Free energy chromium complex

The prismatic cyclopentadienyl and benzene complexes of transition metals are reminiscent of the polycyclic hydrocarbon pris-manes. Figure 3-28 shows ferrocene, (C5H5)2Fe, for which both the barrier to rotation and the free energy difference between the prismatic (eclipsed) and antiprismatic (staggered) conformations are very small [75], Figure 3-28 also presents a prismatic model with Ay, symmetry for dibenzene chromium, (C6F[6)2Cr. 

Replacing one CO group in Cr(CO)3Cp with diazomethane costs 3.6 kcal/mol in terms of free energy. In the presence of free CO, the formation of a ketene complex was observed experimentally. The structures and harmonic vibrational frequencies of the possible ketene tautomers were also computed (see Figure 10.15). Since ketenes possess C=C as well as C=0 Jt-bonds, both the r 2-(C,C) and the r 2-(C,0) coordination modes had to be considered. As chromium is of rather oxophilic character, it is not surprising that the r 2-(C,0) tautomer (Figure 10.15c) was lower in energy by 1.3 kcal/mol. 

The singlet quenching of 10-methylacridinium chloride by chromium(m) complexes is probably an energy-transfer process,149 and a linear free-energy relationship between -values and the rate constants for fluorescence quenching of substituted anthracenes by triphenylphosphine has been recognized.160... 

Photoreduction of chromium(III) polypyridyl complexes by Fe " allows examination of the thermal reverse reaction. For a series of complexes, reactions with Fe show very similar rate constants reflecting internal activation free energies which do not differ markedly. 

The aquation rate for cw-[Cr(en)2(Cl)Br] ion (Br loss) is intermediate between that of the corresponding dichloro and dibromo complexes, and the rate data fit the linear free energy relationship which correlates AG values for corresponding cobalt and chromium complexes. 

Whereas the assignment of mechanism to spontaneous thermal aquations may at times be uncertain, the mechanism of metal-ion-catalysed aquation of halide complexes of cobalt(iii), chromium(ni), and similar cations is unlikely to be other than dissociative as far as the metal(m) centre is concerned. In Volume 2 of this Report it was mentioned that the catalytic effect of metal ions on solvolysis rates of t-butyl halides could be correlated with the stability constants of the respective metal-halide complex formed. Such a correlation is now reported for metal-ion catalysis of aquation of halide complexes of cobalt(m), chromium(m), and rhodium(m). Indeed this correlation is sufficiently general as to embrace such catalysts as H+ and HgCl+ as well as metal ions such as Hg + and A linear free-energy (AG vs. AG°) correlation... 

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