THE PRESENCE AND ABSENCE OF AGOSTIC ELECTROSTATIC SI-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-M INTERACTIONS IN [M(ME2SI(H)N-T-BU)] (M = LI, MG-12) SPECIES/

Agostic Si-H ... Li contacts (i.e, significantly less than similar to 3.0 Angstrom) are not present in the solvent-free X-ray structure of [ Li{Me2Si(H)N-t-Bu}](3); shorter (t-Bu)CH3 ... Li distances (similar to 2.8 Angstrom) are observed instead. The situation in solution is diff erent. While the H-1-Li-6 HOESY spectrum of [Li{Me2Si(H)N-t-Bu}] in to luene at -80 degrees C evidently detects the same trimeric species (la cking Si-H ... Li interactions, but with short (t-Bu)CH3 ... Li contac ts), two major species with strong Si-H ... Li interactions also are p resent. In the Li-HN-SiH3 computational model system, Si-H ... Li inte ractions are favored energetically and result in increased Si-H distan ces and decreased Si-H frequencies. Agostic Si-H ... metal contacts in the solid state are found in the X-ray crystal structure of [Mg-2{Me2 Si(H)N-t-Bu}(4)], where Li''+'' is replaced by the more highly charged Mg''2+''. The two short agostic Si-H ... Mg interactions(2.2, 2.5 Ang strom) which result also are shown by the two low v(Si-H) frequencies (2040, 1880 cm(-1)) in the IR spectrum (Nujol mull) of [Mg-2{Me2Si(H)N -t-Bu}(4)].