The aryl effect in disubstituted dimanganese and dicobalt carbonyls

The energies of the intense bands in the near-UV-vis spectra of a number of axially substituted Mn-2(CO)(8)L-2 and Co-2(CO)(6)L-2 complexes containing non-pi-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 Frock'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 hv = alpha + beta chi + gamma theta + omega E-ar gives an excellent empirical description of the data (100(1 - R-2) = 0.5% and 2.6% for the Mn-2 and Co-2 complexes, respectively) where chi is the Tolman/Bartik measure of the sigma-basicity and theta is the Tolman cone angle of the ligands. The values of hv increase with increasing sigma-basicity and decrease with increasing ligand size and increasing numbers of aryl groups. The maximum increase of 1650 cm(-1) in the hv values of the Mn-2 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 Co-2 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 Mn-2 and Co-2 complexes, respectively. The effect of sigma-donicity is ascribed to the greater concentration of electron density in the region of the M-P bond when the electron is in the sigma* orbital, and the effect of ligand size is ascribed to a lengthening of the M-M and M-PI bonds, the former decreasing sigma-orbital overlap and the latter decreasing the effective sigma-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.