Structures of polymorphic forms of trans-(PyH)[MoCl4(Py)2]: Conformational isomerism of the trans-[MoCl4(Py)2]− ion

Abstract Trans -(PyH)[MoCl 4 (Py) 2 ] was found to crystallize in two polymorphic forms, denoted as 1e and 1s . It was shown by X-ray structure analysis that each of the two forms contains a different conformational isomer of the trans -[MoCl 4 (Py) 2 ] − anion. In 1e , the two trans positioned pyridines are coplanar, whereas in 1s , a dihedral angle of 89.8(1)° is formed by the planes of pyridine rings. The polymorphs also differ in the intermolecular interactions between the pyridinium cations and the trans -[MoCl 4 (Py) 2 ] − anions. Quantum mechanical calculations on the D 2 h and D 2 d conformational isomers of the trans -[MoCl 4 (Py) 2 ] − ion have shown negligible energy differences between the pair.