Pentacoordinate compounds

Today it is widely accepted that fivefold coordinated silicon plays a key role in the reaction mechanisms of the nucleophilic substitution having a trigonal bipyramidal transition state species which ressemble these transition states can be isolated in some special cases. The structural features fit well to kinetic data and possibly explain the significantly higher reactivity (proved by experimental data) of Si-pentacoordinated compounds compared to their tetracoordinated analoga. 

These percentages were determined using 29Si and 13C NMR spectroscopies, the highest rates of Si-O bond formation being observed with X = OTf, whereas chlorosilanes rather led to pentacoordinated compounds. 

Treatment of triphenyllead chloride with the lithium compound 7 afforded first the intramolecular [4 + 2]-coordinated tetraarylplumbane 8 as colorless crystals in 70% yield. Repeated treatment of 8 with triphenyllead chloride finally furnished the pentacoordinated compound 9 in the form of its triphenyllead chloride adduct 10 in 69% yield, also as colorless crystals. The compounds 8 and 10 were characterized by a complete NMR dataset, and by X-ray crystallography (Scheme l).23... 

This behaviour is not restricted to triply- or quadruply- connected compounds as is seen in the pentacoordinated compound (40) (1967,76) but experimental data on this latter type of compound are rather scarce. 

A good illustration of these concepts is provided by the three spirophosphoranes A, B and C in Figure 11. Since all of them are apt to be involved in a P(III) P(V) equilibrium, the proportion of P(III) can be taken as a probe of the stability of the pentacoordinated compound. It was thus found85 that C, with three different substituents in the e subset, is the one for which the proportion of P(III) is highest. 

With the exception of nitrogen, all of the Group Vb elements are expected to form pentacoordinate compounds in their 5+ oxidation state, and this is, indeed, the case with some of the halides, alkoxides, etc. It was not until the pioneering work of Georg Wittig and his collaborators, however, that the first examples of pentaorganyls of these borderline elements between metals and metalloids were reported (95-99, 102, 104). In this early investigation, a complete set of the pentaphenyls could be obtained and characterized (95-99, 102), but apart from the pentamethyl-antimony case, all attempts for the preparation of pentaalkyl derivatives failed (104). 

TABLE 37. Some stmctural parameters for selected germanium pentacoordinate compounds containing a DaxGeCiXeqYax moiety with monoanionic X,D-chelating ligand... 

Pentacoordinate compounds of antimony and bismuth are becoming more prominent as illustrated by the work of Sharutin et al. For example, penta-arylantimony (lOab) and pentaphenylbismuth (16) ary late organotin halides (e.g. RsSnCl (11) and R2SnBr2 3)) to form aryltin derivatives RaSnAr (12), R2SnArX (14) or R2SnAr2 (15), in 78-95% yields. ... 

A remarkable 6/s-phosphorane (25) containing two h)rpervalent centres each with five P-C bonds has been synthesised by Furukawa et.al. by the reaction of dilithioacetylene (24) with two moles of the phosphonium salt (23). Phosphorane (27) may be prepared by an analogous method from (26) and both pentacoordinate compounds react readily with water to form phenylacetylene. 

If 2,3-TNPCoCl is reacted with an alkali metal cyanide, the product is not the bridged compound [2,3-TNPCo(CN)] but the pentacoordinated monomer 2,3-TNPCoCN. The same product is obtained by in situ oxidation of 2,3-TNPCo with oxygen in the presence of excess cyanide [14]. Similarly taaCo forms the pentacoordinated compound taaCoCN as shown by its X-ray crystal structure [126]. 

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