The steric and electronic properties of several sterically demanding tertiary phosphines (dicyclohexylphosphino)biphenyl (2a), 2-dicyclohexylphosphino-2′-methylbiphenyl (2b), 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (2c), 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (2d), 2-diphenylphosphino-2′-(N,N-dimethylamino)biphenyl (2e), 2-di-tert-butylphosphino-2′-(N,N-dimethylamino)biphenyl (2f), and di(cyclohexyl)phenylphosphine (2g) have been studied by synthesizing and characterizing iridium complexes of types [IrCl(cod)(L)] (L = 2a−d) and cis-[IrCl(CO)2(L)] (L = 2a−d and 2g). The infrared stretching frequencies of the carbonyl complexes permit an estimation of the ligand donor properties (basicity) and suggest that the donor properties of ligands 2a−d reside between that of 2g and PCy3. The crystal structures of several [IrCl(cod)(L)] (L = 2a−d) and cis-[IrCl(CO)2(L)] (L = 2a−d and 2g) complexes are reported and used to quantify the ligand steric parameter.