Structure and physicochemical properties of copper(I) complexes of the tridentate ligands L2 (N,N-bis[2-(6-methylpyridin-2-yl)ethyl]phenethylamine) and L3 (N,N-bis[2-(2-pyridyl)ethyl]-β-methylphenethylamine) have been examined to obtain deeper insights into modulation of the coordination chemistry of copper(I) complexes. [CuI(L2)(CH3CN)](CIO4) (2·CH3CN) has a distorted tetrahedral geometry, which consists of three nitrogen atoms of the ligand and one nitrogen atom of the bound CH3CN. Steric repulsion between the 6-methyl group on the pyridine nucleus of L2 and the metal ion of the complex prevents the cuprous complex from adaptation to a three-coordinate geometry which must have a shorter Cu-N(pyridine) distance (@O1.88 Å). Thus, the four-coordinate copper(I) complex (2·CH3CN) with a longer Cu-N bond (1.98@O2.13 Å) becomes favorable, resulting in rather strong binding of CH3CN to the metal ion. In [CuI(L3)](CIO4) (3), there is a CuI-π interaction between the cuprous ion and the phenyl group of the ligand sidearm. Such a copper(I)-arene interaction is essentially weak, but is significantly stabilized in complex 3. The methyl group at the benzylic position of L3 reduces the degree of freedom of sidearm rotation to make the phenyl group stick on the cuprous ion. Thus, the reactivity of the copper(I) complexes of L2 and L3 toward dioxygen is significantly diminished, showing sharp contrast to the high reactivity of the copper(I) complex supported by a similar tridentate ligand L1 (N,N-bis[2-(2-pyridiyl)ethyl] phenethylamine).