Phosphine electronic parameter

Figure 19 Enthalpy of reaction vs phosphine electronic parameter for (COD)PtMe2-I-2PX3 (circles) P(alkyl)3 (slope = 234 R = 0.932) (squares) PMes- Ph (diamonds) P(/2-XC6H4)3 (slope =—0.1S9 R = 0.769) (crosses) PPh3 j (pyrl)j . Carbonyl stretching frequency (cm ) vs enthalpies of reaction (kcalmoD ) for the RhCl(CO)(PR3)2 system (slope = 5.47 R = 0.96)...

Figure 19 Enthalpy of reaction vs phosphine electronic parameter for (COD)PtMe2 + 2PX3 (circles) P(alkyl)3 slope =...

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). 

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. 

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). 

The most basic phosphine considered by Tolman was PBu this produced an A, frequency of 2056.1cm-1 and was made the arbitrary standard for the electronic parameter (Table 12). The electronic parameter, v, and the steric cone angle parameter, d, when plotted together produce a steric and electronic map (Figure 16). 

Minten212 has related the equilibrium constant, KQl, to the cone angle, 6, and the Strohmeier electronic parameter, v, as shown in Figure 17 for the Mn(phosphine)Br2 complexes in toluene at... 

Rates of oxidation of para-substituted arylphosphines with singlet oxygen show good correlation with the Hammett a parameter (p = —1.53) and with the Tolman electronic parameter. The only products are the corresponding phosphine oxides. However, for ortho-substituted phosphines with electron-donating substituents, there are two products, namely a phosphinate formed by intramolecular insertion and phosphine oxide. Kinetic analyses demonstrated that both products are formed from the same intermediate, a phosphadioxirane. VT NMR experiments showed that perox-idic intermediates can only be detected for highly hindered and very electron-rich arylphosphines 243... 

Figure 8 Enthalpy of phosphine exchange reaction (kcal mol ) vs Tolman electronic parameter (cm ) for fra 5-(PR3)2Cl2Ru=CH-CH=CPh2 complexes slope = 1.93 R = 0.98)...

Tolman introduced the cone angle 0 and the electronic parameter to classify phosphine ligands with respect to their steric demand and coordination ability... 

Figure 4 Nolan and co-workers correlation of average values for [(L)lr(CO) CI] complexes with the Tolman electronic parameter (TEP)d° (m) Experimental values for phosphines ( ) experimental values for NEfCs and (a) values obtained by linear regression. (See insert for color/color representation of this figure)...

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