Complex bridging phosphanide

The respective phosphorus resonances are dramatically different with the chemical shifts in tra i-[ CpM(CO) 2( x-PRR )2] shielded by about AS= lOOppm, compared to ctT-[ CpM(CO) 2( J,-PRR )2]- There is no reason why the relative positions of the coligands to the phosphanides should result in such a large chemical shift difference, especially as there is no trani-influence in tetrahedral complexes. Furthermore, the region of 0-100 ppm is the region where we would have to expect the resonance of a bridging phosphanide ligand in the absence of an M-M bond. Indeed, the observed downfield shift from tra i-[ CpM(CO) 2( d-PRR )2] to c(i-[ CpM(CO) 2( J,-PRR )2] is suggestive of the absence of M-M bonds in the trani-isomer. 

Fig. 7.12 Comparison of a cyclic and an acyclic complex with a bridging phosphanide ligand...

Fig. 7.38 Effect of agostic P-H—M interactions on the phosphoms chemical shift in some platinum ethylene complexes with bridging phosphanide ligands...

The role of the phosphorus lone pair is nicely illustrated in the example shown in Fig. 7.26. The bridging, cationic phosphanide complex on the left has an upheld shifted phosphorus resonance at Ap=-151.3ppm (L=CO) and -130.4ppm (L=PMe3), respectively. 

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