Iron oxide is a key multi-functional material in many different fields of modern technology. The β-Fe₂O₃ cubic phase, one of the least studied Fe–O systems, was obtained by Chemical Vapor Deposition (CVD) using for the first time a Fe(II) β-diketonate diamine complex, Fe(hfa)₂·TMEDA, as the molecular source (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′-tetramethylethylenediamine). The strong visible light absorption of β-Fe₂O₃ deposits highlights their possible functional application in photocatalytic hydrogen production under solar light. A comprehensive investigation on the Fe(II) complex, performed by a joint experimental–theoretical approach, explains the molecular origin of its excellent thermal behaviour and reveals why this species is a successful precursor for the CVD of iron oxide nanostructures.