Hi,

In short, I think it could be explained by the difference in electronegativity. The bond between a soft Lewis acid and a soft Lewis base would be covalent and the softest acids have the highest electronegativities.

In your case, Yb appears to have an electronegativity around 1.1-1.2 whereas the value for Zn is 1.65-1.8 ,depending on which source(s) you are using. This would mean Zn is a softer acid than Yb (Zn is actually considered as "borderline" case according to Pearson hard-soft acid base theory...something between a soft and a hard acid).

This is not straight forward, though, because for certain metals, the different oxidation states belong to different groups (e.g., Fe2+/Fe3+ or Ir+1/Ir+3).

There's my 2 cents.

Regards,

JR

Slide #1, Lecture #12 in the link below

http://uncw.edu/chem/Courses/Hancock/CHM445/Course%20Notes.htm

Even though I liked Raitanen responce, Plz. see

---http://www.chemvista.org/hardsoftacidsbases.html

---faculty.uml.edu/.../Hard-Soft%20Acid-Base%20Theory.ppt

Good luck

Thanks so much Jan-Erik and Adel, it has been very helpful.

Situation is not so simple as it was reported by collegues. Hardness is depend on anion (polarizability), so in some cases Yb(+3) is borderline Lewis acid like Zn(+2), see link in attachment.

If the valence of two atoms is same,one can be applied with good approximation an comparison of electronegativity as pointed by Jan-Erik Raitanen.

But in this case, Yb has a valency of three, and zinc - two. Therefore it is not very clear how much of electron density will be pumped by anions on central atom.

In contrary to Jan-Erik Raitanen lecture citation, Cs(+) is SOFT acid and In(+3) is HARD acid, it can be checked by original articles citations.

http://books.google.ru/books?id=cEsmRxs41uUC&pg=PA10&lpg=PA10&dq=soft+hard+lewis+acids+yb&source=bl&ots=7fzgC3Vjf1&sig=hgJBhmbzSQROJO3D88YL8MQuA6g&hl=ru&sa=X&ei=t6HTUvTvNMXc4QS7h4DwCQ&ved=0CGcQ6AEwCQ#v=onepage&q=soft%20hard%20lewis%20acids%20yb&f=false

In the site I recommended clearly pointed out what Andrei mentioned Zn II and Yb III cannot be directly compared from the table presented in lect. but ~ calculation on

Absolute Hardness:

The average of the difference between the ionisation energy and electron affinity is termed as Absolute Hardness (η)

η = (I-A) / 2

The reciprocal of η is termed as Absolute Softness (σ), ARE GIVEN

You can get the I and A values from google search, e.g.

---http://www.lenntech.com/periodic-chart-elements/ionization-energy.htm

Good luck

I agree, the question is not easy. I´m triying to resolve the different behavior of Zn(OTf)2 and Yb(OTf)3 in ring opening reactions with TMSN3

I thought that a good starting point was to compare the properties of both cations as Lewis acids.

Hence my question

@Carlos Vivar

I can not find a direct comparison in the literature. However, an indirect comparison may be possible, if you have access to follow articles. In the first study one described evaluation of the relative Lewis acidities of lanthanide triflates, in the second - cerium and zinc triflate.

http://pubs.rsc.org/en/content/articlelanding/1999/cc/a905569j/unauth#!divAbstract

http://www.rsc.org/suppdata/cc/c0/c0cc02673c/c0cc02673c.pdf

If you trying receive practical results with epoxide ring-opening, you should note that system is not optimal. Best results in terms of yield and regioselectivity was obtained with LiClO4 (hard Lewis acid) as catalyst and NaN3/H2O (p.2277 file in attachment)

From this result can be planned direct experiment about influence of hardness.

LiClO4 is explosive in hydrocarbon organics, so it is better to replace on LiNTf2.By addition of metal chlorides you can get various M(NTf2)n

Dear Andrei

thanks for your interest and support. It has been very helpful. Thanks for the review and links.

This query is not loaded correctly. Hard/soft to what? At example, for oxygen C=O in carbonyl compounds is Yb3+ softer (feed-back between d-orbitals of lanthanide cation and pí*-orbital C=O) than Zn2+, for oxygen R-OH, ethers etc. it is vice versa.

Pavel is absolutly right, the hardness or softness are relative similarly to the acidity or oxidative effect, strongly depends on the softnes sor hardness of the reaction partner.

@Pavel Pazdera and László Kótai

Carlos Vivar question was about ring-opening of oxygen-containing heterocycle (oxirane?) - so about ether-type oxygen.

Anyway, feed-back between d-orbitals and pí*-orbital C=O - it is about carbonyl complexes, M(CO)n type.

Dear Andrei,

I have done search on Yb..O=C type complexes, and Yb(3+)...O(ether) type complexes, after reading the found papers I will inform you whether it is or not information about this type of interaction in these. Please give me some time because in the next days I have to finish a report, and ca. 10-20 papers should collect and read.

Dear Andrei,

I could not find work about studying the back-coordination in the Yb..O=C or Yb-O(ether) systems, although the bond lengths changes in various complexes should be compare to disclose it completely. In the Eur. J. Inrog. Chem. 2012(22), 3541-3545 is a paper, in which a special coodination of polyethers is given to Yb(III), the luminescence properties showed an unusual change of the coordination environment. Perhaps a similar system would be interesting to study the presence/absence of back-coordination.

Dear Andrei,

As you wrote it, the question is opened. I think, comparison of C=O bond length (bond order) in the complexes with the same ligand with various metals (C=O....Me coordination with the same geometry, etc.) could give some information about it - together with the parameters suggested by you. I will think about how I could get data from CCDC.

with best regards

László

László, I have catch downvote for my previous message without any explanation from my "invisible personal RG enemy" :-) , so suggest to transfer our discussion in personal communication if it nedeed.

Some people can made "ideas crap" without literature support and next downvote and go unpunished - it is sad reality of RG :-(

Dear Andrei,

I am sorry for this, my personal mail is [email protected]. Please send the messages to this mail and we can discuss about this topics without qualifications of our discussion.