Tolman Angle

To further illustrate the steric effects induced by these tertiary Group 15 ligands, we have calculated the Tolman angles 6j, using 2.28 A for standardised Ni-P bonds) and the effective cone angles (0g, using true bond distances). 

Keywords Reactive Silicone / Metathesis / Carbene Complexes / Tolman Angle... 

The steric bulk of trisubstituted phosphines can be conveniently assessed by means of the Tolman angle [11]. From this parameter, some conclusions can be drawn about the regiodiscriminating abUities of the relevant hydroformylation catalyst. Attention should be given to the subtle balance between a desired effect... 

The Pt—P (and Pt—Cl) bond lengths correlate with the electron-donating ability of the phosphine (Tolman s Xi factor) rather than steric factors (the cone angle of the tertiary phosphine) [150b]. 

Tmta — see Acetic acid, trimethylenediaminetetra-Tollen s reagent, 5, 780 Tolman s cone angle, 2, 1015 Toluene acetoxylation... 

My question to Dr. J. Kochi is whether it is possible to correlate the steric factor in his equations describing the oxidation of alkylmetal compounds to some measure of the bulkiness of the alkyl groups such as cone angles similar to ones suggested by Dr. C.A. Tolman for tertiary phosphine ligands. 

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. 

As the size of R3P increases so does the value of k. A popular method of assessing the size of R3P is by using Tolman s cone angle. This is the apex angle of the cone centered on P which just encloses the van der Waals radii of the outermost atoms of R3P. The data are shown in Table 2.3 for (2.134) in CO-saturated C2CI4 at 70°C (Ref. 124)... 

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. 

Fig. 15.46 Device For determining cone angles. [From Tolman. C. A. Chem. Rev. 1977. 77. 3I3-34S. Used wuh permission.)...

Definitions of Cone Angle Based on Models or on X-Ray Data 14.3.6.1 Tolman s cone angle... 

For more complex and bulky ligands an accurate geometrical construction was used for measurement (22a) to estimate a value for rs and / s, where s refers to the complex substituent and r,R the sphere enclosing the complex substituent, making it equivalent to the atom B in (21). This procedure yields values for phosphorus cone angles systematically 5 higher than that of Tolman,187 undoubtedly because of the Ni—P distance of 2.23 A chosen by Imyanitov. 

Figure 6 Tolman s cone angle used in mononuclear complexes with the plane of coverage .

Several workers have pointed out that there is a large and growing body of X-ray structural data of transition metal phosphine complexes. Such data can provide real cone angles, which may then be compared to Tolman s cone angles derived from models. Of course this line only applies to crystalline solids. Since the main application of transition metal chemistry is homogeneous catalysis, crystal structure data, though useful, are of limited applicability. Inevitably, cone angles in complexes in solution will be more variable than in crystalline systems. 

Immirzi and Musco209 have proposed a modification of Tolman s definition of cone angle to account for the ability of ligands to intermesh when complexed, e.g. tricyclohexylphosphine in... 

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