Pentacoordinated complex formation

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-... 

As can be seen from the data presented, the high energies of complex formation decrease sharply the endothermicity of the retro-Wittig type decomposition and, moreover, fundamentally change the reaction mechanism. As has been shown for betaines (")X-E14Me2-CH2-E15( + )Me3 (X = S, Se E14 = Si, Ge E14 = P, As), the reaction occurs as bimolecular nucleophilic substitution at the E14 atom. For silicon betaines, the transition states TS-b-pyr with pentacoordinate silicon and nearby them no deep local minima corresponding to the C-b complexes can be localized in the reaction coordinate. 

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. 

At the same time, according to x-ray data for zinc chelate 909, the nitrogen atom is turned to the side of the metal. The distance Npy-Zn is 2.80 A, that allows us to consider the possible participation of the examined donor center in binding with the metal, leading to formation of a hexacoordinated structure (two-capped tetrahedron) [243]. In relation with this result, let s pay attention to the data reported in Refs. 244 and 248. The tetrahedral configuration without coordination of the nitrogen atom of pyridine is attributed to the cobalt complex 907 (X = NTs, M — Co), although this N atom is rotated to the side of the metal [244]. The pentacoordinated complex 910 is described in Ref. 248, in which only one pyridine substituent is coordinated (the distance Npy-Co is 2.45 A) ... 

Immediately upon contact with air, the high-spin complex (g 6.1) attributable to Fe(III) TPP py (pentacoordinated complex) is observed together with the free radicals whose g values are 1.99-2.04 and the low-spin Fe(III) complex (see Figure 4). The formation of a pair of low-spin complexes attributable to the hexacoordinated Fe(III) TPP complexes (g = 2.66, 2.19, 1.80 and g = 2.31, 1.93) occurs with time. Of those, one should be the Fe(III) TPP py2 and another the product-coordinated Fe(III)-TPP complex. The appearance of ESR absorptions attributable to Fe(III) complexes (high-and low-spin) indicates that a part of the Fe(II)-TPP complex is converted to Fe(III) TPP by reaction with molecular oxygen. 

For a better understanding of the nature of solvated silylium ions in nucleophilic solvents the properties of R3Si(S)+ complexes were investigated for solvent prototypes S such as H2O, NH3 and HC1. [41] The possible formation of both tetra- and pentacoordinated complexes was considered. The two S molecules in pentacoordinated complexes were held at equal distances from the Si-atom in order to describe a situation in which the S molecules are part of a solvent shell... 

Overall, the formation of pentacoordinated complexes is favored by chelating and particularly by polydentate macrocyclic ligands. A search in the Cambridge Structural Database reveals that many of the structurally characterized five-coordinated Pd(II) compounds contain macrocyclic or rigid polydentate ligands, S, N, and P donor atoms being dominant. Complexes (23), (24) and (25) are typical examples. 

The contribution of complex formation to MCM copolymerization is more substantial than it is to homopolymerization. The coordinational unsaturation of central metal atoms, for example the pentacoordination state of Sn(IV), plays a definite part. The transfer of an electron from an MCM (electron-donor monomer) to a multiple bond of the comonomer is comparatively easily carried out in the transition state [112] as shown in Eq. 4-36, for example for maleic aldehyde the complex formation constant. 1= 0.17 0.002 L mol". ... 

Platinum(II) 18e pentacoordinate complexes are less stable than square-planar compounds due to a relatively great difference between 6p and 6s orbital energies (see Chapter 1 and discussion of stability of Ni, Pd, and Pt carbonyls). The stable complex [Pt(CN) H(PEt3)2 (TCNE)] possessing CN and H ligands which are in trans position to each other was isolated.Addition of TCNE to fm 5-[PtX(H)(PR3)2] leads to reductive elimination of HX where X = C1 or Br and to the formation of the Pt(0) complex Pt(TCNE)(PR3)2. Unstable olefin compounds possessing trigonal-bipyramidal... 

Although the nature of bonding in pentacoordinated molecules at present is little understood, certain gross structural features recently have revealed themselves in the phosphorus(V)chlorofluorides, Results of studies involving nuclear quadrupole resonance spectroscopy (1), dipole moment measurements (1), IR- and Raman spectroscopy (2), NMR(3), and calorimetric measurements of complex formation (4) coupled with related information on the series POClj F3.j and PClxF3 x have provided the necessary information to allow an initial correlative interpretation concerning stereochemical aspects and the nature of the nonequivalence of bonds in these molecules. 

Donating solvents such as hexamethylphos-phoramide or dimethylpropylurea give rise to upheld shifts which have been interpreted as caused by the formation of pentacoordinated complexes. The solvents shifts seem to follow Gutmann s donor number for solvents. 

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