Phosphine ligands, cone angle

Table 3.3 Cone Angles of Tertiary Phosphine Ligands (Cone Angles of Bidentate Ligands [7]. Cone Angles of Ph2p(CH2) PPh2 are Estimated by Assuming PMP Angles of 74, 85,90° for n = 1,2,3, Respectively)...

In order to assess steric and electronic effects on the bond enthalpies of reaction of phosphine and phosphite ligands to platinum(II), the enthalpies (AH) of reaction of these ligands L on reaction with [PtMe(PPhMe2)2(THF)]PF6 have been measured (equation 412). These authors measured the enthalpies of 34 ligands, and correlated the measured enthalpy with the ligand cone angles.1342,1343 Representative data are shown in Table 8. 

Figure 15 Schematic depiction of ligand cone angle for a generic phosphine ligand and representative Tolman cone angle values...

TABLE 14.1 Ligand Cone Angles and Tertiary Phosphines... 

Phosphines and related P(III) compounds typically serve as ancillary ligands, but the dissociation of these ligands is crucial to the reactivity described in later chapters. Tolman correlated the ligand cone angle with the equilibrium for dissociation from NiL compounds.The extent of ligand dissociation in these nickel complexes and in related palladium complexes increases in the order PMej < PMe Ph < PMePlq < PEtj < PPhj < PPr j < PCyj < PPhBu, . 

Table 9-1. Cone angles for a series of phosphine ligands.

Based on the experience with tertiary phosphines, the importance of the steric properties of NHC hgands in determining chemical behaviour has been immediately recognised. The main practical problem, however, is that NHC hgands substantiaUy present a local symmetry axis, whereas phosphines present a local symmetry axis. This imphes that the well-accepted molecular descriptor used to quantify steric properties in phosphines, the Tohnan cone angle [78], cannot be applied to NHC ligands. 

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. 

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