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The energies of the intense bands in the near-UV−vis spectra of a number of axially substituted Mn2(CO)8L2 and Co2(CO)6L2 complexes containing non-π-acid phosphine substituents have been measured in solution. When combined with data reported previously, the trends show unambiguously that a major contributor to the energies is Giering and Prock's aryl effect (quantifiable by E ar values that are almost proportional to the number of aryl groups attached to the donor P atom). The equation hν = α + βχ + γθ + ωE ar gives an excellent empirical description of the data (100(1 − R2) = 0.5% and 2.6% for the Mn2 and Co2 complexes, respectively) where χ is the Tolman/Bartik measure of the σ-basicity and θ is the Tolman cone angle of the ligands. The values of hν increase with increasing σ-basicity and decrease with increasing ligand size and increasing numbers of aryl groups. The maximum increase of 1650 cm-1 in the hν values of the Mn2 complexes is found when L = P(p-F3CC6H4)3 is replaced by P(n-Bu)3, and this is made up of 49 ± 4% contribution by the less unfavorable aryl effect, 33 ± 5% due to the greater basicity, and 18 ± 1% due to the smaller size of the P(n-Bu)3 ligands. In the Co2 complexes the aryl and steric effects contribute 82 ± 11% and 16 ± 3%, respectively, but the electronic effect is small and not well defined. The dominant contributions of the aryl effect are striking. Indeed, the correlation of the data with the aryl effect alone leads to R 2 values of 0.827 and 0.845 for the Mn2 and Co2 complexes, respectively. The effect of σ-donicity is ascribed to the greater concentration of electron density in the region of the M−P bond when the electron is in the σ* orbital, and the effect of ligand size is ascribed to a lengthening of the M−M and M−P bonds, the former decreasing σ-orbital overlap and the latter decreasing the effective σ-donicity. No explanation is yet available for the origins of the empirically well established aryl effect even though it shows up in a wide range of physicochemical measurements.