Phosphine and related ligands

Bonding between Phosphines and Transition Metals (traditional view) 

35The IUPAC recommends that these compounds be called phosphanes, but the name phosphine is well established and commonly used. 

Similar interactions occur in other phosphorus-containing ligands and related ligands containing arsenic and antimony. Names and formulas of some of these ligands are given in Table 6-4. 

39Values of cone angles have been revised on the basis of analysis of crystal structure information in the Cambridge Structural Database. For a review of these results and other 

As might be expected, the presence of bulky ligands can lead to more rapid ligand dissociation as a consequence of crowding around the metal. For example, the rate of the reaction 

We have already encountered complexes 2.15 and 2.18, where PMe Ph and PPhj are present as ligands. Structures 2.28 and 2.29 are complexes that have been used as catalysts in Ni-based hydrocyanation and Pd-based cross-coupling reactions, respectively (see Sections 5.6.1 and 7.4.1). Notice that 2.29 is a two coordinate complex. 

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. 

The cone is created by the surface that encloses all the Ugand atoms for all orientations resulting from the rotation around the metal-phosphorus bond. The cone angles of phosphines and phosphites can span a very wide range, e.g., the cone angles of PMe Ph, PPhj, and P(Bu )2Ph are about 120°, 150°, and 170°, respectively. 

The electronic contribution by a phosphine was originally measured by IR spectroscopy. The vibrational frequencies of [Ni(CO)3(PR3)] for a number of complexes differing in R were measured. The stretching frequency of the carbonyl is inversely related to the extent of back donation. Since both CO and PRj compete for back donation from the metal, the stretching frequency of the carbonyl is an approximate measure of the donor-acceptor properties of PRj. 

As shown in structure 231, bite angle is defined as the preferred chelation angle determined only by ligand backbone constraints. In other words, bite angle is assumed to be solely based on steric factors, and any electronic preference imposed by the metal center is ignored. The natural bite angles of the ligands dppe and BISBI are about 85° and 113°, respectively. 

Tertiary phosphines, PR3, are important because they constitute one of the few series of ligands in which electronic and steric properties can be altered in a systematic and predictable way over a very wide range by varying R. They also stabilize an exceptionally wide variety of ligands of interest to the organometal-lic chemist as their phosphine complexes (R3P) M-L. Phosphines are more commonly spectator than actor ligands. 

Like NH3, phosphines have a lone pair on the central atom that can be donated to a metal. Unlike NH3, they are also n acids, to an extent that depends on the nature of the R groups present on the PR3 ligand. For alkyl phosphines, the JT acidity is weak aryl, dialkylamino, and alkoxy groups are successively 

FIGURE 4.3 Empty P—R a orbital plays the role of acceptor in metal complexes of PRj. As the atom attached to phosphorus becomes more electronegative, the empty P-X a orbital becomes more stable and so moves to lower energy and becomes a better acceptor from the metal. Shading represents orbital occupation. 

P(NR2)3 is a better donor than it should be based on the argument of Fig. 4.3, probably because the basic N lone pairs compete with the metal d orbitals in donating to PR a.  

FIGURE 4.4 Electronic and steric effects of common P-donor ligands plotted on a map according to Tolman (v in cm , 0 in degrees). Reptx duced from Ref. 26 with permission of the American Chemical Society.) 

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Similar surface structures should exist in large clusters, beginning with a two-layer 55 atom cuboctahedron. The question arises, whether or not the best coverage of a metal surface can be realized by many small ligands or by fewer bulky ligands, such as phosphines. In contrast to CO, phosphines and related ligands can easily be dosed, in order to prevent formation of smaller clusters. 

G. Complexes with Monodentate Phosphines and Related Ligands... 

Another area of interest has included modifying phosphine and related ligands so that reactions can be conducted in non-traditional media, especially those solvents that play a critical role in green chemistry . A number of excellent and comprehensive reviews are now available that provide a detailed summary of work in this area . ... 

Complexes with Dioximes, Schiff Bases, and Other Nitrogen Ligands Complexes with Monodentate Phosphines and Related Ligands Complexes with Bidentate Phosphine, Arsine, and Related Ligands Complexes with Sulfur Ligands Nitrosyl and Thionitrosyl Complexes VII. Technetium(IV)... 

Complex hydrides with tertiary phosphines and related ligands... 

In addition to pioneering the cone angle concept, C. A. Tohnan proposed a parameter X (chi) as a measure of the electronic effect of phosphine and related ligands, based on infrared spectra of complexes containing these ligands (C. A. Tohnan, J. Am. Chem. Soc., 1970, 92, 2953). 

Functionalized Tertiary Phosphines and Related Ligands in Organometallic Coordination Chemistry and Catalysis... 

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