The CuAAC click reaction is greatly accelerated by tris(triazolylmethyl)amine ligands (TL). Using mass spectrometry, we found a trinuclear TL-Cu-3(I)-acetylide complex formed in the reaction. Under catalytic conditions, the trinuclear complex is more active than the proposed dinuclear complex, whereas in a single-turnover reaction, the dinuclear complex is more reactive. Here, this finding is rationalized by analysis of the first single-crystal X-ray structure of a trinuclear TL-Cu-3(I)-acetylide complex and DFT calculation, revealing how TL stabilizes the trinuclear complex, which may accelerate the reaction by promoting coordination with an azide followed by dissociation of a Cu-I to form a more favorable dinuclear transition state. (C) 2018 Elsevier Inc. All rights reserved.