Tolman’s electronic parameter

The preparation of carbonyl-lr—NHC complexes (Scheme 3.1) and the study of their average CO-stretching frequencies [7], have provided some of the earliest experimental information on the electron-donor power of NHCs, quantified in terms of Tolman s electronic parameter [8]. The same method was later used to assess the electronic effects in a family of sterically demanding and rigid N-heterocyclic carbenes derived from bis-oxazolines [9]. The high electron-donor power of NHCs should favor oxidative addition involving the C—H bonds of their N-substituents, particularly because these substituents project towards the metal rather than away, as in phosphines. Indeed, NHCs have produced a number of unusual cyclometallation processes, some of which have led to electron-deficient... 

The El Lever parameter was also shown [71] to correlate linearly with other parameters that measure the net electron-donor character of a ligand (L), namely the Tolman s electronic parameter TEP) [81] for phosphines and a computed electronic parameter CEP) [71] based (as TEP) on the infrared A v CO) frequency in complexes [NiL(CO)3], which is determined by the electronic effect of L. 

Therefore, simple one-parameter relationships can be established with either the carbonyl stretching frequency (vco. Figure 13) or Tolman s electronic parameter (x, Figure 14),... 

Enthalpy values are provided with 95% confidence limits. x is Tolman s electronic parameter. 

Figure 16 Reaction enthalpy vs Tolman s electronic parameter (x) for rhodium systems circles = pPrPNP]RhL diamonds = fra/ 5-RhCl(CO)L2. Letters correspond to the ligands L = PPh3(a), PPhzpyrl (b), PPhpyrL (c), PpyrR (d), and Ppyrlj (e)...

While many workers have used Tolman s steric parameter (the cone angle), not nearly as many have used his electronic parameter. The latter represents the net total of all electronic effects as reflected in the value of the vfCO) vibration (i.e. the contributions of a and n bonding are unresolved) of the LNi(CO)3 (L = phosphine) complex. Tolman further analyzed the contributions of various substituents to v by equation (55). 

For monodentate ligands, e.g., triphenylphosphane, Tolman s cone-angle 0 and the electronic parameter x have a significant influence on the activity and the selectivity of the resulting catalyst system [24,25]. As regards bidentate ligands, which provide two coordination centers for the transition metal, the so-called bite angle fi determines the selectivity of the formed aldehydes. 

Many attempts have been undertaken to define a reliable steric parameter complementary to the electronic parameter. Most often Tolman s parameter 0 (theta) is used. Tolman proposed to measure the steric bulk of a phosphine ligand from CPK models in the following way. From the metal centre, located at a distance of 2.28 A from the phosphorus atom in the appropriate direction, a cone is constructed which embraces all the atoms of the substituents on the phosphorus atom (see Figure 1.6). 

When some other reaction parameter, Z, such as the log of a rate constant, is plotted on to this steric and electronic map on an axis normal to the plane of the paper the comparative contributions of 6 and v should become apparent. A purely steric effect will slope north or south (the reader is encouraged to view Figures 26-28 of ref. 187 to appreciate this fully). Weimann and co-workers211 used Tolman s methodology to show the % steric effect in the oligomerization of butadiene catalyzed by nickel phosphine complexes. 

Figure 8 Enthalpy of phosphine exchange reaction (kcal moC ) vs Tolman electronic parameter (cm ) for tra/ s (PR3)2Cl2Ru=CEI-CEl=CPh2 complexes slope = 1.93 R = 0.98)...

Tolman introduced [27] electronic factors, xij, l or phosphine and phosphite ligands, based upon the totally symmetric CO stretching vibration in a phosphine nickel tricarbonyl derivative. More recently Bartik and co-workers have established this correlation more accurately [28]. These parameters, together with Tolman s cone angle data, have been of great utility in understanding the chemistry and dynamics of phosphine complexes. 

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