Trans-cis isomerization and structure of [R2Ga(mu-NHSiEt3)](2) (R = Me, Et)

Reactions of Et3SiNH2 With R3Ga in a 1:1 ratio have produced dimeric silyla midogallanes, [R2Ga(mu-NHSiEt3)](2) (1, R= Me, trans:cis = 1.5:1; 2, R = Et , trans:cis = 1.3:1), as a mixture of trans (a) and cis (b) isomers. Purifi cation of 1 by either recrystallization or sublimation gives only trans iso mer la as colorless crystals. Colorless liquid 2 has been obtained only as a mixture of the two isomers. The trans --> cis isomerization of 1 has been studied by H-1 NMR spectroscopy. The equilibrium has been observed to foll ow reversible first-order kinetics with Delta H degrees = - 0.64 +/- 0.03 k J mol(-l) and Delta S degrees = - 4.4 +/- 0.2 J K-1 mol(-1). The activation parameters for the trans (1a) --> cis (1b) conversion are Delta H-1(double dagger) = 72.4 +/- 1.3 kJ mol(-1) and Delta S-1(double dagger) = - 38.8 +/ - 4.5 J K-1 mol(-1), and those for the reverse process are Delta H--1(doubl e dagger) = 73.0 +/- 0.4 kJ mol(-1) and Delta S--1(double dagger) = - 34.4 +/- 1.7 J K-1 mol(-1). The isomerization is markedly accelerated in the pre sence of Lewis bases. A crossover experiment indicates that the isomer inte rconversion is a unimolecular process. The thermodynamic and kinetic data h ave been explained based on the solvent effect and the silyl substituent ef fect on the nitrogen atom. The molecular structure of 1a has been determine d by a single-crystal X-ray diffraction study. The molecular geometry of la consists of a centrosymmetric and dimeric unit with two bridging (triethyl silyl)amido groups and two terminal methyl groups bound to each gallium ato m. The two silyl groups are trans to each other with respect to the planar (Ga-N)(2) ring framework. The coordination geometry of both gallium and nit rogen atoms is distorted tetrahedral.