THE EFFECT OF PENTACOORDINATION ON SI-29 NMR CHEMICAL-SHIFTS AND SILICON HYDROGEN COUPLING-CONSTANTS

The coordination of R3SiH compounds with a nucleophile, Nu, to give the adduct R3SiH-Nu produces changes in the Si-29 NMR chemical shift and 1J(SiH) that follow similar trends. Coordination of highly electronegative, 'soft', ligands is accompanied by the strongest low frequency shifts and the greatest increase in 1J(SiH). Formal coordination of 'hard' electropositive ligands such as methyl can result in high frequency shifts and a decrease in 1J(SiH). The Si-29 NMR chemical shift range for the pentacoordinate neutral, anionic and cationic adducts is about delta -110 to -47 ppm, whereas the range for the related four-coordinate silanes is greater at delta +24 to - 76 ppm.