Hexaaqua ions reduction

Consider some vanadium ions in aqueous solution. Pale violet solutions of vanadium(ii) salts contain the [V(H20)6] ion. The vanadium(ii) center is only weakly polarizing, and the hexaaqua ion is the dominant solution species. Aqueous vanadium(ii) solutions are observed to be unstable with respect to reduction of water by the metal center. In contrast, vanadium(ni) is more highly polarizing and an equilibrium between the hexaaqua and pentaaquahydroxy ion is set up. The of 2.9 means that the [V(OH2)6] ion (Eq. 9.17) only exists in strongly acidic solution or in stabilizing crystal lattices. 

The hexaaqua ion, [Cr(OH2)6]3+, is also the final product in the reduction process (7, 12). The complete process using Fe(II) as a reducing agent is shown in the reaction sequence (8)-(ll), where a Fenton-type (15) mechanism is proposed (7,16), with [(H20)5CrIV0]2+ behaving as the chromium equivalent of the -OH radical (16). 

Another class of mthenium aUcene complexes contains those derived from the hexaaqua ion [Ru(H20)6] +. The thermodynamically stable complex [(cod)Ru(H20)4] + (74) forms directly from [Ru(H20)6] + and cod in alcohol at ambient temperature (equation 14). In (74), the redox potential of Ru has shifted more positive for the oxidation to Ru and more negative for the reduction to Ru or Ru°, so as to impose a high stability towards disproportionation see Disproportionation) (in contrast to the readily disproportionating aqua ion [Ru(H20)6] +). The X-ray crystal structure see X-ray Crystallography) of the Tosylate (Ots) salt disclosed quite different R-OH2 distances of 2.095(2) and 2.156(2) A for water gauche or trans to the alkene double bond, showing the structural trans effect see Trans Effect) of the latter on a a- (and tv-) donor ligand trans... 

A similar comparison can be made for the reduction of the hexaaqua ion of Fe + and the cyano complex (equations 8.38 and 8.39), and leads to the conclusion that the overall formation constant for [Fe(CN)g] is 10 times greater... 

Analogous principles apply when metal ions in different oxidation states form complexes with the same ligand usually, the metal ion in the higher oxidation state is stabilized to a greater extent than that in the lower oxidation state. Equations 7.36 and 7.37 show the reduction of hexaaqua and hexaammine complexes of Co(III) remember thatM (aq) represents [M(H20) ] +(aq) (see Section 6.12). 

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