Pearson (1963)

In the lecture on Wednesday we covered the introduction to acids and bases. The paper we are reading specifically concerns itself with Hard and Soft Acids and Bases.


9 Responses to “Pearson (1963)”

  1. Hunter Burgin Says:

    From our reading and lecture Wednesday it has become somewhat clear to me what exactly makes an acid/base “hard” and what makes it “soft”. Hard acids/bases are typically smaller and nonpolarizable in nature while soft bases are typically larger and more polarizable. In addition, Pearson makes it clear that hard acids prefer hard bases while soft acids prefer soft bases. My question focuses primarily on the strong acids made up of a proton and a halogen (HF, HCl, HBr, HI). Hydronium is clearly a very hard base due to its lack of electron shielding and small nature and it would be assumed, based on the HSAB theory, that it would prefer the hard bases such as F- and Cl-. However I fail to understand why hydronium would prefer Br- and I- in terms of Pearson’s theory when both of them are fairly soft bases.

  2. Hunter Burgin Says:


  3. Kevin Greenwood Says:

    I always understood HSAB theory as comparative; being able to say one ion is harder than another is valuable in predicting reaction products or explaining results. However, as hard/soft solvents are described on the last page has me a bit confused.

    Pearson states that the hydroxylic and hydrohalic solvents will be hard. I can’t argue with that. The dielectric constant for water is between 78.5 and 80.1 (depending on who you ask) and around 39 for methanol. While methanol’s dipole strength is about half of that of water, it is still considered a hard solvent. As organic chain lengths increase, dielectric constants decrease and solvents get softer. I looked up the solubility of triacetone triperoxide (peroxides are defined as ‘soft’ on the second page) just to see if the data agrees with the theory. I found a paper* that shows TATP as being insoluble in water but soluble in chloroform (111 g/100mL – dielectric constant 4.8) and hexane (11.1 g/mL – dielectric constant 1.9). Being a soft acid, I can see why TATP is nearly insoluble in hard solvents. However, when comparing solubilities in the soft solvents chloroform and hexane, this trend reverses and the peroxide is more soluble in the harder solvent. It makes sense that TATP would be more soluble in the chloroform as the stronger dipole in the solvent is interacting with the peroxide oxygens. Am I comparing apples to oranges by using dielectric constants with HSAB? Can HSAB theory be applied this specifically, or is it more useful when used more generally?


    • profhurst Says:

      I probably cannot recommend looking up too many details on TATP on the internet. There is a link to dielectrics, although things can get complicated quickly for what (as noted below) is a more qualitative approach.

  4. Porter Marsh Says:

    The text says that in order to decide which properties are of importance it is necessary to examine the theories. Some properties considered were polarizability, ionization potential, and unsaturation. My questions is which factor(s) does the paper determine to be important? Some of the properties mentioned above in the paragraph are dependent on each other so determining which one(s) are important and which are side effects of the important one(s) seems difficult. The paper says examination of the theories will help with determination without then saying exactly which properties they decide on.

  5. Adam Settimo Says:

    I agree with Kevin that I thought of this theory more as a comparative; something to be more looked at quickly to confirm chemical behavior, but now am more interested in the borderline cases.
    Like Porter above me I am curious of the criteria that govern how something is considered hard (a) or soft (b) with the more borderline cases? When we get down to these borderline cases is there a specific order that say whether oxidation state is more important that nuclear size? Is there a blanket order of importance?
    For example, on the third page there is reference to Ir(III) being a hard acid. This makes sense when we look at the oxidation state, but the size of the atom is very large, being right next to Pt and the beginning of the soft acids. We have not only the 6s shell, but the most of the 5d’s, and the 4f block. Surely the addition of shielding from the f block must have some influence to maybe have “offset” the oxidation state a bit, right? or is the oxidation state king over the nuclear size? Here is a link to an ACS site listing Rh(III) as a boarderline case:

    Surely if Rh(III) is a borderline case and doesn’t have the shielding that the provided by the addition of the F block, then the Ir(III) Should be at least a borderline case as well, or am I missing something?

  6. Tony Says:

    Last class I felt a parallel might exist between hydrophobic and hydrophilic interactions in organic chemistry and soft and hard acid in inorganic chemistry. London forces, or van der waals forces, are used to describe hydrophobic interactions while ionic or dipole or both describe the hydrophilic interactions. I was under the impression that maybe soft and hard acids worked similarly. Luckily this paper addresses this to some extent. The electron correlation effect proposes that London forces might be in play in soft acid/bases but orbital hybridizations have a bigger role. Yet London forces are still a factor since softness concerns with polarizability. From my understanding ionic and dipole effect do play a role in determining soft/hard acid effects because of the same reason that they induce polarization; this is affected so much that polar solvents weaken hard acids/bases and enhance soft ones.

    My first question involves the issue of gas phase equilibrium. If no solvent is involved does polarizability become harder to achieve? If so do some soft acids/base or borderline ones become hard? Some of these might seem impractical to think of in gas phase but could it be possible that biological enzymes that employ dissolvation mimic gas phase to favor a reaction by changing the softness of a borderline compound?”

    Lastly I cannot imagine why F>I>Br> is the order of more to less stable for the halogen of reaction of equation 11 and the following paragraph. Can someone explain?

  7. Josh Ellsworth Says:

    My question also relates to the concept of solvent influence on the hard or soft nature of the acid/base. It seems possible that the right solvent system could make an unfavorable pairing slightly less so. If one were to use an ionic liquid such as 1-butyl-3-methylimidazonium hexafluorophosphate (which is completely polar as well as aprotic), would it be possible to make the generation of Mercury(II) Fluoride (or some other soft acid/hard base combination) more favorable? Could this be used for the preparation of Hydrides or Fluorides that would ordinarily fall apart in solution?

  8. Daniel Begay Says:

    It’s interesting how the authors of this paper were able to build a functioning table to describe hard and soft acids using data and information from many different sources.

    My question comes from the second page, second paragraph. The authors state that the Chatt criterion of categorizing hard and soft bases is comparable to their substrates, they could it to identify that from the phosphate ester, the phosphorus atom was an electrophilic center. That left me very confused as I thought by definition that electrophilic atoms had to be an “electron lover”; wanting to accept electrons. Which by definition would be a Lewis Acid. From what I’ve read up on and what has been discussed in class, phosphorus is treated as a base. Is it considered a hard acid because of the ligands that surround it? Do the oxygens draw electronegativity away from the P center atom? I have the same question for the peroxide that was described as a soft acid. is the O–O bond making it “bigger”, in a matter of speaking?

    Second question, since this paper was published in 1963, how much of these rules and standards are still used to this day? Are there new criterions used to categorize these acids and bases?

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