A pseudo-Jahn-Teller distortion in an Mo(2)(mu(2)-O)(2) ring having the shortest Mo(IV)-Mo(IV) double bond

Four structures of edge-sharing bioctahedral compounds of the type Mo(2)(mu(2)-DArF)(2)(eta(2)-L-L)(2)(mu(2)-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 Mo(2)(mu(2)-O)(2) are very similar with very short Mo-Mo distances of 2.306[2] A. These are consistent with the presence of a Mo=Mo double bond of the type sigma(2)pi(2). As expected for these electronic configurations, the compounds are diamagnetic. The most striking characteristic is the distortion of the Mo(2)(mu(2)-O)(2) ring where a set of two Mo-O distances are significantly shorter then the other set (by ca. 0.05 A). This D(2)(h)--> C(2)(h) distortion is explained on the basis of a pseudo-Jahn-Teller effect.