The cyclononatetraenyl (Cnt) ligand is a large monoanionic ligand. It is easily synthesized by ring expansion after cyclopropanation of the cyclooctatetraenyl (Cot) ligand. The Cnt ligand can be reported as the cis–cis–cis–cis (cis) isomer, where the aromatic ring is flat, and all carbon atoms form a homogenous ring, and as the cis–cis–cis–trans (trans) isomer, where one carbon places itself inside the ring. The isomerization from the trans to the cis form has been reported numerous times in previous articles, but no quantitative analysis has been proposed due to contradictory data. This article proposes a detailed analysis involving light to rationalize this intrigue concerning isomerization. A careful synthesis at low temperatures and with light protection yields the ligand in its trans form (Cnt-trans). The controlled photo-isomerization of the Cnt-trans ligand is reported herein. A series of divalent or trivalent rare earth complexes, (Cnt)2Sm, and (Cot)(Cnt)Ln (Ln = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Er, Ho), which synthesis, solid-state X-ray diffraction and solution 1H NMR and UV-visible measurements, have been revised according to the synthesis using the Cnt-trans ligand. The photo-isomerization of the (Cnt-trans)2Sm evolves to the intermediate (Cnt-cis)(Cnt-trans)Sm and the (Cnt-cis)2Sm complex as the thermodynamical product. The photoisomerization of the trivalent (Cot)(Cnt)Ln complexes highlights the formation of a photostationary state (PSS) after several minutes of irradiation, in which both Cnt-trans and Cnt-cis ligands are present. The ratio of these two forms varies according to metal and irradiation wavelength: low-energy wavelengths favor the cis isomer, and high-energy wavelengths favor the trans isomer. DFT and TD-DFT were performed to provide a tentative orbital explanation.