Tin II Phosphandiides

In contrast to magnesium phosphandiides, analogous tin(II) derivatives possess more covalent metal-phosphorus bonds. This basic difference is also apparent for dilithium versus dicopper(I) phosphandiides (see Section II and III). It is, therefore, interesting to assess the structural and electronic features of such species in a similar way. To date, only three tin(II) phosphandiide derivatives have been prepared 

The Sn6P6 cages 19c and 19d are accessible by two different Bronsted acid-base reaction pathways Reaction of lc and Id, respectively, with two different stannanediyl derivatives furnished in 80-89% yield red-black crystals of the aggregates (Eq. 12) (39). The tin(II) phosphandiides are somewhat related to the previously described oligomeric bis (phosphaneyl) stannanediyls of the type PkSn, which easily form intermolecular aggregates (50, 51) or remain monomeric, if the phosphorus atoms bear very crowded organosilyl substituents (52). 

The transient (RPSn)221, which possibly represents a reactive intermediate during the synthesis of the hexameric Sn6P6 clusters 19c and 19d, has been trapped by the transformation of 20d in the presence of 2,3-dimethylbuta-l,3-diene, which solely gave 19d and the corresponding lff-l,l,-dichlorostannol (Eq. 14). 

The first Sn4P4 cube 22f has been prepared in an analogous procedure, starting from the very bulky silylphosphane and Lappert s stan-nanediyl (Eq. 15) (49), which has been isolated in the form of dark red crystals in 95% yield. 

DFT-IGLO calculations of the 31P chemical shift without spin-orbital corrections yielded a value of S = 274, whereas additional spin- 

The unusual Sn3P2Cl2 cluster 20d has been isolated in 44% yield in the form of yellow crystals (Eq. 13) (39), simply by the same reaction of Id with the stannanediyls but in the presence of SnCl2. 20d represents formally an adduct of the cyclic bis(stannanediyl) phosphandi-ide 21 and SnCl2, which coordinate to each other upon their complementary Lewis acid/Lewis base sites. 

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The tin(II) phosphandiides 19c, 19d, and 22f show, with exception of the SnCU-adduct 20d, unusual high-field positions of the NMR singlet signals (Table II). However, the relatively small CP, Sn) coupling constants also reflect considerable electronic features. DFT calculations of the parent compound HeSngPe revealed that spin-orbital coupling has a strong influence on the P chemical shift (39). 

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