The low-energy isomers of Ir(CO)(NHC) (x = 1, 2, 4) are investigated with density functional theory (DFT) and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The structures, relative energies, ligand dissociation energies, and natural charges are calculated. The energies of tetrairidium cluster are predicted at the CAM-B3LYP level that best fit the CCSD(T) results compared with the other four functionals in the benchmark calculations. The NHC's behave as stronger σ donors compared with CO's and have higher ligand dissociation energies (LDEs). For smaller isomers, the increase in the LDEs of the CO's and the decrease in the LDEs of the NHC's as more NHC's are substituted for CO's are due to π-back-bonding and electron repulsion, whereas the trend of how the LDEs change for larger isomers is not obvious. We demonstrate a μ-CO resulting from the high electron density of the metal centers in these complexes, as the bridging CO's and the μ-CO's can carry more negative charge and stabilize the isomers. Comparison of calculations for a mixed tetrairidum cluster consisting of two calixarene-phosphine ligands and a single calixarene-NHC ligand in the basal plane demonstrated good agreement in terms of both the ligand substitution symmetry (C derived), as well as the infrared spectra. Similar comparisons were also performed between calculations and experiment for novel monosubstituted calixarene-NHC tetrairidium clusters.