Recrystallization of [MoO(2)Cl{HC(3,5-Me(2)pz)(3)}]Cl [where HC(3,5-Me(2)pz)(3) is tris(3,5-dimethyl-1H-pyrazol-1-yl)methane] led to the isolation of large quantities of the dinuclear complex dichlorido-2κ(2)Cl-μ-oxido-κ(2)O:O-tetraoxido-1κ(2)O,2κ(2)O-[tris(3,5-dimethyl-1H-pyrazol-1-yl-1κN(2))methane]dimolybdenum(IV) acetonitrile monosolvate, [Mo(2)Cl(2)O(4)(C(16)H(22)N(6))]·CH(3)CN or [{MoO(2)Cl(2)}(μ(2)-O){MoO(2)[HC(3,5-Me(2)pz)(3)]}]·CH(3)CN. At 150 K, this complex cocrystallizes in the orthorhombic space group Pbcm with an acetonitrile molecule. The complex has mirror symmetry: only half of the complex constitutes the asymmetric unit and all the heavy elements (namely Mo and Cl) are located on the mirror plane. The acetonitrile molecule also lies on a mirror plane. The two crystallographically independent Mo(6+) centres have drastically different coordination environments: while one Mo atom is hexacoordinated and chelated to HC(3,5-Me(2)pz)(3) (which occupies one face of the octahedron), the other Mo atom is instead pentacoordinated, having two chloride anions in the apical positions of the distorted trigonal bipyramid. This latter coordination mode of Mo(VI) was found to be unprecedented. Individual complexes and solvent molecules are close-packed in the solid state, mediated by various supramolecular contacts.