Unraveling an Unusual Covalent C–Sb Bond Formation in a Co3Sb4CO6F6 Pseudo-superstructure

The development of a novel transition metal carbon oxyfluoride, Co3Sb4CO6F6, i.e., an [M-L-C-O-F]-type compound, leads to the formation of a 2-fold ∼(√2a × √ 2b × c) pseudo-superstructure compared to the basic structure (a × b × c) of Co3Sb4O6F6, i.e., an [M-L-O-F]-type compound, and a unique C–Sb covalent bond has been revealed for the first time. The covalent bond parameter, i.e., distance between carbon and antimony, not only could be included in the directory of the chemical bond in the bond valence sum (BVS) model of Brese and O’Keeffe [Acta Crystallogr. B47, 1991, 192–197] but also can open up research in inorganic chemistry. A single crystal of Co3Sb4CO6F6 has been obtained by employing a greener hydrothermal synthetic technique, and table sugar (sucrose, C12H22O11) acted as the source of carbon while incorporating into Co3Sb4O6F6, a transition metal oxyfluoride system comprising a stereochemically active lone pair cation (Sb3+). The smallest atomic radii of carbon were perfectly suited to occupy the tetrahedral sites of the Sb4 pseudo tetrahedron in Co3Sb4O6F6 and generated distortion in the structure, which resulted in lowering the symmetry to Fmm2 from I4̅3m. The participation of one electron from the lone pair of antimony cation and one electron from carbon led to the formation of a CSb4 covalent unit, which is well supported by the electron localization function (ELF) study. The DFT study also confirmed the lowering of the band gap energy (E g = 1.16 eV) in Co3Sb4CO6F6 compared to Co3Sb4O6F6 (E g = 1.48 eV).