Phosphides Terminal and Bridging

we are faced with phosphorus resonances that are frequently very far downheld indeed. Most of these phosphorus atoms are subject to very large shift anisotropies, making accurate predictions and simple explanations difficult. However we are not concerned with accurate predictions. We are looking for an approximate chemical shift value and the best advice for the practitioner in this context is to ask the NMR operator to open the window a few hundert ppm further to the left. 

As the structures have aU been assigned a metal-metal bond by the original authors and thus possess a three-membered M P metallacycle, we would expect the chemical shifts to be in the downfield region at around S = 150-300ppm for 

Note The [ CpM (jU-PRR ) (/U-CO)] series has phosphorus resonances that are deshielded by AS= 15-45ppm, compared to the respective values in the l CpM(CO)j2(/a-PRR )J series. This is attributed to the higher metal-metal bond order in the former complexes. 

The case of [ CpM(CO) j( j,-PRR )j] is not that straightforward. There are two isomers, one in which both Cp-ligands are on one side of the M-M vector and both 

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. 

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In a similar vein, it was shown that molybdenum amido derivatives of the isolobal terminal phosphide and carbide, " and related chalcogenide atoms,could be obtained. Detailed mechanistic studies of the Mo N(R)Ar 3/N2 system involving X-ray, EXAFS, magnetic, Raman, and isotopicaUy labelled NMR spectroscopy showed that the reaction proceeded through an intermediate involving an end-on bound N2 bridging the molybdenum centres (Scheme... 

The appearance of a bridging phosphide is seen downfield from a terminal one, and compounds with well defined P=M double bonds exhibit chemical shift values that are very significantly downfield. An example is [Cp2Mo=PMes ] at < p=799.5ppm. 

The tripalladium mixed-valence Pd3(2/3) and Pd3(4/3), Pd(II) and Pd(0) clusters may be formed with a range of phosphines, isocyanides, or carbonyls as terminal ligands and carbonyls, nitrosyls, isocyanides, sulfur dioxide, phosphides, phosphines or diphosphines, halides, thiolates as doubly or triply bridging ligands. These clusters exhibit a wide range of steric and electronic environments. 

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