The dimerizations of the Al-N analogues of ethylene, acetylene, and cyclobutadiene are studied by ab initio methods. The dimerizations occur without activation barriers, and the resulting structures are in good agreement with the experimental X-ray structures of known derivatives. The dimerizations of H2AlNH2 and [HAlNH](2) are intermolecular dative processes with energies of 30-35 kcal/mol per Al-N bond formed. The dimerization of HAlNH forms normal Al-N bonds, with 70-75 kcal/mol of energy released per Al-N bond created.