Pentacoordinated hypervalent

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... 

Hypervalent Structures with a Pentacoordinated Phosphorus and Two Five-Membered Rings... 

The triphenylmethyl-substituted germanium halide 31 can be prepared in modest yield using GeCl4 and contains a formally seven-coordinate germanium metal center (Scheme 9). The structure of 31 (Figure 2, Table 11) is a tricapped tetrahedral structure where all the three Ge-O distances are shorter than the sum of the van der Waals radii (3.62 A). A series of hypervalent germanium halides 32-34 can be prepared, as shown in Scheme 10.95 The fluoride 32 is hexacoordinate, whereas the chloride 33 and bromide 34 are pentacoordinate. 

In 2006 Terada and Kouchi reported the investigation of phosphonium salts in catalysis [112]. A pentacoordinated phosphorus atom is a hypervalent [113] atom, which has a formal valence shell of more than eight electrons. As shown in Scheme 40, it is possible for the lower lying o orbital of a P -EWG (Electron Withdrawing Group) bond to take up a free electron pair of a Lewis base, in order to form a new bond. If the new formed bond is trans to the EWG, the formed complex is more stable. 

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. 

Apart from fluoro ligands at hypervalent silicon complexes, also oxo and aza ligands are known to support penta- and hexacoordination in silicon compounds1-8. Among the various pentacoordinate oxo complexes, those with bidentate ligands (1,2-diols, aliphatic or aromatic, as well as a-hydroxycarboxylic acids) are most readily prepared, and are... 

The azide Me3PbN3 forms a linear chain polymer with a p2-N atom symmetrically hypervalent bridging the pentacoordinate lead centers (N—Pb 2.54 A). The N—Pb—N angle (178.6°) deviates only slightly from the ideal value of 180°77,78. The carbodiimide Me3SnNCNSnMe3 forms an infinite helical network, too79. 

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... 

In diorganotin adducts D — SnR2X2 the Sn—X bond length differences between axial and equatorial positions in the pentacoordinate species 118 are of particular interest since they conform quite well to expectations of hypervalent D — Sn—X bonding (Section V.B.3). The longer Sn—Clax bond length by ca 0.06 A due to N — Sn donor interaction is consistent with the expected lower s character of the axial bond compared to... 

The Ge atoms in the germocanes (Table 38) are pentacoordinated due to hypervalent D —> Ge—X interaction. The coordination environments of these atoms are distorted TBP with the donor D (D = N, O, S) and the X in axial positions. The intramolecular coordination D —Ge bond belongs to both 5-membered heterocycles. 

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). 

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