Complexes pentacoordinated

Let us examine the properties of molecules where a central atom M is surrounded by five ligands A, B, C, D, E. We assume that the ligands are at the vertices of a trigonal bipyramid. This assumption is adequate for most pentacoordinate complexes but we ougth to mention that the description of stereoisomerization we propose could be applied if another polytopal form—the tetragonal pyramid, for example—was the stable one. The same type of description has already been undertaken for hexacoordinate octaedral complexes . ... 

Mechanism b) applies if a second solvent molecule is added at the tin atom of this pentacoordinate complex. The influence of the halogen atom can safely be predicated if mechanism a) is operative. The equilibrium constant for the formation of the trigonal bipyramidal complex is not very different for X=C1 and for X=Br31). Furthermore, since Cl( ) is harder than Br( the cleavage of the tin-... 

Tris(2-aminoethyl)amine (tren) and hexamethyl tren form pentacoordinate complexes with a bound water. The enthalpies of ionization have been determined.242 These results dispute the data from the systems that led to the view that solvent structure mediates the pAia of zinc bound water in zinc hydrolytic enzymes. The effect was shown to be entirely enthalpic. 

Hi. Pentacoordinated complexes. The structures of tris(stannyl)amines could not be determined exactly. On the one hand, IR spectroscopic investigations indicate a pyramidal... 

Roewer et al. have used salene-type ligands for the preparation of neutral penta- and hexacoordinated silane complexes.834-836 Interestingly, the reaction of the acid form of the salene ligand reacts with organotrichlorosilanes in the presence of diethylamine to form pentacoordinated enamine silane complexes 849-851, whereas the disodium salt of the salene ligand reacts with phenyltrichlorosilane providing the hexacoordinated chlorosilane complex 852 (Scheme 119).834 The pentacoordinated complex 849 was also obtained when the hexacoordinated complex 852 was reacted with triethylamine (Scheme 119). 

We will examine here exclusively octahedral manganese complexes in that they have been more extensively characterized. It is noted though that there exists a large number of pentacoordinate complexes with trigonal bipyramidal or square pyramidal geometry. 

There are numerous iron(II) pentacoordinate complexes. An example is constituted by [Fe(saldpt)], the N302 coordinated iron(II) complex of the Schiff base bis(salicyldeneaminate-3-propyl)amine, Scheme 9. 

As for iron, cobalt forms pentacoordinate complexes of N3O2 coordination with the already mentioned bis(salicyldeneamine-3-propyl) amine affording the series [Co(R-X-saldpt)], Scheme 12. 

In a recent study, Fernandez-Bertran et al. used mechanochemical reactions to prepare a number of hemin complexes with amino acids such as arginine, histidine, lysine, methionine and tryptophan. The basic amino acids react with the hemin peripheral propionic acid groups, while arginine is also able to form a pentacoordinated complex at the Fe(III) centre. The reactions were followed by IR and Mossbauer spectroscopies [77a]. The solid-state reaction of hemin with KCN, Na2S and various substituted imidazoles has also been investigated [77b]. 

The greater effectiveness of hydrogen peroxide is also shown in the reactions of the pentacoordinated complexes [M(A -L )(cod)] supported by N-donor hgands (L = dpa-R [72,79], Cn [79], PysSs [81]) (Fig. 5). While these complexes are unreactive with molecular oxygen, they react with H2O2 to systematically give the l-hydroxy-2-metalla (5,6,7)-allyl derivatives (Scheme 12) as the thermodynamic products. 

The second water molecule to be dissociated is Wat2, with a reaction enthalpy for 2a — + H2O amounting to 9.2 kcal mol-1. The pentacoordinate complex 3a (Fig. 9), with Wat2 removed, is... 

The fluorine atom, which is the most electronegative group attached to silicon, shows a low capability for forming pentacoordinate complexes. 

Another case of bridging was also reported recently in a neutral poly-silicon complex, and is discussed in the section on neutral pentacoordinate complexes (Section III.C.2). 

A second method for the preparation of neutral pentacoordinate complexes consists of an exchange reaction between chloromethylchlorosilanes (ClCH2SiMe Cl3 , n = 0-2) and N-trimethylsilylated compounds, as shown for example in equation 2989. The compounds of types 96-99 were prepared by this method89-93. The same reagent was used extensively for the synthesis of O Si complexes, and this is discussed in more detail in the corresponding section (III.B.l). 

Nitrogen-coordinated pentacoordinate complexes have been used as stereoselective reducing agents in the preparation of erythro-(meso)- 1,2-diols from diketones and a-hydroxyketones109. The reducing agent was the (l-naphthylamino-8)trihydridosilane 92e. After formation of the dioxo chelate from the diketone (equation 32), the diol was obtained from the pentacoordinate silicon complex by reduction with LiAlILt. 29 Si NMR spectroscopy was used for the product-ratio analysis in this reaction, which was found to yield primarily the erythro diols. 

TABLE 12. Barriers for pseudorotation and Si—N cleavage in neutral pentacoordinate complexes... 

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