Molybdenum(V) complexes with formate: Geometric isomerism of the [Mo2O4Cl2(Py)2(HCOO)]− ion

Reactions of (PyH)5[MoOCl4(H2O)]3Cl2 with formate resulted in trans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)] (1) and cis-(PyH)[Mo2O4Cl2(Py)2(HCOO)] (2), whereas the bromide starting material (PyH)[MoOBr4], yielded (PyH)3[Mo2O4Br4(HCOO)]·2CH3CN (3) and cis-(PyH)[Mo2O4Br2(Py)2(HCOO)] (4) (where Py stands for pyridine, PyH+ for pyridinium cation and (Py)2H+ for a hydrogen-bonded PyH+⋯Py ion). In all, the dinuclear metal–metal bonded { Mo 2 O 4 } 2 + core may be recognized with its six coordination sites distributed among halides, pyridine ligands and formate. The latter is coordinated via both oxygen atoms, with each to a different metal ion. The [Mo2O4Cl2(Py)2(HCOO)]− ion exhibits geometric isomerism: the pyridine ligands, on each metal ion one, are either trans or cis to each other. The trans isomer crystallized with (Py)2H+ countercations, whereas the cis isomer as a PyH+ salt. In the crystal lattice of cis-(PyH)[Mo2O4Cl2(Py)2(HCOO)] (2), as confirmed by the X-ray structure analysis, pyridinium cation forms a hydrogen bond with the doubly-bridging oxide of the cis-[Mo2O4Cl2(Py)2(HCOO)]− ion. The countercations of trans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)] (1) cannot participate in hydrogen-bonding. The DFT calculations on the isomers of the [Mo2O4Cl2(Py)2(HCOO)]− ion show the trans isomer to be by ca. 15 kJ/mol more stable than the cis isomer. The calculations on the hydrogen-bonded PyH+⋯[Mo2O4Cl2(Py)2(HCOO)]− ion-pairs show a reversed order of stability. Hydrogen-bonding and weak CH⋯Cl interactions between PyH+ cations and the cis-[Mo2O4Cl2(Py)2(HCOO)]− ion increase the stability of the cis compound.