Lewis Acid/Base & Oxidation and Reduction

[blazinfury]

In Lewis A/B, an acid is an e- pair acceptor and a base is an e- pair donor. So when an acid accepts e- it becomes reduced in its charge. However, acids are usually found with high ox numbers leading one to believe that they get oxidized. I find the OIL RIG and Lewis Acid/Base definitions to be counter-intuitive.

 

For example, if one asks which of the following is a stronger acid: Ca2+ or Li+, one would say it is the one with the higher ox number, but based on the Lewis definition, wouldn't it be the one with the lower ox number since you are gaining e- (ie - charge) and should decrease in ox #?

[Suxamethonium]

Hey

 

So, true a lewis base is an electron pair donor (and vice versa), and true a lewis acid is technically reduced during electron transfer.

 

As for OIL RIG (Oxidation Is Loss - Reduction Is Gain). If theres any confusion, to clarify: In oxidation, electrons are lost to form an oxidised product. And in reduction electrons are gained to produce a reduced product.

 

From your post I am of the impression, that these of themselves you don't have much trouble with, rather its how you've brought them together?

I think it is more the redox than the lewis base, so I'm going to try and outline that and see how you go?

 

So, If you have Cu⁺ and oxidise it, you take away an electron (or -ve charge). So charge sum would be +1 - -1 = +2 and you get Cu²⁺.

Likewise, if you have Cu⁺ and reduce it, you add an electron and so get, +1 + -1 = 0, Cu_(s).

 

As such if you had Li⁺ as a lewis acid, which is reduced (accepting 2 electrons), you would get something akin to Li^- (though in reality it would exist as a covalent or dative bond).

Likewise, Ammonia (lewis base) reacts with borontrifluoride (lewis acid) to give the adduct, which has a dative bond between the nitrogen (also + charged) and boron (also - charged). The nitrogen (base) was oxidised, and the boron (acid) reduced.

 

I hope this shows that the theories are compatable??

 

As for which is a stronger lewis acid of base, I was under the impression it was much more complex than which has the highest oxidation number. Possibly linked to oxidation/electronic potentials?

 

Hope I helped out a bit.

[mississippichem]

As Suxamethonium stated, oxidation/reduction processes result in the transfer of an electron(s) from one distinct chemical species to another where as a Lewis acid/base reaction will yield an adduct containing either a quite polar covalent bond or coordination bond.

 

It's also worth noting that the electron's shared in the Lewis acid/base adduct will be spin paired [there will be a HOMO-LUMO interaction for those that care] where as a redox process will occur electron by electron from reductant to oxidant.

 

It's interesting to note that there is some grey area here. For example, some bi-metallic coordination complexes can undergo an oxidation/reduction process where one metal center can oxidize the other through an intermediate excited state involving their common bridging ligand, an intramolecular redox process. This is actually a pretty trendy research topic in the inorganic chemistry world right now. These processes are well documented but the theory behind them is still a bit hazy. How does one define the oxidation state of the metal in such a complex? No one knows...fractional oxidation states have been proposed but are also criticized for implying unphysical "pieces" of electrons, i.e. they really just represent the statistical average of the oxidation states but this generates problems as well.

 

Sorry for the tangentially related rant. Many chemists live and die without ever seeing a bi-metallic complex.

 

As for which is a stronger lewis acid of base, I was under the impression it was much more complex than which has the highest oxidation number. Possibly linked to oxidation/electronic potentials?

 

Correct. I suppose you could draw a loose correlation between oxidation number and Lewis acidity/basicity. I'm sure that radial distance of the highest occupied molecular orbital from the nucleus [of the Lewis coordinating atom] and therefore polarizability of the molecule play just as much of an important role. I'm sure one might even be able to consider ligand field effects in more "ionic like" adduct cases. It's actually an interesting topic as well.

 

I don't think that actual redox potentials will provide much insight as they must be taken with respect to a reference electrode.

[Suxamethonium]

 

Sorry for the tangentially related rant. Many chemists live and die without ever seeing a bi-metallic complex.

 

 

Haha, I could be wrong- But is this like prussian blue (I only say it because it would mean we unknowingly see it all the time)? If not, I probably just havent grasped what you said clearly (Highly likely- organic chem is more my area). Thanks for elaborating on what I said, I was worried that I wasn't detailed enough.

[hypervalent_iodine]

Prussian blue is the first thing I thought of too; and yes, I think that sort of case is what he's talking about.

[Essay]

As Suxamethonium stated, oxidation/reduction processes result in the transfer of an electron(s) from one distinct chemical species to another where as a Lewis acid/base reaction will yield an adduct containing either a quite polar covalent bond or coordination bond.

 

It's also worth noting that the electron's shared in the Lewis acid/base adduct will be spin paired [there will be a HOMO-LUMO interaction for those that care] where as a redox process will occur electron by electron from reductant to oxidant.

 

It's interesting to note that there is some grey area here. For example, some bi-metallic coordination complexes can undergo an oxidation/reduction process where one metal center can oxidize the other through an intermediate excited state involving their common bridging ligand, an intramolecular redox process. This is actually a pretty trendy research topic in the inorganic chemistry world right now. These processes are well documented but the theory behind them is still a bit hazy. How does one define the oxidation state of the metal in such a complex? No one knows...fractional oxidation states have been proposed but are also criticized for implying unphysical "pieces" of electrons, i.e. they really just represent the statistical average of the oxidation states but this generates problems as well.

 

Sorry for the tangentially related rant. Many chemists live and die without ever seeing a bi-metallic complex.

 

 

Correct. I suppose you could draw a loose correlation between oxidation number and Lewis acidity/basicity. I'm sure that radial distance of the highest occupied molecular orbital from the nucleus [of the Lewis coordinating atom] and therefore polarizability of the molecule play just as much of an important role. I'm sure one might even be able to consider ligand field effects in more "ionic like" adduct cases. It's actually an interesting topic as well.

 

I don't think that actual redox potentials will provide much insight as they must be taken with respect to a reference electrode.

Real life is not a reference electrode, but it is full of relativity!

 

...I also noticed, "Many chemists live and die without ever seeing a bi-metallic complex." and felt compelled to add:

 

 

Look beneath your feet! Humus in the soil is composed of many such molecules; and more even with 3, 4..., n+1, metallic moieties. Fractional redox fields abound, organize, and form superwaves ...iwstm.

 

~

"Redox cycling in rhizospheres of relatively well-aerated soils is a little-studied process with considerable potential impact on soil acidity." --p.191

 

The Rhizosphere: An Ecological Perspective. 232 pages, Elsevier Science, Academic Press,

 

$75.00 - 95.00, Library-o-Congress Call Number: QK644 .R445 2007

"The Rhizosphere" ...[meaning the extended root zone--especially in land use and evolution].

 

"Thus small changes in the equilibrium between inputs and decomposition could have significant impact on atmospheric CO2 concentrations...." --p.31 ...which in turn affects soil acidity, which in turn affects....

 

"These calculations reinforce the importance of conceiving of the rhizosphere broadly in space and time." --p.194

 

"The close correspondence of rhizospheres and soil redoximorphic features (Fimmen 2004)... hypothesize that rhizosphere-stimulated Fe-redox cycling [add] significant controls [to] soil acid-base reactions." --p.194

 

"Reduction of Fe^III increases iron solubility with respect to oxide/hydroxide phases by as much as eight orders of magnitude." --p.193

 

"Contrary to the bulk soil environment with generally abundant O2, rhizospheres can be reducing environments due to the turnover of decomposable organic compounds... and root respiration, [where] steep redox gradients can develop...." --p.191

 

see also, google: rhizosphere-induced mottling

===

 

++ Teaser... for insights on the "smell" of rust ...or paths of lightning....

"Reaction kinetics of adsorbed Fe^II at pH < 5 is relatively rapid compared to aqueous Fe^II (Wherli 1990). --p.193

[...adsorbed onto organic acids/odors/humic substances?]

 

~

Edited January 19, 2012 by Essay

[mississippichem]

Essay,

 

I'm not sure I follow. Care to elaborate?

[Essay]

Essay,

I'm not sure I follow. Care to elaborate?

 

...about humus in general, as a fractional redox moiety; or about my wild speculations on structured or super-structured effects ...or airborne humus?

 

~

[hypervalent_iodine]

Essay, can you start making sense and can you do so on topic?

[Essay]

Essay, can you start making sense and can you do so on topic?

 

 

Sorry about that. Reading the comment about molecules with two redox sites ("bi-metallic complex"?) as being rare, reminded me how surprised I was to learn about the multiple redox sites-- coordinating Fe and Mn, Cu, etc. --in humic molecules. These aren't easily studied in the lab, so they may be irrelevant; but that diversity in molecules -and even parts of molecules- does exist.

 

In trying to "second" the remark about fractional redox charge, I got way off topic. I also suspect a connection between this phenomenon and smell (olfaction), and since Captain Panic & John Cuthber had talked about the smell of rust last year... I thought the reference might stimulate some more exchanges.

 

Sorry to be so oblique (...thought you guys could read minds, eh?); I'll try to stay more "on topic" in the future. Thanks for your work here.

 

~