A comprehensive calculational investigation has been carried out on a series of complexes of the type [(terpyridyl‐R1)Pt(C≡C‐R2)], where terpyridyl‐R1 is a series of substituted 2,2':6',2''‐terpyridyl ligands and C≡C‐R2 is a series of substituted acetylide ligands. In one series of complexes (I), the energy of the electronic excited state is varied by changing the substituents on the terpyridyl ligand (R1). In a second series of complexes (II), this electronic structure variation is obtained by changing the para substituents (R2) of the acetylide ligand. The effect of varying the substituents on the lowest‐energy excited states of the complexes has been assessed by calculating their electronic structures and excitation energies. We anticipated that introduction of electron‐withdrawing substituents on the terpyridyl ligand will benefit the LLCT (or MLCT) and prohibit the nonradiative pathways via d‐d transitions in these complexes; introduction of electron‐donating substituents on the acetylide ligand can also prohibit the nonradiative pathways by increasing the energy gaps between the HOMO–LUMO and d‐d transitions. The results also reveal that the lowest‐energy excitations of all complexes of series I and IIa–b complexes are dominated by a π(C≡C) → π*(terp) (LLCT) transition mixed with some energetically dπ(Pt) → terpyridyl (MLCT) transition. However, for the complexes IIc–IId, in which phenyl rings are introduced on the acetylide ligand, the lowest‐lying absorptions of IIc and IId are predominately LLCT in character, with less MLCT mixture, due to a lower contribution of the Pt(d) orbital to the HOMO, while for IIe, with a stronger donor on the acetylide, the lowest‐lying absorption is completely LLCT in character. The absorption and emission calculations using the TDDFT method are based on the optimized geometries obtained at the B3LYP/LanL2DZ and CIS/LanL2DZ levels, respectively. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)