I learned in my textbook that zinc forms more stable complex ion with NH3 than OH- because NH3 has only one lone pair and that makes repulsion between d orbital electron in zinc and lone pair electron in ligand smaller in NH3 complex ion than in OH- complex ion. But I also learned that Al forms only OH- complex ion. I know Al doesn't have d orbital electron but why it can't make NH3 complex ion? Does this mean more than 1 lone pairs are involved in ligand bonding in Al(OH)4- complex ion?
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1$\begingroup$ Aluminium ions have strong hydrolysing tendency and aluminium has generally affinity to oxygen. $\endgroup$– PoutnikCommented Sep 23, 2022 at 8:00
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1$\begingroup$ You can check: chemistry.stackexchange.com/questions/138924/… or this sciencemadness link: sciencemadness.org/talk/viewthread.php?tid=10977 $\endgroup$– Nilay GhoshCommented Sep 23, 2022 at 9:43
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2$\begingroup$ @satorukurita Generally, hydrolyzing tendency progressively grows with the ion charge. All $\ce{M^3+}$ and $\ce{M^4+}$ have strong hydrolysing tendency. $\ce{Ce^4+}$ is known to exist as a dimer with oxide or hydroxide bridges. $\endgroup$– PoutnikCommented Sep 23, 2022 at 12:41
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1$\begingroup$ @satorukurita With high intensity of electrostatic field around Al^3+, it is a strong advantage for OH- to have negative charge, compared to neutral NH3. Furthermore, there is possibility to form hydroxide and oxide bridges. Similar for Fe^3+. $\endgroup$– PoutnikCommented Sep 25, 2022 at 6:25
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2$\begingroup$ There could be also said that coordinated H2O molecules around Al^3+ and Fe^3+ have due the strong field high tendency to release H+, becoming OH-, what starts to happen in mildly acidic pH, where is practically no free NH3. $\endgroup$– PoutnikCommented Sep 25, 2022 at 7:23
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1 Answer
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The whole problem comes from the high charge of $\ce{Al^{3+}}$ ion : $+3$. As such it attracts strongly $\ce{H2O}$ molecules from the solution. But the central Oxygen atom of these $\ce{H2O}$ molecules is more strongly attracted than the $\ce{H}$ atoms, due to the high electronegativity of $\ce{O}$ atoms. Hydrogen atoms from water are not so strongly attracted and they are even repelled by $\ce{Al^{3+}}$, due to the repulsion of the $+3$ charge on $\ce{Al^{3+}}$ and the partial positive charge on $\ce{H}$ atom. As a consequence, the bond $\ce{O-H}$ which exists in water and in the structure $\ce{Al···O-H}$ will be broken between $\ce{O}$ and $\ce{H}$ when water is approaching $\ce{Al^{3+}}$. And this rupture is leaving a new bond $\ce{Al^{3+}- O^{-}}$ which is charged $2+$ and repels the $\ce{H^+}$ ion into the solution. Of course the triply charged $\ce{Al^{3+}}$ is stronger than $\ce{H^+}$ to hold the intermediate Oxygen atom. Matter of fact, all solutions of triply charged ions like $\ce{Al^{3+}}$ (or $\ce{Fe^{3+}}$, etc.) are hydrolyzed and produce a strongly acidic aqueous solution. As a consequence, if ammonia is introduced into such an acidic solution, it will immediately be neutralized and transformed into ammonium ions $\ce{NH4^+}$ which cannot make a complex, for lack of available doublet. This is the reason why $\ce{Al^{3+}}$ (and $\ce{Fe^{3+}}$) ions cannot make complexes with ammonia.
