Coordinated solvent molecules

The processes of complex-ion formation referred to above can be described by the general term complexation. A complexation reaction with a metal ion involves the replacement of one or more of the coordinated solvent molecules by other nucleophilic groups. The groups bound to the central ion are called ligands and in aqueous solution the reaction can be represented by the equation ... 

Those in which solvent molecules are directly involved in formation of the ion association complex. Most of the solvents (ethers, esters, ketones and alcohols) which participate in this way contain donor oxygen atoms and the coordinating ability of the solvent is of vital significance. The coordinated solvent molecules facilitate the solvent extraction of salts such as chlorides and nitrates by contributing both to the size of the cation and the resemblance of the complex to the solvent. 

We should note at this point, that the above reaction implicitly refers to aqueous solutions, and that, for convenience, we have explicitly excluded free and coordinated solvent molecules. Strictly, the above relationships should be written as in Eq. (8.2). 

The definition of solvent exchange rates has sometimes led to misunderstandings in the literature. In this review kjs 1 (or fc2lsolvent]), sometimes also referred to as keJ s 1, is the rate constant for the exchange of a particular coordinated solvent molecule in the first coordination sphere (for example, solvent molecule number 2, if the solvent molecules are numbered from 1 to n, where n is the coordination number for the solvated metal ion, [MS ]m+). Thus, the equation for solvent exchange may be written ... 

Rate constant for the exchange of a particular coordinated solvent molecule. 4 Neat solvent. 

Electronic configuration = (2g6. Six-coordinate ionic radius = 73 pm (169). b Rate constant for the exchange of a particular coordinated solvent molecule. c All three H20 and MeCN are equivalent. 

Figure 13-7. A snapshot of the MD simulation of the Orf2 ternary complex looking down the PT-barrel. The protein backbone is shown as ribbons (red) superimposed on top of the crystal conformation (gray). GPP and the magnesium ion are shown as licorice as well as 1,6-DHN, proximal protein side chains (within 3 A) and 4 Mg-coordinating solvent molecules...

The HCN exchange itself proceeds through a trigonal bipyr-amidal intermediate [Li(NCH)5]+ reached via a late transition state. The entering HCN molecule approaches the lithium cation directly, and pushes three coordinated solvent molecules away toward the equatorial positions. In line with the experimental observation of a very fast exchange process, the computed... 

From the method of preparation of [BeCl(12-crown-4)]+ (179), it is known that the Cl ligand can be substituted by a solvent molecule. We applied our most common test solvents water and NH3 to a Be2+ cation, where most coordination sites are occupied by a chelating ligand, in this case the crown ether 12-crown-4. In contrast to the tetrahedral [Be(solvent)4]2+ solvated complexes, the precursor complexes [Be(solvent)(12-crown-4)]2+ are quadratic pyramidal, where four oxygen donor atoms of the crown ether form the quadratic basis, while Cl- or a coordinating solvent molecule occupies the apical position. Addition of one water or ammonia molecule to [Be(12-crown-4)]2+ is exothermic (see Table IX). 

Insertion of non-coordinated solvent molecules in these networks can stabilise the LS state. For instance, the transition temperatures of the system [Fe(hyetrz)3] (3-nitrophenylsulfonate)2 Solv depend dramatically on the nature of the solvent as follows Solv=0 (T1/2 105 K)room temperature) [43]. A similar influence of non-coordinated solvent molecules was first reported for the mononuclear complexes [Fe(2-picolylamine)3]Cl2 Solv [44]. The hydrate [Fe(hyetrz)3](3-nitrophenylsulfonate)2-3H20 loses water when heated and this is accompanied by a change to the HS state [45]. The significant feature of this system is that the HS anhydrous sample does not re-ab-sorb water under a normal atmosphere. Thus it can be the basis for single use applications. Several other examples of changes in spin state induced by water removal have been identified for ID polymeric chain compounds following this discovery [46-49]. 

So far, the reduction of metal ions into the metallic state was discussed involving a complete removal of the coordinated solvent molecules in the reduction process. We shall now consider such redox-systems in which both the oxidized and the reduced species are solvated. The polarographic reduction of Eu(III) to Eu(II) in different solvents occurs at such halt-wave potentials which are again related to the donicity of the solvent molecules118). In the Ei/j-DN plot a straight line is observed. Analogous results were obtained for the redox complexes Sm(III)-Sm(II) and Yb(III>Yb(II) 118> 120> (Fig. 27). 

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