Quadrupole splitting as a probe for bonding

In view of the assumptions and estimates involved in the derivation of the values quoted above, it is likely that they have a considerable uncertainty, probably of around 10 to 20%. 

Gold is intermediate between transition and non-transition metal behaviour, since gold(I) has a closed shell configuration, 5d Based on the isomer shift for gold metal (assumed to be 5d °6s ) and the alkali metal aurides (Au, 5d 6s ), it has been suggested that unit increase in n, corresponds to an increase in isomer shift of about 8 mm/s (Sham, Watson Periman, 1981). 

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Another experimental probe of BD versus a donation is Mossbauer spectroscopy of a series of iron complexes.54 Plots of isomer shift versus quadrupole splitting for [FeH(PPXL)]+ for L such as H2, N2, CO, MeCN, and Cl show that the H2 complexes fell way off the straight line defined by the a donors, more so than even CO. It was concluded that it bonding in H2 complexes is very significant, more so than in N2 or CO complexes. Theoretical analyses concur that H2 is a much better -acceptor ligand than early calculations showed. More quantitative measures of BD are provided by charge decomposition analysis (CDA) and extended transition state (ETS) analysis.41,55-60... 

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