PI-BOND VS. AGOSTIC INTERACTION IN 3-COORDINATED ALKOXY AND THIOLATE DERIVATIVES OF ALUMINUM, BORON AND CATIONIC CARBON - AN AB-INITIO STUDY OF H2X-YR SYSTEMS (X = AL, B, C+ Y = O, S R = H, CH3)

Rotational barriers and pi conjugation effects have been studied in H2 X-YR systems (X = Al, B; Y = O, S; R = H, CH3) by means of ab initio c alculations at the MP4/6-311G*//MP2/6-311G** level. In non-substitute d systems H2X-YH, it is shown that the rotational barrier depends on t hree factors: (i) pi conjugation strength between the X vacant p orbit al and the Y p-lone pair; (ii) the opening ability of the Y-valence an gle and (iii) a possible direct interaction between X and the Y hydrog en substituent. The conjugation stabilization has been estimated throu gh Valence Bond calculations and is found to decrease in the order BO (22.6 kcal/mol), BS (16.6 kcal/mol), AlO (10.2 kcal/mol), and AlS (8.1 kcal/mol). Study of the methylated systems confirm the results found in unsubstituted species. The most important feature is the location o f a deconjugated secondary minimum for H2B-SCH3 species. An acute B-S- C angle (69.6 degrees) is found, thus evidencing a direct H ... B agos tic interaction with a four-membered (BSCH) heterocycle. To further ev idence our analysis, additional calculations have been performed on ca rbocationic species H2C+-YR (Y = O, S; R = H, CH3) for which the H2Cmoiety is a strong pi acceptor. As expected, the C+-O and C+-S conjuga tion are strong (about 60 kcal/mol). Only two minima have been located for the H2C+-OCH3 species. In the sulfur case, the Potential Energy S urface (PES) is more complicated and six stationary points have been c haracterized. A strong agostic interaction is found for a secondary de conjugated minimum for which an hydrogen symmetrically bridges the two carbon atoms. A schematic energy profile connecting the various extre ma is given for this cationic species.