Questions tagged [hsab]

HSAB - Hard and Soft Acids and Bases is Pearson's theory classifying Lewis acids.

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Rationalisation of stabilising dispersion interactions in HgS via QM

It is well-known that mercury(II) sulfide, HgS, is extremely inert- for example, it has one of the lowest solubilities of nearly all known compounds in water(check Wikipedia). There are two well-known ...
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Parallel between polarisability of donor atom and CFS strength

Observing the strength of crystal field splitting by different ligands, can the following be considered?: Most of the members in the series are ligands where the donor atom is nitrogen, oxygen or ...
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Why does thiocyanate bind to Fe³⁺ with the S side instead of the N side?

According to hard and soft acids and bases (HSAB) theory, "soft" Lewis bases bind to "soft" Lewis acids, and "hard" Lewis bases bind to "hard" Lewis acids. $\ce{Fe^3+}$ is a hard Lewis acid. In $\ce{...
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Is there a mathematical basis behind the HSAB theory?

I learnt about the HSAB concept about a year ago, and found that it helped rationalise many of the solubilities/reactivities that “pure” Lewis acid/base theory could not. Is there a mathematical ...
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Is Using HSAB to Explain Michael Donors and Michael Acceptors Valid?

Basically, I've recently been thinking about using HSAB theory to rationalize the concept of michael donors and michael acceptors. Essentially, if we have a deprotonated diketone (with a lone pair on ...
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Why does iron(III) form the trithiocyanate complex instead of the triisothiocyanate one?

As far as I know, $\ce{Fe^{3+}}$ is a hard acid, and thus shouldn't it favor complexation with the nitrogen's lone pair rather than the softer S?
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Why is potassium cation harder than copper?

According to HSAB theory, the smaller radius and the less polarizable an acid is, the harder it is. According to Wikipedia, the ionic radii of $\ce{ K+}$ is much larger than $\ce{Cu+}$, which makes ...
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Why does strong Lewis acid-strong Lewis base interactions prevail over hard-soft acid-base interactions?

In Pearson's paper, he mentions that strong acid-strong base interactions are more favourable compared to hard-soft acid-base interactions. He cites the example of $\ce{H+ + H- -> H2}$. In this ...
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HOMO-LUMO and perturbation theory

I have some confusion about perturbation theory, and just how it fits into the hard-hard/soft-soft interactions within organic reactions. I have come across a formula for perturbation that is ...
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Hard and Soft Acid and Bases in terms of HOMO and LUMO

It has been given in a book and on Wikipedia that: hard bases have highest-occupied molecular orbitals (HOMO) of low energy, and hard acids have lowest-unoccupied molecular orbitals (LUMO) of high ...
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Why is WF6 stable whereas CrF6 is unknown?

$\ce{F-}$ is a hard base since it is small and relatively polarizable. Both $\ce{Cr^6+}$ and $\ce{W^6+}$ are hard acids, but shouldn't $\ce{CrF6}$ be favored since it would be a smaller hard acid?
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Carbonyl oxygen as nucleophile

Aldehydes and ketones have electrophilic centres at their carbonyl C, since the carbonyl O polarises the bond and attracts electron density towards itself on account of its high electronegativity. But ...
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Hard-hard and soft-soft acid/base interactions [duplicate]

How does one conclude if a given compound or a complex is a hard or soft acid or base? What are the factors that contribute to hard-hard, soft-soft, soft-hard, and hard-soft interactions?
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Why doesn't EDTA complex with alkali metal ions?

EDTA complexes with all the other metal ions in the periodic table except those from the group 1. Why is this so ? What is the coordination chemistry behind this ?
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How is charge density a useful measure of polarising power and polarisability?

How do you use charge density ($Z/r$) as a measurement of polarising power and polarisability, and hence hard/soft acid character? So far I've been looking at different polarising power and ...
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Why is diborane cleaved symmetrically by trimethylamine but unsymmetrically by ammonia?

I know that soft bases cleave diborane symmetrically but hard ones don't. However, what makes ammonia harder than trimethylamine? And why does hardness/softness affect this?
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Why does Silver form bonds with covalent character?

Compounds of silver form particularly strong bonds which is accounted for by the significant covalent character of the bonding. Furthermore, is the tendency to form covalent bonds linked to the fact ...
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Why is Ag+ a soft acid?

A soft species is typically large, polarizable and has a low charge (magnitude). The silver cation is not highly charged (this supports softness) however, it is anomalously small due to high nuclear ...
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Why is aluminium(III) considered a hard acid if it forms covalently bonded compounds like aluminium oxide?

If something is a hard acid, it tends to form ionic bonds as it is polarizing. So why is the aluminium ion a hard acid when it forms bonds with covalent character. A similar argument follows for why ...
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Is hydrogen bonding generally defined to include only three period two elements?

Traditionally, hydrogen bonding has been defined to only include interactions between a positively polarized hydrogen and three period-two elements: nitrogen, oxygen, and fluorine. Why was this ...
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Hardness and softness are terms used to describe which aspect of acidic behaviour?

What do hardness and softness mean? Is it something to do with the polarisability.
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Strongest chloride complex as predicted by HSAB theory

Why won't $\ce{Ni^2+}$ form a complex with $\ce{Cl-}$, while the ions $\ce{Cu^2+}$ and $\ce{Co^2+}$ form the complexes $\ce{[CuCl4]^2-}$ and $\ce{[CoCl4]^2-}$? According to the HSAB theory, $\ce{Cl-...
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Why is lead sulfide found in nature, whereas lead oxide is less common?

This question probably applies to other heavier metals as well. The only rationalization I can figure out is that lead in the +2 oxidation state (which is most common) is a borderline soft acid and ...
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Why do thiols have such a propensity for bonding with mercury?

Historically, thiols $\ce{-SH}$ were known as "mercapto-" due to their strong propensity to bond with mercury. Certain drugs still carry this designation, though mercaptopurine has an $\ce{=S}$ group ...
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