Base-Free Molybdenum and Tungsten Bicyclic Silylene Complexes Stabilized by a Homoaromatic Contribution

Unprecedented base-free bicyclic silylene complexes of molybdenum and tungsten 3a,b (a, M = Mo; b, M = W) were prepared by the reaction of the calcium salt of 1,2,3,4-tetrakis(di-tert-butylmethylsilyl)tetrasilabicyclo[1.1.0]butane-2,4-diide 2(2-)center dot Ca2+ with Cp2MCl2 (M = MO, W). The unusual structural features of 3b (long Si=W double bond, short skeletal (Sp(2)-Si)-(sp(3)-Si) bonds and extremely long Si-Si bridge) are indicative of a contribution from the zwitterionic resonance form W delta--Si delta+, in which the electrophilic silicon center is stabilized by the Si-3-homoaromatic conjugation. This was supported by the theoretical calculations (NBO, NPA) revealing the presence of pi(W=Si) and sigma(Si-Si bridge) interactions.