Trigonal bipyramidal pentacoordination

In the crystal structure of PCDase, the iron has trigonal bipyramidal pentacoordination with one water ligand. Recent extended X-ray absorption fine structure (EXAFS) studies indicate that the bound water is more... 

The abundance of five-coordinate copper complexes corresponds overwhelmingly to the Cu(ll) ion, since Cu(l) has little tendency to go beyond fonr coordination. Hence, the tripod ligand tris(pyridy]methyl)amine forms complexes fliat are essentially four-coordinate tetrahedral with Cu(l), with distances to the amine nitrogen atom longer than 2.40 A, but trigonal bipyramidal pentacoordinate with Cu(ll), with distances to the apical N shorter than 2.20 A. 

Fig. 1. Berry pseudorotation about pentacoordinate ( ) phosphorus, where (Q) represent fluorine atoms, (a) Original trigonal bipyramid (b) square...

The pentacoordinate molecules of trigonal bipyramidal form, like PF5, are a very nice example for the study of the formal properties of stereoisomerizations. They are characterized by an appreciable nonrigidity and they permit the description of kinetics among a reasonable number of isomers, at least in particular cases (see below). Therefore the physical and chemical properties of these molecules have been thoroughly investigated in relation to stereoisomerization. Recent reviews may be found in the literature on some aspects of this problem. Mislow has described the role of Berry pseudorotation on nucleophilic addition-elimination reactions and Muetterties has reviewed the stereochemical consequences of non-rigidity, especially for five- and six-atom families as far as their nmr spectra are concerned. 

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 . ... 

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. 

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-... 

Today it is widely accepted that fivefold coordinated silicon plays a key role in the reaction mechanisms of the nucleophilic substitution having a trigonal bipyramidal transition state species which ressemble these transition states can be isolated in some special cases. The structural features fit well to kinetic data and possibly explain the significantly higher reactivity (proved by experimental data) of Si-pentacoordinated compounds compared to their tetracoordinated analoga. 

Pentacoordinate phosphorus offers an example of the application of EHT to covalent compounds that do not contain carbon (34). There are two possible high-symmetry structures for PHS, namely, a D3h trigonal bipyramid and a C4v square pyramid. The energies and shapes of tire MO s for each of the two are given in Fig. 26. For the latter, the optimal value of a was found to be 99.8°. Still another structure was considered ... 

As demonstrated by single-crystal X-ray diffraction, the /-coordination polyhedra of 85-87 are distorted trigonal bipyramids, with each of the axial positions occupied by the oxygen atoms. This is shown for compound 86 in Fig. 11. In all cases, the crystals are formed from pairs of (A)- and (A)-enantiomers. Selected geometric parameters for 85-87 are listed in Table XIII. As can be seen from the Si-O [1.8004(10)-1.829(6) A], Si-N [1.741(7)-1.764(6) A], and Si-C distances [1.867(8)-1.915(2) A], the A/02N2C frameworks of 85-87 are built up by five normal covalent bonds and do not involve a bonding system in the sense of the 4+1 coordination usually observed for pentacoordinate silicon species with Si-N bonds. 

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. 

Let us now examine the pentacoordinate Ni(II) complexes with the already cited Schiff base bis(salicylideneimmine-3-propyl)amine, [Ni(R-X-saldpt)]. As for the cases of iron(II) and cobalt(II) complexes, this ligand constrains the central nickel(II) ion to assume a trigonal bipyramidal geometry, Figure 105.156... 

For example, conversion of the hexacoordinate (octahedral) derivatives [MnHCl(PP3)] to the corresponding deprotonated pentacoordinate (trigonal-bipyramidal or square pyramidal) derivatives [MnCl(PP3)]+ (M = Fe, Ru, Os), Scheme 6, follows electrode mechanisms that are more or less complicated depending upon the nature of the central metal.16... 

---