative residual peaks are located at about 0.9 Å from P2 and W1, respectively; no chemical meaning could be attributed to these features.

Figure 1
The molecular structure of (I) with the atom-labeling scheme (displacement ellipsoids are drawn at the 20% probability level). The unlabeled atoms are derived from the corresponding labeled atoms by the 1 -x, -y, 1 -z symmetry transformation. Only one component of the disorder is shown for the nitrosyl/bromide groups as well as for the carbon atoms of the dmpe ligand. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 2.37 e Å −3 Δρ min = −1.59 e Å −3

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.

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