The interactions between aluminum atoms and model molecules representing trans-polyacetylene are studied quantum chemically by a local density functional method. We focus on the chemical and electronic structure of the organoaluminum complexes. Special emphasis is put on a comparison between results at the local spin density approximation and ab initio Hartree-Fock levels. In unmetallized polyenes, the density functional method provides a very good description of the carbon-carbon bond lengths of conjugated systems; in the case of hexatriene, it reproduces the bond dimerization in very good agreement with experimental measurements. Upon metallization, a strong covalent interaction between aluminum and carbon is found. The Al-C bond formation induces an interruption of the bond alternation pattern and reduces the pi-conjugation in the oligomer, in qualitative agreement with photoelectron spectroscopy data and previous theoretical results at the Hartree-Fock level. Notably, the pi-electron levels in the organoaluminum complexes maintain delocalization. In contrast to Hartree-Fock results where an aluminum atom binds to a single carbon, the interactions calculated with the local spin density approximation lead to (i) formation of multicenter aluminum-carbon bonding; (ii) near planarity of the polyene molecule; and (iii) a lower degree of charge transfer from the metal atom to the polymer.