Three novel ruthenium(II) complexes, RuX2(IMesH2)[N,N-diethyl-N′-(2-benzylidene)triazene] [IMesH2 = 1,3-dimesitylimidazolin-2-ylidene; X = Cl– (2), CF3COO– (3), C6F5COO– (4)] are described. Single-crystal X-ray structure analysis of compounds 2–4 revealed that the triazene ligand exists in the trans form and that the aryl-connected nitrogen atom of the triazene binds to the ruthenium atom to form a five-membered chelate ring. Compounds 2–4 were applied to the UV-induced ring-opening metathesis polymerization (ROMP) of cis-cyclooctene (5) and dicyclopentadiene (6). Whereas all three precatalysts show pronounced thermal latency in the presence of 5 or 6, only compound 2 displays significant photo-ROMP activity. The photoreactivity of precatalysts 2–4, which decreases with increasing electron-withdrawing character and with decreasing pKa of the X groups, is discussed and correlated with the X-ray data of 2–4, with low-temperature 1H NMR measurements as well as with cyclovoltammetric measurements.