Effects of the Molecular Environment and Isotopes

The large temperature dependence of Co chemical shifts was noticed in 1951, and subsequent investigation showed it to be associated with the variation of dE. Benedek et al. ascribed this to the changes in the mean vibrational energies of the ground and excited electronic states (which form part ofzlE). However, with the more recent estimates of the magnitude of a , the temperature dependence of 

Brown and Cohen state that the temperature coefficient is proportional to 7. However, the equation of Benedek et a/. shows it to be proportional to and hence where K is not expected to be constant between complexes [but is 

When comparing results at different temperatures it is desirable to make some estimate of the change in b due to the temperature difference. An assessment of the likely magnitude may be made from the temperature coefficients given in Table 16. 

Benedek et al. found that the resonances of [Co(CN)g] , [Co(N02)6], and [Co(acac)3] moved 22.3, 46.4, and 15.1 ppm, respectively, to low frequency when a pressure of 1000 kg cm was applied. The behavior is not strictly linear, dajdP decreasing by ca. 25 /o by 10,000 kg cmThey interpret the effect of pressure as reducing the metal-ligand distance and hence and so tr.  

The sensitivity of metal shifts to weak interactions with solvent and other species present in solution provides a convenient means of studying these processes. In this way the preferential solvation of [Co(acac)3], hydrogen bonding between [Co(CN)g] and acidic solvents, the formation of outer sphere complexes between [Co(en)3] and PO and and ion pairing of tetraalkyl 

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