Phosphanide bridging

The Ca/Sn-mixed metalated phosphanide-phosphandiide cluster 43 crystallizes in the triclinic space group PI (71). It consists of a trigonal Ca2SnP2 bipyramid, where the phosphandiide P centers serve as ju -bridging centers to the metals. The two phosphanide ligands, however, are. -bridging between two Ca and Ca/Sn centers (Fig. 32). Both Ca ions are octahedrally coordinated. 

Whereas at 253 K only the monomeric molecule is detectable, at 283 K a small signal arises from the dimeric species (Fig. 2). At 313 K coalescence is observed between these two species, llirthermore, an intramolecular exchange reaction of the terminal and the bridging phosphanide ligands of the dimer broadens the singlets. The latter process is fast on the NMR time scale. The spectroscopic data are compared in Table 1. For the barium derivative shown in Fig. 3 the bicyclic structure has been proven by the small Vp p coupling of 6.7 Hz at 213 K which leads to a quartet for... 

The size of the trialkylsilyl groups determines the appearance of the dimeric molecule Thus, the dimethyl-i-propylsilyl substituent enforces a monocyclic dimeric barium bis(phosphanide) with two terminal and two bridging phosphanide ligands (Fig. 4) [4]. 

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...

Substitution of a bridging phosphanide with a bridging hydride in [CpFe-CO ( j,-PPhj)]j (see Fig. 7.13) reintroduces the Fe— Fe bond, accompanied by a dramatic downfield shift of about A<) =200ppm due to the replacement of a four-membered metallacycle with a three-membered metallacycle. 

Table 7.13 chemical shift values in a series of nickel-coordinated bridging secondary phosphanides... 

Fig 7.16 chemical shift values for some related nickel bridging primary and secondary phosphanides... 

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. 

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

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