DFT calculations (B3LYP/LANL2DZ/6-31G*) were used to investigate the ways in which 1-methyl-4-phenyl-1-azabuta-1,3-diene and 4-phenyl-1-oxabuta-1,3-diene bind to a Fe(CO)₄ moiety. As possible coordination modes, η²-coordination across the CC or CN/CO bond, σ-coordination to the lone pair of the heteroatom, or η³-coordination through the CC–C or the NC–C/OC–C moiety were considered. The latter forms involve coupling of the non-coordinated atom of the heterodiene with one of the carbonyl ligands to an acyl species. The calculated geometric parameters of all structures compare well with X-ray crystallographic data of similar complexes. The species in which the ligand is transoid and σ-coordinated is lowest in energy, for both compounds studied. However, the η²-alkene bound 1-oxabuta-1,3-diene complex is practically equal in energy to the σ-transoid form and thus competes. This agrees with experimental observations that the heterodiene is σ-bonded in Fe(CO)₄(1-methyl-4-phenyl-1-azabuta-1,3-diene) but η²-coordinated in Fe(CO)₄(4-phenyl-1-oxabuta-1,3-diene). The solvent dependence was estimated from single point PCM calculations, for CH₂Cl₂ as solvent. For the 1-azabuta-1,3-diene complexes, the relative energies of η²-olefin and η³-allyl forms are inverted, with the η³-allyl form being more stable in polar solvents. The 1-oxabuta-1,3-diene complexes in their η²-olefin and σ-O forms change order of relative energy, and conversion to the σ-O form is expected in a polar medium for these complexes. Calculated IR vibrational stretching frequencies of the carbonyl ligands and the CN/CO bond were compared with experimental data, to produce the best fits for the σ-transoid form of Fe(CO)₄(1-methyl-4-phenyl-1-azabuta-1,3-diene) and η²-olefin bonded Fe(CO)₄(4-phenyl-1-oxabuta-1,3-diene). These results are again consistent with the experiment and show that the DFT method applied in this work can be used as an aid for structural validation.