Anion-Anion Interactions Involving the [Mo₃Se₁₃]²⁻ Cluster. Syntheses and Characterization of (Me₄N)₂Mo₃Se₁₃, K₂Mo₃Se_12.5O_0.5, and K₆M_o6Se₂₇•6H₂O

The hydrothermal reaction of M0O₃, Na₂Se₂ and Me₄NCl (molar ratio, 1:3:2) at 135 °C for 3 days gave dark red needles of (Me₄N)2Mo₃Se₁₃ (I); the reaction of M0O₃ and K₂Se₂ (molar ratio of 1:2 for one month and 1:3-5 for 3 days) at 135 °C gave black needle-like crystals of K₂Mo₃Se_12.5O₀.₅ (II) and black chunky crystals of K₆Mo₆-Se₂₇6H₂O (III) respectively. The structures were determined by single crystal X-ray diffraction techniques. Compound I crystallizes in the trigonal space group P3₁m (No. 157), Z = 1, a = 11.578(1) Å, c = 6.110(2) Å, V = 709.4(3) ų. d_calc = 3.42 g/cm³, μ = 177.99 cm⁻¹Number of data measured: 992. Number of unique data: 502. Number of data having I> 3.0σ(I): 438. Number of variables = 49. R/R_w = 6.0/7.5%. Compound II crystallizes in the monoclinic space group Cm (No. 8), Z = 2, a = 11.874(3) Å, b = 16.834(3) Å, c = 5.820(3) Å, β = 116.65(2)°, V = 1040(1) ų, d_cale = 4.45 g/cm³, μ = 246.64 cm⁻¹. Number of data measured: 997. Number of unique data: 951. Number of data having I > 3.0σ(I): 849. Number of variables = 89. R/R_W = 4.3/5.3%. III crystallizes in the monoclinic space group P2₁/n (No. 14), Z = 4, a = 10.21(2) Å, b = 17.84(1) Å, c = 27.04(2) Å, ß = 98.8(1)°, V = 4868(21) ų, d_Calc = 4.15 g/cm³. μ = 219.98 cm⁻¹. Number of data measured: 8706. Number of unique data: 7920. Number of data having I> 3.0σ(I): 3520. Number of variables = 391. The [Mo₃Se₁₃]²⁻ in I and [Mo₃Se_12.5O_0.5]²⁻ in II are isostructural with [Mo₃S₁₃]²⁻, while III is the cocrystallization of two K₂Mo₃Se₁₃ clusters and one K₂Se. The anions of I and II are the same except that the triply bridging Se site is partially occupied by O atoms in II. Solid state IR and UV/vis spectra are reported. The [Mo₃Se₁₃]²⁻ cluster has been found to possess significant affinity for anionic species, and this has been manifested in all currently known compounds containing this trinuclear cluster. The new compounds reported in this paper also exhibit clearly defined anion-anion weakly bonding interactions. Extended Hiickel calculations were performed to explain the unusual [Mo₃Se₁₃]²⁻Se²⁻ interactions in these compounds. The calculations show that the Se atoms (of the bridging Se₂²⁻), which form the points of interaction of the cluster with the anionic species, are weakly positively charged.