[PhLiNCH₂CH₂NLiPh]· 3 HMPA, a Monomeric Dilithiumethylenediamide Complex with N₂Li₂ Double Bridging: Synthesis, Crystal Structure, and Model MO Calculations

An Li2N2O bicyclo[1.1.1]pentane-type structure is characteristic of the trilithiated N,N'-diphenylethylenediamine in the presence of hexamethylphosphortriamide (HMPA). Li⊕ ions bridge the two N⊖ centers in this “ion triplet” in an-asymmetric fashion. Each of the Li⊕ ions is also coordinated to a terminal HMPA ligand (only the O atom of which is shown in the illustration to the right) and further bridged by a third HMPA ligand. The structural principles underlying this complex were predicted by ab initio calculations, and they can be rationalized in terms of a simple electrostatic model.