In their lower oxidation states, the early transition metals of the fourth and fifth periods tend to form metal clusters {M_x} for two reasons. One, 4d and 5d orbitals are larger than 3d orbitals and are, thus, capable of forming metal-metal bonds. Two, the sublimation enthalpies of the metals are high; part of it is saved when metal clusters are retained. The virtually simple binary halide MoCl2, obtained by a synproportionation reaction, features a crystal structure [1,2] which contains octahedral molybdenum clusters {Mo₆} which are surrounded by eight inner (i) and six outer (a) chloride ligands; four of the latter bridge to neighboring clusters producing a layer structure, according to the Niggli formulation, {Mo₆}Clⁱ₈Cl^a₂Cl^a-a_4/2.

In attempts to synthesize ternary chlorides containing the [{Mo₆}Cl₁₄]^2- cluster-complex anion in a molten-salt system, MoCl₂ faced a disproportionation reaction and red crystals of Na₃[MoCl₆] were obtained.

Red single crystals of Na3MoCl6 were obtained from the attempted dissolution of MoCl₂ = {Mo₆}Clⁱ₈Cl^a₂Cl^a-a_4/2 in a NaCl/AlCl₃ flux (45:55 mol%, close to the eutectic [3]) at 350 °C in a sealed Pyrex ampoule. In this melt the {Mo₆} cluster must have been disrupted during a disproportionation reaction, under the influence of the acidic flux. Hexagonal red crystals were embedded in essentially white crystalline material (Figure 1); some black powder (molybdenum) could also be recognized.

Na₃MoCl₆ crystallizes with the Na₃CrCl₆ type of structure [4], trigonal, space group P-31c (No. 163) with a = 687.1(1), c = 1225.3(2) pm, Z = 2. Previous data from powder diffraction, a = 692.0(8), c = 1222.2(5) pm, agree sufficiently well. It was also assumed that Na₃MoCl₆ and Na₃VCl₆ are isotypic with Na₃CrCl₆ [4]. Other hints at the existence of Na₃MoCl₆ are from preparative investigations or from phase diagram determinations where the crystal structure was apparently of no concern [5,6].

The structure of Na₃MoCl₆ consists of hexagonally closest-packed layers of chloride anions, 4H–…B | ABAB | A…. Octahedral voids between these layers are filled in a way that half of them are filled between double layers BA by Na⁺ cations, and half of the voids are filled by Na⁺ and Mo³⁺ in an ordered fashion between double layers AB, see Figure 2. Thus, chains of face-sharing octahedra run parallel [001] and are filled with Na⁺, Cr³⁺, Na⁺, □, where □ denominates a void. Neighboring chains are displaced by ½c in the [001] direction. Therefore, the Mo³⁺–Mo³⁺ distance is 729.9(1) pm. In the triple octahedron (Cl⁻)₃Na⁺(Cl⁻)₃Mo³⁺(Cl⁻)₃Na⁺(Cl⁻)₃, Mo³⁺ resides in a perfect octahedron when distances are concerned, 245.2(1) pm, 6×, but the octahedron is somewhat compressed along the −3 axis giving rise to Cl-Mo-Cl angles of 88.71(3)° and 93.75(3)°, respectively. The Na⁺ ions are, however displaced from the octahedral center with Na⁺-Cl⁻ distances of 274.8(1) to 291.4(2) pm, 3× each. The Cr³⁺–Cl⁻ distances in Na₃CrCl₆ are with 235.3(2) pm 10 pm smaller, roughly in accord with Shannon's ionic radii for Cr³⁺ (CN 6, 62 pm) and Mo³⁺ (CN 6, 69 pm) [7].

It is interesting to note that the Na₃CrCl₆ type of structure is only adopted with M = V, Cr, Mo, wheras the lighter and larger M = Sc, Ti, Y [8-11] as well as the lanthanides R = Dy−Lu [8,12,13] adopt the cryolite type of structure, Figure 3. The cryolite type of structure (Na₃AlF₆ type) is a monoclinic structure in which Na⁺ and F⁻ in a 1:3 ratio form layers between which octahedral voids are occupied by Na⁺ and Al³⁺. The Na₃GdCl₆ structure, on the other hand, is a stuffed LiSbF₆ type structure [14] in which Cl⁻ ions form, again, a hexagonal closest packing and Na⁺ and Gd³⁺ occupy octahedral voids. One Na⁺ and Gd³⁺ center rather regular octahedra, the remaining two Na⁺ are statistically distributed over the remaining four octahedral voids. There is a close relationship between the cryolite and the Na₃GdCl₆ type [11]; Na₃GdCl₆, for example, undergoes a reversible first-order phase transition from Na₃GdCl₆-I (stuffed LiSbF₆) to Na₃GdCl₆-II (cryolite type) at 205 °C [8].

All reactions and handling were carried out under a dry nitrogen atmosphere using dry box equipment (MBraun, Garching, Germany). MoCl₂ was prepared by synproportionation of Mo (Chempur, Karlsruhe, Germany, 99.95%) and MoCl₅ (Sigma-Aldrich, Munchen, Germany, 99.99%) in a 3:2 molar ratio with a slight excess of MoCl₅. MoCl₂ was filled into a Pyrex ampoule together with an excess AlCl₃ (Sigma-Aldrich, München, Germany, 99.99%) / NaCl (Chempur, Karlsruhe, Germany, 99.99%) flux, 55:45 mol%. The Pyrex ampoule was sealed under reduced pressure. The following temperature program was applied in a tubular furnace: heated to 623 K with 20 K/h, kept at that temperature for 3 days, then cooled slowly to 298 K (2 K/h). The Pyrex tube was transferred to a dry-box and the contents inspected with the aid of a microscope.

Na₃MoCl₆ forms well-faceted, polygonal red crystals. Some of these were selected under a microscope and sealed in thin-walled glass capillaries. After their quality had been checked by Laue diffraction patterns, the single crystals were transferred to a single-crystal X-ray diffractometer (Stoe Image Plate Diffraction System, IPDS I) to collect a complete intensity data set at ambient temperature. Structure solution and refinement was performed with the programs SHELXS-97 (direct methods) [15] and SHELXL-97 [16], scattering factors were from International Tables for X-ray Crystallography [17]. Data corrections were carried out for Lorentz and polarization factors and absorption (numerical with the aid of the programs X-RED [18] and X-SHAPE [19]). Further details of the crystal structure determination may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; e-mail: crysdata@fizkarlsruhe.de), on quoting the depository number ICSD-422981, the authors and the journal citation.

Crystal data for Na₃MoCl₆ (377.64 g mol⁻¹); diffractometer IPDS-I, Stoe, Darmstadt; Mo-K_α (graphite monochromator, λ = 71.073 pm); T = 293(2) K; 2θ_max = 56.3°; 100 images, 0° ≤ φ ≤ 200°; Δφ = 2°; indices: −9 ≤ h ≤ 9, −9 ≤ k ≤ 9, −15 ≤ l ≤ 16; transmission (min, max) = 0.0872, 0.1363; ρ_calc = 2.503 g cm⁻³; 4490 reflection intensities measured of which 416 were symmetrically independent, R_int = 0.0543, F(000) = 354, μ = 17.76 mm⁻¹. Trigonal, P-31c (no. 163), a = b = 687.1(1), c = 1225.3(2) pm, V = 501.0(1) × 10⁶ pm³, Z = 2. R values: R₁/wR₂ for 318 reflections with [I₀ > 2σ(I₀)]: 0.0238/0.0671 and for all data: 0.0350/0.0706; S_all = 1.062.

Red single crystals of Na₃MoCl₆ were obtained from the solution of the cluster chloride {Mo₆}Cl₁₂ in a slightly acidic NaCl/AlCl₃ melt at 350 °C upon cooling. The crystal structure was first observed for Na₃CrCl₆; in a hexagonal closest-packing of chloride spheres, half of the octahedral voids are occupied by Na⁺ and one sixth by Mo³⁺ ions such that these are 729.92(7) pm apart. Mo³⁺–Cl⁻ distances (245.2(1) pm) are 10 pm longer than for homologous Cr³⁺-Cl⁻.