SIMPLE SILYLHYDRAZINES AS MODELS FOR SI-N BETA-DONOR INTERACTIONS IN SINN UNITS

Two simple silylhydrazines, H3SiMeNNMe2 and (H3Si)(2)NNMe2, have been prepared by the reaction of bromosilane with the corresponding NH-func tional hydrazine in the presence of a base and the appropriate lithiat ed hydrazine. The intermolecular attractive interactions (beta-donor-a cceptor interaction) between the silicon and beta-nitrogen atoms of H3 SiMeNNMe2 and (H3Si)(2)NNMe2 has been demonstrated by low-temperature X-ray crystallography and ab initio calculations (MP2/6-311G). The con tributions to the strength of this two-bond interaction are discussed in the light of a new series of calculations on RSiH2NR'NR2 '' molecul es (R=H, F; R'=H, Me, SiH3; R ''=H, Me), which show electronegative su bstituents at the silicon acceptor center to exert the largest effect, while the electronic nature of the substituent at the P-nitrogen atom is also important. As the resulting structures cannot be described sa tisfactorily by either the VSEPR concept or Bartell's two-bond radius model, a two-bond interatomic attraction has to be taken into account in addition; this leads to an extension of the common models for empir ical structure prediction.