Octahedral aqua complexes

Somewhat better data are available for the enthalpies of hydration of transition metal ions. Although this enthalpy is measured at (or more property, extrapolated to) infinite dilution, only six water molecules enter the coordination sphere of the metal ion lo form an octahedral aqua complex. The enthalpy of hydration is thus closely related to the enthalpy of formation of the hexaaqua complex. If the values of for the +2 and +3 ions of the first transition elements (except Sc2, which is unstable) are plotted as a function of atomic number, curves much like those in Fig. 11.14 are obtained. If one subtracts the predicted CFSE from the experimental enthalpies, the resulting points lie very nearly on a straight line from Ca2 lo Zn2 and from Sc to Fe3 (the +3 oxidation state is unstable in water for Ihe remainder of the first transition series). Many thermodynamic data for coordination compounds follow this pattern of a douUe-hunped curve, which can be accounted for by variations in CFSE with d orbital configuration. 

Hydration enthalpies are closely related to formation enthalpies of octahedral aqua complexes. Because the water molecule is a weak ligand, the resulting configurations are of the high-spin type. 

Rates of water exchange in octahedral aqua complexes... 

Rates of water exchange in octahedral aqua complexes can vary up to 20 orders of magnitude. Modem techniques enable determination of rate constants in the mentioned rate span. There are several reasons for these rate differences. Thus, the replacement rates for univalent cations increase in the order Li" " < Na" " < K" < Rb" < Cs" ". Figure 2.6.1 shows that the ionic radii increase in the same manner. 

Studies of solid state conversions of chloride and bromide complexes of the types [Ni(dl-Bua)2(H20)2] X2 (Bua = 2,3-butanediamine)47 and [Ni(rf/-Stien)2-(H20)2]X2 (Stien = l,3-diphenyl-l,2-diaminoethane X = Cl, Br, NO2) have revealed similar reactions. Dehydration is the initial step in the process and results in conversion of the octahedral aqua-complexes into the square-planar complexes, [Ni(d/-Bua)2]X2 and [Ni(Stien)2]X2. Further heating results in rearrangement to form new octahedral complexes, Eq. 11.3 ... 

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

No zirconium(III) complexes with oxygen donor ligands have been isolated. However, the electronic absorption spectra of aqueous solutions of Zrl3 have been interpreted in terms of the formation of aqua complexes (equation 4).29 The spectrum of a freshly prepared solution of Zrl3 exhibits a band at 24 400 cm-1, which decays over a period of 40 minutes, and a shoulder at 22000 cm-1, which decays more rapidly. The 24400 cm-1 band has been assigned to [Zr(H20)6]3+, and the 22000 cm-1 shoulder has been attributed to an unstable intermediate iodo-aqua complex. If it is assumed that the absorption band of [Zr(H20)6]3+ is due to the 2T 2Ee ligand-field transition, the value of A is 24 400 cm. This corresponds to a A value of 20 300 cm-1 for [Ti(H20)6]3+ 30 and 17 400 cm-1 for the octahedral ZrCl6 chromophore in zirconium(III) chloride.25... 

Aqua complexes of the ions R3Sn+ and R2Sn2+ have been studied in aqueous solution,3 5 and bipyramidal Bu3Sn(OH2)2+ (equation 9-48)6 and octahedral R2Sn(OH2)42+ (R = Me7 or Bu8) have been identified crystallographically. 

The aqua complex of the nickel(II) ion retains its green color and its octahedral structure in aqueous solution even in the presence of the SCN ions from the KSCN, which compete with the Cl" ions however, these SCN ions displace... 

The aqua complex changes from the octahedral structure with coordination number 6 to the chloro complex during the reaction. This shows a tetrahedral structure and exhibits the coordination number 4. A structure or coordination number is not to be seen as a permanent constant for a metal cation. Depending on size, charge and required space of the ligand, the coordination number can change it is usually 2, 4 or 6. Further data can be taken from appropriate specific literature. 

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