Dimeric (E)-4-lithio-1,4-diazabut-1-ene and N,N’-dilithioethylenediamine complexes [cis-{Li[mu-N(t-Bu)CH(t-Bu)CHN-t-Bu]}(2)] (1) and [{Li[rac-N(t-B u)CH(Me)CH(Me)N-t-Bu]Li}(2)] (2) have been prepared by carbolithiations of the 1,4-diazabuta-1,3-diene precursor using t-BuLi in pentane or MeLi in Et(2)O. Hydrolysis of 1 and 2 yields the substituted (E)-1,4-didzabatut-1-ene HN(t-Bu)CH(t-Bu)CHNt-Bu (3) and the racemic 2,3-dimethylethylenediamine rac-HN(t-Bu)CH(Me)CH(Me)N(H)t-Bu (4). Crystals of 1 are monoclinic, space group C2/c (No. 15), a 10.507(5), b = 15.419(2), and c = 20.075(9) A, beta = 92.13(2)degrees, V = 3250(2) Angstrom(3), Z = 4. Crystals of 2 are orthorhombic, space group Fddd (No. 70), a = 9.562(6), b = 18.36(1), and c = 33.04(2) Angstrom, V = 5799(6) Angstrom(3), Z = 8. Ab initio calculations on the dimerization of the unsubstituted analogue of 1 (E)-4-lithio-1,4-diazabut-1-ene give relative stabilization energies of 31.91 and 31.99 kcal mol(-1) for the amido nitrogen-bridged dimers of C-i and C-2 symmetry, respectively, relative to the monomer of C-s symmetry containing a planar five-membered chelate ring.