Chromium , aquation reaction

The major contributions falling within the scope of this section are those of Frankel, Langford, King and others,who have concentrated on the thermal and photochemical reactions of chromium(III) complexes. Additional interesting results relevant to cobalt(III) aquation reactions have been produced by Barraclough and McTigue " while a recent review by Amis on the solvation of a range of main block elements is relevant. 

More closely related to kinetics and mechanism are three reviews from Danish chemists, one on the state of the art with respect to Cr(III) substitution reactions and the other two ° survey /t-diol dinuclear Cr(III) systems. Chromium chemistry is also discussed in a general review on metal-ion-assisted aquation reactions " and in a review on the effect of pressure on reaction rates. In the following sections, units for and AH are in kJ mol A5 in J K mol A V in cm mol visible absorption spectral maxima (A ax) in nm, and molar extinction coefficients (e) in cm Unless otherwise stated, all rate constants are reported at 25.0 (298.2 K). 

Rate laws for aquation of aquo-chromium(m) contain terms in [H+] when aquation of the conjugate base makes a significant contribution to the overall aquation reaction. Thus the rate laws for aquation of cis- and trans-[Cr(en)2(OHa)Cl] +, of [Cr(NH3)4(OH2)Cl] +and of [Cr(OHa)6-(3-picoline)] + (ref. 165) are... 

Another rather unusual case of rapid aquation is provided by those oxoanion ligands MO in which the M-0 bond is sufficiently weak for it to break rather than the Co-0 bond. This time we have an intramolecular reaction of a coordinated ligand. A recent example is provided by aquation of the [Co(NH3)5(OMo03)]+ cation (57). This aquation pathway is actually more commonly encountered for chromium(III) complexes. 

Chromium(II) must catalyze the aquation of Cr X by the reverse of the two steps (5.62) and (5.63). Traces of lower oxidation state may be the cause of apparent lability in the higher one e.g. Fe(II) labihsing Fe(III). Exchange of Co(NH3)5 with NHj(aq), catalyzed by Co(II) has been examined using N-labelled NHj and nmr monitoring. The treatment is complicated by side reactions. 

The products of reduction of Fe(NCS)2+ by Cr2+ in aqueous solution containing thiocyanate ions (equation 36), include the isomers of [Cr(NCS)(SCN)(H20)4]+, which contains both bland S-bonded thiocyanate ligands.641 These isomers undergo spontaneous decomposition by parallel aquation (loss of the S-thiocyanato ligand) and isomerization (Cr—SCN— Cr—NCS) reactions. Details of the solution visible spectra of these isomers as well as those of similar chromium(III) complexes for comparison are listed in Table 68. From the compiled data it is apparent that the S-bonded thiocyanate ligand lies very close to Br- in the spectrochemical series while the N-bonded form lies between Nj and NO. 

Cotton and Mott3 reported the reaction of chromium(II) acetate with Me2NCH2CH2ONa to form an unusual tetrameric product in which each V,lV-dimethylaminoethanolate group is N—O bidentate but bridges through O to other Cr atoms in the tetramer. A recent protonation and aquation study on Crni/ethanolamine complexes4 has shown a sequence of reactions involving initial... 

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