Recrystallization of [MoO₂Cl{HC(3,5-Me₂pz)₃}]Cl [where HC(3,5-Me₂pz)₃is tris(3,5-dimethyl-1H-pyrazol-1-yl)methane] led to the isolation of large quantities of the dinuclear complex dichlorido-2κ²Cl-μ-oxido-κ²O:O-tetraoxido-1κ²O,2κ²O-[tris(3,5-dimethyl-1H-pyrazol-1-yl-1κN²)methane]dimolybdenum(IV) acetonitrile monosolvate, [Mo₂Cl₂O₄(C₁₆H₂₂N₆)]·CH₃CN or [{MoO₂Cl₂}(μ₂-O){MoO₂[HC(3,5-Me₂pz)₃]}]·CH₃CN. At 150 K, this complex cocrystallizes in the orthorhombic space groupPbcmwith 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 Mo⁶⁺centres have drastically different coordination environments: while one Mo atom is hexacoordinated and chelated to HC(3,5-Me₂pz)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^VIwas found to be unprecedented. Individual complexes and solvent molecules are close-packed in the solid state, mediated by various supramolecular contacts.