Bridging phosphinidenes

Fig. 7.21 P chemical shift values of a series of complexes with a bridging phosphinidene ligand...

As we have just learned, the attempt to synthesize a bridging phosphinidene complex without zr-donor contribution from the phosphorus atom lone pair failed for the W/Fe couple due to the ligand lability of the tungsten fragment. The... 

Note A bridging phosphinidene possesses only one lone pair. It can therefore only contribute two electrons through a n-donor interaction. This electron shortage can lead to the formation of metal-metal bonds and thus three-membered metalla-cycles with a significant ring contribution, resulting in a large downfield shift. 

The difference in the phosphorus resonance for a terminal and a bridging phosphinidene complex can be seen in a series of zirconium complexes shown in Figs. 7.29 and 7.30. 

Fig. 7.30 Bridging phosphinidenes - influence of ring size on the phosphorus resonance...

R1 and R2 are weak donors a radical mechanism dominates whereas, if they are strong donors, the carbenes 30 are formed by way of a zwitterionic intermediate26. In some cases such as 31 the fate of the carbene is uncertain and FVP at 600 °C gave dimethyl phthalate as the only isolable product27. Retro-Diels-Alder reaction of the phosphole dimers 32 at 400 °C gives mainly the phospholes 33 in over 90% yield but a minor product is the dihydrophosphindole 34, suggesting at least some extrusion of the bridge in the form of the phosphinidene R1 P 2X. Finally, it should be noted that the retro-Diels-Alder reaction... 

Phosphine sulfides, with trinuclear Ru clusters, 6, 748 Phosphinidene-bridged ditantalum(IV) complexes, preparation, 5, 183 Phosphinidenes... 

The phosphinidene-bridged system 140 was obtained by reaction of W(CPh)( / -CjH5)(CO)2 with the tungsten phosphine complex shown in [Eq. (127) (150). CuCl-induced elimination of the substituted benzene... 

Transition metal phosphinidene complexes were originally prepared in order to access what was expected to be a rich chemistry of phosphorus(I)T However, terminal phosphinidenes have been found to be difficult to prepare, partly because they tend to either catenate or bridge. This is a manifestation of the double-bond rule for the main group elements. However, breakthroughs in syntheses of these complexes came about through the use of devious synthetic techniques in combination with sterically encumbered ligands. 

Heating toluene solutions of 25-27 to 70°C in the presence of Pd/C resulted in dehydrogenation and formed neutral dinuclear phosphinidene-bridged zirconocene(iv) complexes (Scheme 12). In the case of 27, dehydrogenation proceeded in refluxing toluene without a catalyst. 

The bridging zirconium phosphinidene complexes in Fig. 7.30 are expected to resonate upheld from the terminal ones in Fig. 7.29, since terminal phosphinidene complexes are far better suited for. -donor interactions of the phosphorus atom towards the metal than are the bridging ligands. 

Rather fewer bridged phosphide complexes of type (8.54b) have been prepared, and known phos-phinidene complexes of type (8.54c) are even fewer in number. Phosphide groups function as p-type ligands, whereas in phosphinidene complexes the P atom may be linked to various numbers of metal atoms although it is usually 3 or 4. 

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