EFFECT OF POLYDENTATE DONOR MOLECULES ON LITHIUM HEXAMETHYLDISILAZIDEAGGREGATION - AN X-RAY CRYSTALLOGRAPHIC AND A COMBINATION SEMIEMPIRICAL PM3 SINGLE POINT AB-INITIO THEORETICAL-STUDY/

Addition of 1 equiv of polydentate amine or ether donor solvent to hyd rocarbon solutions of lithium hexamethydisilazide (LHMDS) yields a var iety of complexes with different aggregation states. X-ray crystallogr aphic analyses have been carried out on six new compounds revealing th ree-and four-coordinate monomers, eta(1)-coordinated mono-and disolvat ed dimers, and polymers of dimers. PM3 calculations were able to locat e minima for a variety of possible structures for the ligands N,N,N',N '-tetramethylethylenediamine and 1,2-dimethoxyethane. However, the hea ts of formation from these calculations are found to be unreliable in predicting the relative stabilities of the isomers. Single point ab in itio calculations at the 6-31G level on the PM3 optimized structures give energies which correspond well to the known aggregation states of LHMDS species. Deaggregation from dimers to monomers appears to be dr iven by a combination of steric, electronic, and chelate effects.