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

• Two polymorphs of trans-(PyH)[MoCl4(Py)2].
• The trans-[MoCl4(Py)2]− ion exists as two conformational isomers, one possesses an eclipsed and the other a fully staggered conformation of pyridine rings.
• Quantum mechanical calculations show only negligible energy differences between the pair.

Trans-(PyH)[MoCl4(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-[MoCl4(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-[MoCl4(Py)2]− anions. Quantum mechanical calculations on the D2h and D2d conformational isomers of the trans-[MoCl4(Py)2]− ion have shown negligible energy differences between the pair.

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