Trends in Ionization Potentials

Trends in ionization potentials for the transition metals in groups 4-10. Data from Douglas, B. E. McDaniel, D. H. Alexander, J. J. Concepts and Models of Inorganic Chemistry, 3rd ed. Wiley New York, 1994. 

Although the origins of the trends in acidity and basicity are currently being studied, in many cases the third-row metal complexes are more basic than the second, which are more basic than the first. As discussed in more detail in Chapter 3, Os(CO) (H)2 is less acidic than Ru(CO) (H)2, which is less acidic than Fe(CO) (H)2, and CpW(CO)3H is less acidic than CpMo(CO)3H, which is less acidic than CpCr(CO)3H. 

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Although the trends in ionization potentials and electron affinities are similar, some differences likely explained by electron repulsion effects, appear among the various sugar radicals. The difference between the IP and the EA of a free radical is mainly a result of the repulsion of the electrons in the HOMO in the anion (EA) as shown in the schematic representation above. Thus any delocalization in this state serves to ease this repulsion and increase the electron affinity of the radical. Indeed, the radical has a more delocalized structure than more localized radicals such as the 05 species. This results in the energy difference in the IP between 05 vs. 01 to disappear on going to the EA, as seen in the Figure 6. 

Although substantial experimental work has been performed on the redox properties of the radical intermediates [3, 11, 12, 139, 140, 146], no consistent scheme has been presented. Experimental data show that G(8)OH, G(5)OH, C(5)0H, A(5)0H, G(8)H, C(5)H and C(3)H have reducing capabilities mostly toward tetranitromethane (TNM), while C(6)OH, T(6)OH, C(6)H, T(6)H, A(4)0H and G(4)0H can oxidize TMPD (N,N,N, N -tetramethyl-p-phenylenediamine). The trends in ionization potentials and electron affinities of the various DNA radical adducts obtained from scaling to experimental values of smaller model radical compounds (described above) are in excellent agreement with those data and provide us with a consistent and complete list of redox properties for the DNA radical intermediates (Figure 6). 

In the second ionization potentials the same upward trend continues from left to right in a period. As a consequence of these trends, positive ions are produced chemically only from elements at the left hand side of the periodic table. The increase in ionization potential towards the right makes positive ion production energetically too expensive. Elements on the right side of the periodic table nevertheless can react, either through the formation of negative ions or of covalent bonds. 

It is impractical to measure the gas phase ionization potential for Fe(CNBF3)8-4. However, it is possible to obtain closely related information from charge-transfer spectra and oxidation potentials for Fephen2(CN)2and its adducts. Solvation effects maybe minimized by the determination of oxidation potentials in a solvent of relatively low dielectric constant. When methylene chloride is used as a solvent, the observed trends in oxidation potentials correlate well with expected acidity of the acceptor ... 

It is thus seen that for the first property which we have considered, the dipole moment, quantitative predictions could be made with reasonable accuracy, even in the -electrons era, with sufficient care and some training. The more elaborate computations have essentially brought confirmation of the main trends. For ionization potentials, the precision gained is more considerable, especially as concerns the relative locations of the ir and lone-pair ionizations. (The separation of the highest ir orbital and highest o orbitals being over 1.5 eV in non-empirical computations, it is probable that correlation effects would not modify the relative ordering of the ir and o ionization potentials)... 

Harris and Jones also presented results for spin-flip energies and for ionization potentials. Again, trends are reasonably well interpreted. For the spherical GL potential used, the energy to flip an s spin is overestimated for all of the atoms except Ni and Cu, the largest discrepancies, of about 0.5 eV, occurring near the middle of the series. On the other hand, the spin-flip energy for a d electron in Cr " is underestimated in this approach by about 1 eV. Errors in ionization potentials are about 0.4 eV. 

The elements all easily form positive ions M and consequently are highly reactive (particularly with any substrate that is oxidizing). As the group is descended there is a gradual decrease in ionization potential and an increase in the size of the atoms the group shows several smooth trends that... 

For larger clusters much spectroscopic detail is lost, but measurements of the photoionization thresholds provide information concerning cluster ionization potentials. Sodium clusters show a relatively smooth decrease in ionization potential from 5.1 eV for the atom to 3.5 eV for the 14-atom cluster. This is still significantly above the 1.6-eV work function of bulk sodium. For the smaller clusters the odd sizes have a lower ionization potential than the neighboring even-sized clusters because of effects due to open versus closed shell configurations. Recently measurements have been made of the ionization potential of iron clusters up to 25 atoms. In this experiment the ionization potentials were bracketed by the use of various ionizing lasers. There is a decrease in the ionization potential from 7.870 eV for an iron atom to the 4.4-eV work function of the bulk metal, but the trend is by no means linear. Thus, the ionization potential of Fea is about 5.9 eV, while those of Fes and Fe4 are above 6.42 eV. Clusters in the range of 9-12 atoms have ionization potentials below... 

A second anomaly is evident in the trends in the ionization potentials of the / -block elements. From nonrelativistic theory we would expect an increase in ionization potential as the occupation of the valence p shell goes from to p. This is due to an increase in nuclear charge that is only partly compensated by the screening from the other valence electrons. From p to p we expect a decrease in ionization potential due to the loss of exchange energy with electron pairing. Finally, from p to p we expect a similar increase as from p io p for the same reason. 

Attempts were made at explaining the trends in reactivity through the use of both an electron-transfer model85 and a resonance interaction model,86,87 but without success. It seems that the trends in reactivity on a fine scale cannot be easily explained by such simple models, but instead depend on a multitude of factors, which may include the ionization potential of the metal, the electron affinity of the oxidant molecule, the energy gap between dns2 and dn+1s1 states, the M-O bond strength, and the thermodynamics of the reaction.57-81... 

The comparison of coronal and photospheric abundances in cool stars is a very important tool in the interpretation of the physics of the corona. Active stars show a very different pattern to that followed by low activity stars such as the Sun, being the First Ionization Potential (FIP) the main variable used to classify the elements. The overall solar corona shows the so-called FIP effect the elements with low FIP (<10 eV, like Ca, N, Mg, Fe or Si), are enhanced by a factor of 4, while elements with higher FIP (S, C, O, N, Ar, Ne) remain at photospheric levels. The physics that yields to this pattern is still a subject of debate. In the case of the active stars (see [2] for a review), the initial results seemed to point towards an opposite trend, the so called Inverse FIP effect , or the MAD effect (for Metal Abundance Depletion). In this case, the elements with low FIP have a substantial depletion when compared to the solar photosphere, while elements with high FIP have same levels (the ratio of Ne and Fe lines of similar temperature of formation in an X-ray spectrum shows very clearly this effect). However, most of the results reported to date lack from their respective photospheric counterparts, raising doubts on how real is the MAD effect. 

Figure 16a shows the progressive bathochromic shift in the CT absorption bands (hvct) obtained from PyN02+ with aromatic donors with increasing donor strength (or decreasing ionization potential). A similar red shift is observed in the CT absorption bands (hvCj) of hexamethylbenzene complexes with various para-substituted JV-nitropyridinium cations (X-PyNO ) as shown in Fig. 16b. Such a trend in the hvct is in accord with the increasing acceptor strength of X-PyNO in the order X = OMe < Me < H < Cl < C02Me < CN.

These are some of the general trends that relate the ionization potentials of atoms with regard to their positions in the periodic table. We will have opportunities to discuss additional properties of atoms later. 

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