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In hexaaquaions it is generally true that aqueous solutions of complex ions with a higher positive charge on the central metal ion will have a lower pH. Explanations typically note that the greater charge density of the central metal ion has a stronger polarising effect on the OH bonds in the aqua ligands, which weakens the O-H bonds, increasing the dissociation of H+ ions.

How I'm struggling with this is that it's also common to predict that bond strength increases with polarity, to the point that Bond dissociation energy values are used to calculate Pauling values.

So we end up with apparently contradictory statements such as:

"The bonds to H also follow the expected trend of increasing bond strength with increasing electronegativity difference." (1.4: Bond Polarity and Bond Strength, Chemistry Libre Texts, https://bit.ly/3FbzexP)

and

"For a series of mononuclear oxoacids of an element E, the strength of the acids increases with increasing number of oxygen atoms. This trend can be explained qualitatively by considering the electron-withdrawing properies of oxygen. The oxygen atoms withdraw electrons making the O-H bond weaker." (Inorganic chemistry, Shriver and Atkins, 4th, p.120)

Taken in tandem, my reading of all this is that bond strength generally increases with polarity, yet when a nearby electron-withdrawing moeity polarises the O-H bond in an acid, the bond is weakened, not strengthened.

I'd be grateful for any explanation anyone can offer. Thanks.

EDIT: I may have answered my own question. Viewed in the sense of repulsion between the positive central metal ion and the delta positive hydrogen atom(s) in the aqua ligands, a more polar O-H bond will produce greater repulsion between the metal ion and the H atom. So a bond is not intrinsically stronger for being polar, it's context specific.

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    $\begingroup$ "Bond strength" as measured by dissociation energies is a measure of the energy required for homolytic cleavage of the bond, where one electron goes with each atom. Acidity is a measure of heterolytic cleavage, where both electrons stay with the base and the H+ ion separates. $\endgroup$
    – Andrew
    May 3, 2022 at 11:44

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Your edited conclusion is not correct. The apparent contradiction results from the definition of bond dissociation energy which refers to homolytic cleavage of a bond, where one electron from the bond goes with each atom:

$$\ce{A-H -> A. + H.}$$

When an acid dissociates, the bond is broken heterolytically, with both electrons staying on the base:

$$\ce{A-H -> A- + H+}$$.

A highly polarized bond tends to be more difficult to break homolytically, but easy to break heterolytically, and a simple explanation for this is that the second electron is at a much lower energy if it stays with the electronegative atom rather than going with the electropositive hydrogen.

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  • $\begingroup$ Thanks for that. Just to clarify, the incorrect part of my conclusion is that the effect of polarity on bond strength is context specific, right? It's not incorrect to reference the greater repulsion between the higher charged central metal ion in an aqua complex and the H atom of the more polarised O-H bond in the aqua ligands is it? $\endgroup$
    – thetada
    May 5, 2022 at 11:24
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    $\begingroup$ @thetada Right. $\endgroup$
    – Andrew
    May 5, 2022 at 12:29

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