Pentacoordinate centers

Pentacoordinate centers are stereochemically far more complex than tetracoordinate centers. Idealized geometries for such centers are the trigonal-bipyramidal (XVIII) and tetragonal-pyramidal (XIX) arrangements. 

Pentacoordinate centers are mainly exemplified by phosphorus, whereas some pentacoordinate sulfur derivatives exist, and a few other elements can be envisaged [36], In the case of phosphorus derivatives (phosphoranes), the trigonal-bipyramidal arrangement is the low-energy geometry, while interconversion of isomers occurs by pseudorotation through the tetragonal-pyramidal transition state [23,36]. 

The number of stereoisomers generated by a pentacoordinate center varies with 

In the case of [Ma3b3] complexes, there are again two diastereoisomeric forms (diagrams XXVI). The former has each triplet of identical ligands occupying the 

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The solid-state structure of tetrabutylbis(2,3 4,6-diisopropylidene-2-keto-L-gulonato)distannoxane has been determined. The [Bu2Sn(IV)] complex of D-lactobionic acid also proved to be oligomeric, containing octa- (within the chain) and pentacoordinated (at the end of the chain) Sn centers in a ratio of 2 4. ... 

The symmetry properties of the pentacoordinate stereoisomerizations have been investigated on the Berry processes. They have been analyzed by defining two operators Q and The operator / is the geometrical inversion about the center of the trigonal bipyramid. Since this skeleton has no inversion symmetry, / moves the skeleton into another position. Moreover, if the five ligands are different, it transforms any isomer into its enantiomer, as shown in Fig. 3. 

We should mention the recent progress in Au(I) and Au(III) catalyzed organic reactions [24, 297]. A number of groups of theoretical chemists are currently investigating the reaction mechanisms using quantum chemical methods [298-301]. Li and Mia published DFT calculations on AU5H5X hydrometal pentagons with Dsh planar pentacoordinate nonmetal centers (X = Si, Ge, P, S) [302]. The introduction of the nonmetal centers X introduces p aromaticity to MHX complexes. 

The role of the ligands is both to stabilize the Pd(0) state and to tune the reactivity of the palladium. The outline mechanism above does not specify many detailed aspects of the reaction that are important to understanding the effect of ligands, added salts, and solvents. Moreover, it does not address the stereochemistry, either in terms of the Pd center (tetracoordinate pentacoordinate , cisl, transl) or of the reacting carbon groups (inversion , retention ). Some of these issues are addressed by a more detailed mechanism.190... 

The cyanide exchange on [M(CN)4]2 with M = Pt, Pd, and Ni is a rare case in which mechanistic comparisons between 3d, 4d, and 5d transition-metal complexes. Surprisingly, the behavior of these metal square-planar centers leads to mechanistic diversity involving pentacoordinated species or transition states as well as protonated complexes. The reactivities of these species are strongly pH-dependent, covering 15 orders of magnitude in reaction rates.85... 

According to the 29Si NMR spectral data (vide infra), the silicon center of 36 is assumed to be pentacoordinated due to its intramolecular interaction with both terminal amino groups attached to the substituents. This kind of two-fold coordination of the nucleophilic side arm of the 2-(dimethylaminomethyl)phenyl substituent to... 

Alternatively, unreactive mixtures of organosilicon hydrides and carbonyl compounds react by hydride transfer from the silicon center to the carbon center when certain nucleophilic species with a high affinity for silicon are added to the mixture.76 94 This outcome likely results from the formation of valence-expanded, pentacoordinate hydrosilanide anion reaction intermediates that have stronger hydride-donating capabilities than their tetravalent precursors (Eq. 6).22,95 101... 

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. 

Various hybridization states of the phosphorus atom have been parameterized in various force fields (Table 9). If a force field is to be useful for pentacoordinated phosphoranes, the relative energies of the two basic configurations, trigonal bypyramid (62) and square pyramid (63), must be differentiated. For this purpose, a 1,3-non-bonded interaction term centered only at the central phosphorus atom was added to MMI. In this modification, account was ts en of... 

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