Steric parameters cone angle

Many of the properties of metal complexes with monodentate PRj ligands can be rationalized in terms of its steric and electronic contributions. A quantitative estimation of the steric demand of PRj can be made in terms of its cone angle, a parameter originally proposed by Tolman. As shown in structure 230, it is the angle of an imaginary cone with its vertex at the metal atom and a fixed average metal-phosphorus distance. 

Unfortunately, for all these reasons the conclusions cannot be applied quantitatively for description of the pH effects in the RCH-RP process. There are gross differences between the parameters of the measurements in [97] and those of the industrial process (temperature, partial pressure of H2, absence or presence of CO), furthermore the industrial catalyst is preformed from rhodium acetate rather than chloride. Although there is no big difference in the steric bulk of TPPTS and TPPMS [98], at least not on the basis of their respective Tolman cone angles, noticable differences in the thermodynamic stability of their complexes may still arise from the slight alterations in steric and electronic parameters of these two ligands being unequally sulfonated. Nevertheless, the laws of thermodynamics should be obeyed and equilibria like (4.2) should contribute to the pH-effects in the industrial process, too. 

Strohmeier showed that the IR carbonyl frequencies of metal complexes could be used as a measure of the electronic properties of the ligands [8]. Tolman introduced a systematic approach to describe electronic and steric ligand effects [9]. The electronic parameter / is based on the difference in the IR frequencies of Ni(CO)3L and the reference compound Ni(CO)3(P Bu3), similar to the method introduced by Strohmeier. For phosphorus ligands the cone angle 6 is defined as the apex angle of a cylindrical cone, centered at 2.28 A from the center of the P atom, which touches the outermost atoms of the model. 

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). 

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

A series of monocarbonyl complexes were tested with phenyl groups on the diamine and various substituents on the phosphorus atoms (complexes 64-72). The electronic effects of the ligand can be quantified by use of the carbonyl stretching wavenumber of the complex while the steric effects can be expressed by the Tolman cone angle for groups around the phosphorus atoms. Figure 8 shows a plot of these two parameters versus the TOF for the ATH of acetophenone in isopropanol at 30 °C catalyzed by complexes 64-72 (0.02 mol%) which have been activated by KOtBu (0.16 mol%). 

Figure 8 The plot of catalyst activity for the ATH of acetophenone (TOF, h ) catalyzed by complexes 64-72 (0.02 mol% 0.2 mol% KOtBu) at 30 °C versus CO stretching frequency, as an electronic parameter, and Tolman cone angle, as a steric parameter. Reproduced from with permission from the American Chemical Society. (See insert for color/color representation of this figure)...

Certain reaction conditions and properties of the nickel complexes promote hydrocyanation. The oxidative addition of HCN requires an open coordination site. Catalysts containing P(0-o-Tol)3 as ligand are particularly reactive because the imsatu-rated L Ni complex is the most stable form of the Ni(0) complex, whereas the saturated 18-electron L Ni complex is the most stable form of the catalysts containing smaller phosphite ligands. The need to imderstand the steric and electronic properties of the ligand on the dissociation of phosphine led to the classic work of Tolman on cone angles and electronic parameters. ... 

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