Isomers coordination

It can be seen that other intermediate isomers are feasible but in the above cases they have not been isolated. Substantial differences in both physical and chemical properties are to be expected between coordination isomers. 

Figure 16.18 summarizes the types of isomerism found in coordination complexes. The two major classes of isomers are structural isomers, in which the atoms are connected to different partners, and stereoisomers, in which the atoms have the same partners but are arranged differently in space. Structural isomers of coordination compounds are subdivided into ionization, hydrate, linkage, and coordination isomers. 

FIGURE 16.21 The compounds in parts (a) and ib) are coordination isomers. In these compounds, a ligand has been exchanged between the cationic and anionic complexes. 

Attempts to prepare related hpp complexes of tantalum were only partially successful. Reaction of 2 equivalents of either (hpplSiMes or in situ generated Li[hpp] with Tads afforded a solid that analyzed correctly for the compound (hpp)2TaCl3. X-ray structural analysis of the crystallized product identified the coordination isomer [Ta(hpp)4][TaCl6], with a distorted dodecahedral cation and a regular octahedral anion (Figure 28). °... 

Coordination isomers occur when complex cations and complex anions are present and ligands are exchanged between anions and cations, for example ... 

A coordination isomer involves the exchange of ligands between a complex cation and a complex anion of the same compound, forming another complex cation and complex anion. 

The effect of /rfive-coordinate aluminum alkyls have been discussed.94 Studies have also focused on the equilibrium between four-coordinate and five-coordinate isomers in compounds of the type [R2Al /i-0(GH2) ER1x ]2 (n = Z, 3 ER1x = OR1, SR1, NRX2) (shown in Figure 7(h) for n = Z). Factors that control this equilibrium and hence the coordination around the aluminum include the steric bulk of the substitutent at the aluminum (R), and the Lewis base donor R1, the basicity of the neutral donor group ER1, and the chelate ring size (as determined by n).95... 

The coordination of styrene is expected to be strongly influenced by substituents that are neglected in the minimal QM model A. Thus, for sake of clarity, we do not present the styrene coordination results using model A. Depicted in Figure 8 are the three most stable styrene coordinated isomers, 8a-c. The coordination energies, which are also shown in Figure 8 in kcal/mol, reveal that the initial formation of the tt-complex is slow and reversible. In fact, only for isomer 8a is the styrene coordination exothermic and here it is only exothermic by 0.5 kcal/mol. Isomers 8a-c all have the olefinic bond of the styrene lying parallel to the plane defined by the P-Pd-Si atoms. No other sterically accessible isomers could be located where this bond lies parallel to this plane. Due to steric reasons, complexes with the olefinic bond perpendicular to this plane were found to be at least 8 kcal/mol less stable. 

Chrominm(III) and Cobalt(III) Nucleotide Complexes. Coordination isomers of Cr(III)- and Co(III)-nucleotides can be separated and used to test for specificity. Since Cr(III) is diamagnetic, EPR and NMR experiments can supplement the chirality probes . ... 

Exchange-inert complexes of Cr(III) with nucleotide ligands are very stable toward hydrolysis. Such complexes have proven to be extremely useful as chirality probes in that different coordination isomers can be prepared and separated These nucleotide complexes have also proved useful as dead-end inhibitors of enzyme-catalyzed reactions. Because Cr(lII) is paramagnetic, distances can be measured by measuring the effects of Cr(lll) on the NMR signals of nearby atoms when the Cr(lll)-nucleotide complex binds to the surface of a mac-romolecule. See Exchange-Inert Complexes... 

Coordination isomers of metal ion-nucleotide complexes. For most metal ions, for example Mg, the metal... 

FIGURE 1. Schematic representation of octahedral coordination isomers. Top two enantiomers formed by iron(III) complexation by a nonchiral chelator. Bottom incorporation of chiral centers to the ligating groups generates two diastereomers... 

The reaction of mandelic acid with ZrCLj in acetonitrile yields a mixture of [Zr ( )-PhCHOHC02 3C1] and [Zr ( )-PhCH0HC02 4].421 The latter soluble product is believed to be a monomeric, seven-coordinate isomer of the polymeric tetrakis(mandelate) obtained in aqueous media it exhibits an IR band at 1700 cm-1, which is attributed to v(0=0) of a monodentate mandelate ligand. 

When M0O2- coordinates to a metal ion, as in [Co(NH3)5Mo04]Cl or [Co(NH3)4Mo04]N03, the IR and Raman spectra of the M0O4- grouping are perturbed.22 An instructive example considers the coordination isomers [Co(NH3)5C1]Mo04 and [Co(NH3)sMo04]C1.22... 

The tris(2-cyanoethyl)phosphine can act either as a monodentate phosphorus donor ligand or as a bidentate mixed donor ligand. The red square planar [NiX2L<2] complexes (X = Cl, Br) transform in the solid state into blue six-coordinate isomers which have a polymeric structure involving bridging bidentate ligands (Scheme (2).1178,1184,1192... 

The NiL4X2 complexes (L = Ph3PO, Ph3AsO X = C104, BF4) have been obtained either as five-coordinate or as tetrahedral species.1682,1802 The interconversion from the five-coordinate to the four-coordinate isomers can be performed in the solid state by heating.1682... 

Benzotriazole reacts with the complex anion [Fe(CN)5(H20)]3 to afford a mixture of N-2- and N-3-coordinated isomers [Fe(CN)5(btaH)]3 characterization by H and 13C NMR spectroscopy yielded an equilibrium constant K = 1.9 + 0.3 favoring the less symmetric N-3 isomer (209). Relaxation times obtained from cyclic voltammetry data indicate an intramolecular mechanism for the (N-2) (N-3) isomeri-... 

Coordination isomers occur when one or more ligands are exchanged between a cationic complex and an anionic complex (Fig. 16.26). An example of a pair of coordination isomers is... 

FIGURE 16.26 Coordination isomers. In this compound, ligands have been exchanged between the cation and anion complexes. 

---