A polymeric Mo polyselenide, K2Mo3Se18, and a large molecular cluster Mo polyselenide, K8Mo9Se40.4H2O, were synthesized by the hydrothermal method. The reaction of Mo, K2Se4, and H2O in a 1:1.5:22.2 ratio in a vacuum-sealed Pyrex tube at 135-degrees-C for 3 days yielded black platelike crystals of K2Mo3Se18 (I). The reaction of MoO3, K2Se2, and H2O in a 1:2:22.2 ratio under the same conditions as above yielded black chunky crystals of K8Mo9Se40.4H2O (II). Both compounds were obtained in approximately 20% yield. The structures were determined by single-crystal X-ray diffraction techniques. Crystal data for K2Mo3Se18: monoclinic P2(1), Z = 2, a = 10.277 (6) angstrom, b = 12.66 (1) angstrom, c = 10.624 (8) angstrom, beta = 116.82 (5)-degrees, V = 1233 (2) angstrom 3; 2-theta(max)(Mo K-alpha) = 45-degrees; number of data measured 1824, number of unique data 1608, number of data having F(o)2 > 3-sigma(F(o)2) 1382, number of variables 207, number of atoms 23; mu = 267 cm-1; final R = 0.052 and R(w) = 0.086. Crystal data for K8Mo9Se40.4H2O: triclinic P1BAR, Z = 2, a = 10.312 (8) angstrom, b = 18.55 (3) angstrom, c = 18.57 (2) angstrom, alpha = 87.6 (1)-degrees, beta = 87.57 (7)-degrees, gamma = 84.0 (1)-degrees, V = 3523 (8) angstrom 3; 2-theta(max)(Mo K-alpha) = 45-degrees; number of data measured 9888, number of unique data 8638, number of data having F(o)2 > 4-sigma(F(o)2) 3592, number of variables 546, number of atoms 65; mu = 224 cm-1; final R = 0.092 and R(w) = 0.108. I and II have rather complicated structures. I contains the [Mo3(Se)(Se2)3(Se3)(Se4)2]n2n- polymeric anion which contains [Mo3Se7]4+ clusters cross-linked by Se4(2-) ligands. The zigzag chains are then arranged side by side and interact through weak Se...Se contacts generated by a Se atom of a Se3(2-) ligand of one chain and the triangular face of Se atoms from the three bridging Se2(2-) ligands of a [Mo3Se7]4+ cluster in another chain. [Mo9Se40]8- contains three trinuclear [Mo3Se7]4+ subclusters which are "glued" together by the two monoselenide ions. One monoselenide ion belongs to a [Mo3Se7]4+ core being coordinated to three Mo atoms and at the same time interacting with the triangular face of three Se atoms of Se2(2-) bridging ligands of another [Mo3Se7]4+ subcluster. The other monoselenide ion interacts with six Se atoms of Se2(2-) bridging ligands of two [Mo3Se7]4+ subclusters. Two separate [Mo9Se40]8- clusters come in close contact (2.98 angstrom) in the solid state through two terminal Se2(2-) ligands related by an inversion center.
