Ab Initio SCF MO Study of the Interaction of Lithium with Ammonia

An ab initio molecular orbital study of the stability and electronic structure of lithium–ammonia complexes was made. A complex of lithium with one ammonia molecule is predicted to be stable by 20 kcal/mole relative to a free lithium atom and a free ammonia molecule. The bonding in the complex is primarily through sharing of the lone pair of electrons on ammonia between the ammonia molecule and the lithium atom. The unpaired electron orbital is rearranged upon formation of the complex so that it has very low density in the region of the ammonia with the exception of a sharp peak in density near the nitrogen nucleus. The complex with two ammonia molecules is nearly isoenergetic with the one ammonia complex plus an ammonia molecule. Addition of a second ammonia causes an increase in the preferred lithium–nitrogen distance and an increase in the occupancy of the nitrogen orbitals by the unpaired electron. Ammonia appears repulsive to the unpaired electron in the diammonia complex, also. The implications of these results for models of the monomer in metal–ammonia solutions are discussed.