Chemical isomer shift absence

The only real exception to this is dicyclopentadienyltin(II) [199], Cp2Sn, which has a chemical isomer shift of 3-73 mm and a quadmpole splitting of 0-65 mm s" It is, however, unstable at room temperature, even in the absence of air, being converted to polymeric [Cp2Sn] with a shift of 0-72 mm s, i.e. the tin becomes oxidised. 

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. 

The absence of a red shift in the UV absorption spectrum (292 nm) in alkaline medium indicates that the hydroxyl function is in ortho position with respect to the acyl side chain. This is confirmed by the chemical shift (5 14.61) of the enol signal at low field in the H NMR spectrum (65). Conclusive evidence of structure 263, implying exclusion of the isomer in which the dihydrofuran ring and the dihydropyran ring are interchanged, is provided by the conversion of the oxidized product 223 (Rg. 93, see 13.1.4.) to compound 263 in acidic medium. 

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