The Aryl Effect in Disubstituted Dimanganese and Dicobalt Carbonyls

Abstract

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.

σ→σ* Transitions in Mn2(CO)8L2 complexes (L = P-donor ligands): π-acidity and oxygen pendant group effects in phosphite ligands

“…The energies of the r→r* transitions in the bis axial Mn 2 (CO) 8 L 2 complexes have already been shown to fit well to an equation containing parameters that characterize the rbasicities (pK a ) and sizes (cone angles h) of the 14 ligands involved, together with the effects of the number of pendant aryl groups on some of the donor phosphorus atoms. 3 It has been shown here that the enhanced energies that were originally found 2 to be associated with the presence of oxygen atoms attached to the donor phosphorus atoms in the ligands cannot be accounted for simply by their p-acidities as originally suggested. 2 However, an analysis that allows for there being an effect simply related to the number of pendant oxygen atoms on the phosphorus atoms, in addition to the effect due to their p-acidity, is very successful, although a steric threshold at 130 • also has to be incorporated into the analysis.…”

“…A detailed QALE analysis 1 of the dependence of the energies assigned to the r→r* transitions 2,3 in bis-axial Mn 2 (CO) 8 L 2 complexes on the nature of the alkyl or aryl phosphine ligands (L) showed 3 that there was a clear and substantial effect ascribable to the presence of aryl groups 4 on the P-donor atoms in the ligands. The original data 2 showed clearly that phosphites and phosphinites considerably enhanced the energies of these transitions, and this was loosely ascribed to p-acidity effects.…”

The tuning of P-donor ligands: the aryl and other pendent group effects (PGEs) revisited

“…A given property will therefore be governed by eqn (7), as opposed to eqn (8) For a given P-donor the difference between the two approaches is therefore quantified by eqn (9) (i.e. ( 8) -( 7)) or, more generally, by eqn (10).…”

“…9 Other major effects of pendent aryl groups have been observed. 10 However, G & P later proposed another method of quantifying the aryl effect. 11 The new values were not simple integers, several of the ligands PPh n R 3-n with n = 1 or 2 having apparent E ar values significantly different from 1 or 2.…”

Multi-mechanism linear free energy relationships and isoequilibrium or isokinetic temperatures

“…there was a so-called 'aryl effect'. 39,40 This led to the introduction of another term into such MMLFERs, e.g. dE ar where E ar 40 was equal (or close) to the number of pendant aryl groups on the phosphorus atom.…”