Di-μ-chlorido-μ-(dimethyl sulfide)-bis{dichlorido[(dimethyl selenide-κSe)(dimethyl sulfide-κS)(0.65/0.35)]niobium(III)}(Nb—Nb)

The dinuclear compound, [Nb2Cl6(C2H6S)1.7(C2H6Se)1.3], features an NbIII=NbIII double bond [2.6878 (5) Å]. The molecule lies on a twofold rotation axis that passes through the middle of this bond as well as through the bridging dimethyl sulfide ligand. The NbIII ion exists in an octahedral coordination environment defined by two terminal and two bridging Cl atoms, and (CH3)2Se/(CH3)2S ligands. The (bridging) ligand lying on the twofold rotation axis is an ordered (CH3)2S ligand, whereas the terminal ones on a general position are a mixture of (CH3)2Se and (CH3)2S ligands in a 0.647 (2):0.353 (2) ratio (the methyl C atoms are also disordered).

The dinuclear compound, [Nb 2 Cl 6 (C 2 H 6 S) 1.7 (C 2 H 6 Se) 1.3 ], features an Nb III Nb III double bond [2.6878 (5) Å ]. The molecule lies on a twofold rotation axis that passes through the middle of this bond as well as through the bridging dimethyl sulfide ligand. The Nb III ion exists in an octahedral coordination environment defined by two terminal and two bridging Cl atoms, and (CH 3 ) 2 Se/(CH 3 ) 2 S ligands. The (bridging) ligand lying on the twofold rotation axis is an ordered (CH 3 ) 2 S ligand, whereas the terminal ones on a general position are a mixture of (CH 3 ) 2 Se and (CH 3 ) 2 S ligands in a 0.647 (2):0.353 (2) ratio (the methyl C atoms are also disordered).

Experimental
Crystal data [Nb 2 Cl 6 (C 2 H 6 S) 1  The chemistry of the lower oxidation states of niobium in discrete complexes remains relatively unexplored. Our research group has already carried out X-ray crystallographic determinations of the complexes of the general formula [Nb 2 Cl 6 L 3 ] (L= tetrahydrothiophene C 4 H 8 S, dimethyl sulfide (Kakeya et al., 2006a(Kakeya et al., , 2006b. These complexes have a triply bridged face-sharing dioctahedral structure with one thioether as a bridging ligand and two terminal Cland thioether. A series of ligand substitution reactions of these complexes with monodentate oxygen donors and substituted phosphanes has been explored. The structures of the face-sharing dioctahedral complexes preserve their original geometry in the case of reaction with monodentate ligands and ligand substitution occurred only at terminal positions (Cotton et al., 1985). We report here the first success in determining the structure of [Nb 2 Cl 6 (C 2 H 6 Se) 1.3 (C 2 H 6 S) 1.7 (Scheme I), which has selenoether as ligands at terminal positions.
The molecule has dinuclear bridging unit [Nb 2 (µ-Cl) 2 (µ-Me 2 X)] (X = mixture of S, Se) with the terminal Me 2 X ligands in a trans orientation to the bridging Me 2 S (Fig. 1). The average Nb-(µ-Cl) and Nb-(µ-Me 2 S) distances fall within the range of those for [Nb 2 (µ-Cl) 2 Cl 4 (µ-Me 2 S)(Me 2 S) 2 ], which has the same bridging unit (Kakeya et al., 2006a(Kakeya et al., , 2006b. The terminal Nb-Cl lengths are shorter than the corresponding distances to the bridging atoms. If the terminal metalchalcogen bond were purely ionic, the distance should coincide with the sum of ionic radii of metal and chalcogen. Since the ionic radii of the trivalent niobium, Se 2and S 2given in the literature are 0.72 Å, 1.98 Å and 1.84 Å, the bond distances of the metal and chalcogen is 2.70 Å for Se 2and 2.56 Å for S 2-. We find that the difference between bond lengths and sum of the those radii is smaller in the title compound than in [Nb 2 Cl 6 (Me 2 S) 3 ]. This difference is ascribed to the influence of the covalency of the metal-chalcogen interactions. Other geometrical parameters also lie within the same ranges as in analogous dinuclear niobium complexes (Kakeya et al., 2006a(Kakeya et al., , 2006b.

S2. Experimental
All the reactions were carried out under a dry argon atmosphere by using standard Schlenk tube techniques.

S3. Refinement
The H atoms were placed in calculated positions, with C-H = 0.98 Å for CH 3 , and refined using a riding model, with U iso (H) = 1.5U eq of the carrier atoms.
The chalcogen ligand on the general position is disordered; the atom refined to a 0.647 (2)Se : 0.353S mixture. The Se1-C distances were restrained to 1.95±0.01 Å and the S1′-C distances to 1.80±0.01 Å. The temperature factors of Se1 and S1′ were made identical.
The refinement led to an Nb1-Se1 distance of 2.72 Å but a much longer Nb1-S1′ distance of 2.79 Å (a normal Nb-S bond is approximately 2.40 Å). Refinement then proceeded by setting the Nb1-S1 (ordered sulfur) and the N1b-S1′ (disordered sulfur) bond distances to be within 0.01 Å of each other. The two methyl groups were each split into two components, and the temperature factors of the primed atoms were set to those of the unprimed ones. Additionally, the anisotropic temperature factors were tightly restrained to be nearly isotropic. This model gave distances of 2.420 (1) Å for the ordered atom and 2.543 (6) Å for the disordered atom.
The failure of the Hirshfeld test for the Nb1-Se1 and Nb1-S1′ bonds is attributed to the tight restraints imposed on the disordered ligand.