B. Goldfuss et al., 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/, Organometallics, 16(26), 1997, pp. 5999-6003
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)].