A series of mononuclear heteroleptic copper( i ) halide complexes, CuL(PPh 3 )X (X = Cl, Br, I), based on 4-(3,5-diphenyl-1 H -pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine (L) and triphenylphosphine, have been synthesized by reaction between CuX (X = Cl, Br, I), L and PPh 3 in a molar ratio of 1/1/1 in MeCN solutions. The copper atom, showing the distorted tetrahedral environment, is bound by the N , N -chelating ligand L, triphenylphosphine and a halide ion. The complexes CuL(PPh 3 )Cl and CuL(PPh 3 )Br are isostructural. In CH 2 Cl 2 solutions, L and the complexes CuL(PPh 3 )X (X = Cl, Br, I) display a luminescence band with λ max = 377 nm and a lifetime of 1.9 ns (ligand-based luminescence (LL*)). However, the complex CuL(PPh 3 )I has an additional weak luminescence band with λ max = 681 nm and a lifetime of 96 ns of 3 MLCT origin. In the solid state, L shows the splitting of the luminescence band to λ max = 365 and 384 nm and a slight increase of the lifetime to 2.66 ns. Solid samples of the complexes CuL(PPh 3 )X demonstrate 3 MLCT luminescence bands at 620 nm (X = Cl), 605 nm (X = Br) and 559 nm (X = I) with lifetimes in the range 3.6-11.2 μs, whereas the LL* band (377 nm) is absent. Quantum yields and rate constants of radiative and nonradiative processes were determined in CH 2 Cl 2 solutions and in the solid state for all complexes. The luminescence quantum yield and lifetimes for the solid samples increase in the order CuL(PPh 3 )Cl < CuL(PPh 3 )Br < CuL(PPh 3 )I. This is due to the increase of radiative decay and simultaneous suppression of nonradiative decay. The complex CuL(PPh 3 )I shows a high quantum yield of 29.4% and an excited state lifetime of 11.2 μs. Correlations between spectral properties and structural parameters are discussed.