A Pseudo-Jahn−Teller Distortion in an Mo₂(μ₂-O)₂ Ring Having the Shortest Mo^IV−Mo^IV Double Bond

Four structures of edge-sharing bioctahedral compounds of the type Mo2(μ2-DArF)2(η2-L-L)2(μ2-O)2, where DArF is an anion of an N,N‘-diarylformamidine and L−L is a chelating acetate or DArF group, are reported here. The cores of the ring formed by the Mo2(μ2-O)2 are very similar with very short Mo−Mo distances of 2.306[2] Å. These are consistent with the presence of a MoMo double bond of the type σ2π2. As expected for these electronic configurations, the compounds are diamagnetic. The most striking characteristic is the distortion of the Mo2(μ2-O)2 ring where a set of two Mo−O distances are significantly shorter then the other set (by ca. 0.05 Å). This D2h → C2h distortion is explained on the basis of a pseudo-Jahn−Teller effect.