Synthesis, structural characterization, and electronic structure of monoclinic α-[SeCl3][MoOCl4].

The crystal structure of the monoclinic α-[SeCl3][MoOCl4] polymorph, trichloridoselenium tetrachloridooxidomolybdate, is reported from a complete single-crystal X-ray diffraction study. Black-brown moisture-sensitive crystals were obtained from reactions of MoOCl4 and Se2Cl2 in sealed glass ampoules at 100 °C. The compound crystallizes in the space group P21/c and is isostructural with the tellurium analog, α-[TeCl3][MoOCl4]. Its structure consists of trigonal-pyramidal [SeCl3]+ cations and square-pyramidal [MoOCl4]- anions that dimerize through asymmetric Mo...Cl...Mo bridges to form centrosymmetric [Mo2O2Cl8]2- units. The SeIV centers exhibit characteristic 3 + 3 coordination, with three short primary Se-Cl bonds and three longer secondary Se...Cl contacts, yielding distorted [SeCl6] trigonal antiprisms. These contacts connect adjacent [Mo2O2Cl8]2- dimers into layers parallel to the bc plane. The electronic structure calculations show that α-[SeCl3][MoOCl4] has a band gap of ca 0.88 eV. Crystal orbital Hamilton population (COHP) and crystal orbital bond index (COBI) bonding analyses confirm that short primary contacts are covalent, whereas longer secondary Se...Cl and Mo...Cl contacts are significantly weaker yet structurally important for layered connectivity. The natural bond orbital (NBO) analysis further supports charge polarization between the [SeCl3]+ and [MoOCl4]- fragments, placing α-[SeCl3][MoOCl4] in the realm of layered ionic solids or even van der Waals materials. Bonding of the broad [ChCl3][MoOCl4] series (Ch = S, Se, Te) are discussed as well.