Pentacoordinated hypervalent compounds

Hypervalent compounds with coordinative phosphorus —> tin bonds are rare. An intermolecular P —> Sn coordination in Me2ClSnCH2CH2PPh2 and its bromo-substituted analogue was suggested based on NMR and 119Sn Mossbauer spectroscopic data201,202. The X-ray crystal structure analyses showed pentacoordinate tin atoms with the phosphorus and the chlorine atoms located in axial positions203. The P —> Sn distance amounts to... 

Pentacoordinated (lO-B-5) and hexacoordinated (12-B-6) boron compounds are available from the reaction of BCI3 with the dilithio salt of the bis addition product of HFA and pyridine (265). These hypervalent compounds show signals in the B NMR spectrum at very high field (173a). 

The development of the hypervalency concept was discussed in general. This model was concerned with the group 14 elements . The ability of the tin atom to expand the coordination sphere in organotin compounds was established in the early 1960s, when an oligomeric structure was suggested for trimethyltin fluoride and trimethyltin carboxylates" on the basis of IR data. It should be pointed out that the trimethyltin chloride-pyridine adduct was the first pentacoordinate organotin compound structurally characterized by X-ray diffraction in 1963". ... 

Hypervalent compounds of organic germanium, tin and lead derivatives 989 TABLE 8. X-ray data for pentacoordinate anions R3SnHal2 ... 

Attempts have been made to model hypervalent, pentacoordinate phosphorous compounds. The first, by Holmes et al., was accomplished by adding a 1,3 van der Waals energy term around the phosphorus atom to the terms already present in This 1,3 term is composed of exponential and... 

Summary Our investigations on silicon compounds of etfaylene-MiV"-bi (2 -hydroxyacetophenoneiminate) led to the synthesis and X-ray structure analysis of a new kind of salen complex — hypervalent silicon compounds with a threefold deprotonated salen ligand and an enamine structure. This stmctural unit provides access to new routes for synthesizing hypercoordinate silicon complexes. Addition reactions between various Brpnsted acids and these new pentacoordinate silicon compounds were carried out to precipitate complexes bearing hexacoordinate silicon atoms. 

More recendy, Akiba and coworkers have synthesized pentacoordinated hypervalent boron heterocyclic species 4—6 by employing a sterically rigid anthracene skeleton (Scheme 1) (2000ACE4055,2005JA4354). Compounds 6—8 were prepared by lithiation of bromoanthracene 4 followed by the addition of E-chlorocatecholateborane derivatives 5. Products 6—8 were... 

In a more recent work, Kawashima reported the preparation and structural studies of a series of four-membered lO-S-4 and lO-S-5 heterocycles 51—54 (Figure 11) (2011PSS1046). These compounds containing a tetra-coordinated or pentacoordinated hypervalent sulfur atom together with other heteroatoms were isolated as thermally stable products and structurally... 

There has been an interest in the existence of hypervalent pentacoordinate boron compounds for some time. In part this interest stems from the possibility that these could be akin to transition states in SN2-type reactions at boron atoms, for example the reaction of the complex BHs O with NMcs. The synthesis has recently been reported of a tridentate anthracene Hgand, l,8-dimethoxy-9-bro-moanthracene, which has been used in the formation of l,8-dimethoxy-9-borylanthracene, whose X-ray structure is reported. This represents the first fully characterised hypervalent pentacoordinate boron compound. 

A new direction in the chemistry of hypervalent silicon compounds has recently been developed by Tacke and coworkers the chemistry of zwitterionic organosilicates28-31. In these compounds the silicon is formally negative, with a cationic nitrogen attached to it by an alkyl chain. One of the groups of products of this general class is the pentacoordinate zwitterionic fluorosilicates31. 

Neutral compounds of heavy group 14 elements having donor atoms in monodentate ligands can be tetrahedral in the solution. However, in this case the environment for the element in the solid state can be five-coordinate due to hypervalent bridges. Selfassociation can be also observed for pentacoordinate species when the donor atom forms a unidentate bridge across the six-coordinate centers. The self-association is also characteristic of anionic species (Sections IV.A and IX.A). For a tin atom the intermolecular coordination is observed very frequently . As a rule it results in a polymeric structure mainly via the oxygen atom . However, dimers and other simple cyclic hypervalent structures are formed only seldom. 

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