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6.8 The following Table lists stability constants for a range of substituted tris-(1,10phenanthroline)iron(II) complexes \( \left(\log \beta_{3}\right. \) ), together with the \( \mathrm{pK}_{\mathrm{a}} \) values of the respective ligands, some Hammett \( \sigma \) values for substituents, and redox potentials \( \left(\varepsilon^{0}\right) \) for the respective iron(II)/iron(III) complex couples. Construct graphs showing the relations between the given stability constants and the other parameters, and comment on the correlations (or non-correlations) which become apparent. \begin{tabular}{lccccccc} & \( 5 \mathrm{NO}_{2} \) & \( 5 \mathrm{Cl} \) & \( 5 \mathrm{Ph} \) & none & \( 5 \mathrm{Me} \) & \( 5,6 \mathrm{Me}_{2} \) & \( 4,7 \mathrm{Me}_{2} \) \\ \( \log \beta_{3} 17.8 \) & 19.7 & 21.1 & 21.2 & 21.9 & 23.0 & 23.1 \\ \( \mathrm{p} K_{\mathrm{a}} \) & 3.25 & 4.26 & 4.80 & 4.97 & 5.28 & 5.60 & 5.94 \\ \( \sigma \) & +0.78 & +0.23 & +0.01 & 0 & -0.17 & - & - \\ \( \varepsilon^{\mathrm{o}} / \mathrm{V} \) & 1.26 & 1.11 & 1.08 & 1.07 & 1.02 & 0.97 & 0.88 \end{tabular} The stability constant for the formation of tris-(1,10-phenanthroline)iron(III) is \( 10^{14} \) (i.e. \( \log \beta_{3}=14 \) ). Why is this value smaller than that for the analogous iron(II) complex?

          6.8 The following Table lists stability constants for a range of substituted tris-(1,10phenanthroline)iron(II) complexes \( \left(\log \beta_{3}\right. \) ), together with the \( \mathrm{pK}_{\mathrm{a}} \) values of the respective ligands, some Hammett \( \sigma \) values for substituents, and redox potentials \( \left(\varepsilon^{0}\right) \) for the respective iron(II)/iron(III) complex couples. Construct graphs showing the relations between the given stability constants and the other parameters, and comment on the correlations (or non-correlations) which become apparent.
\begin{tabular}{lccccccc} 
& \( 5 \mathrm{NO}_{2} \) & \( 5 \mathrm{Cl} \) & \( 5 \mathrm{Ph} \) & none & \( 5 \mathrm{Me} \) & \( 5,6 \mathrm{Me}_{2} \) & \( 4,7 \mathrm{Me}_{2} \) \\
\( \log \beta_{3} 17.8 \) & 19.7 & 21.1 & 21.2 & 21.9 & 23.0 & 23.1 \\
\( \mathrm{p} K_{\mathrm{a}} \) & 3.25 & 4.26 & 4.80 & 4.97 & 5.28 & 5.60 & 5.94 \\
\( \sigma \) & +0.78 & +0.23 & +0.01 & 0 & -0.17 & - & - \\
\( \varepsilon^{\mathrm{o}} / \mathrm{V} \) & 1.26 & 1.11 & 1.08 & 1.07 & 1.02 & 0.97 & 0.88
\end{tabular}

The stability constant for the formation of tris-(1,10-phenanthroline)iron(III) is \( 10^{14} \) (i.e. \( \log \beta_{3}=14 \) ). Why is this value smaller than that for the analogous iron(II) complex?

6.8 The following Table lists stability constants for a range of substituted tris-(1,10phenanthroline)iron(II) complexes (logβ3. ), together with the pKa values of the respective ligands, some Hammett σ values for substituents, and redox potentials (ε^0) for the respective iron(II)/iron(III) complex couples. Construct graphs showing the relations between the given stability constants and the other parameters, and comment on the correlations (or non-correlations) which become apparent.

5 NO2 5 Cl 5 Ph none 5 Me 5,6 Me2 4,7 Me2

logβ3 17.8 19.7 21.1 21.2 21.9 23.0 23.1

p Ka 3.25 4.26 4.80 4.97 5.28 5.60 5.94

σ +0.78 +0.23 +0.01 0 -0.17 - -

ε^o / V 1.26 1.11 1.08 1.07 1.02 0.97 0.88

The stability constant for the formation of tris-(1,10-phenanthroline)iron(III) is 10^14 (i.e. logβ3=14 ). Why is this value smaller than that for the analogous iron(II) complex?

Added by Morgan J.

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