[nell]

hi all
does anyone know why SnCl2 is a weaker Lewis acid than ZnCl2? and how I can get the general postulate for another metal halides?
thanks

[CaverKat]

Since both have an oxidation of II, then it may be because Sn is more electronegative than Zn (1.96 vs 1.65).
Could possibly have something to do with the difference in electronegativity of the metal compared to the halide.  Not sure.

...not even considering sigma bonding character and orbital overlap.

[orgopete]

hi all
does anyone know why SnCl2 is a weaker Lewis acid than ZnCl2? and how I can get the general postulate for another metal halides?
thanks

I did not answer in hopes someone more knowledgeable would. As inexpert as I am, I shall give an answer. First off, I certainly don't know of any general postulate or if there is one. I would also worry that compound specifics might be more important than a general postulate.

As has been noted in this forum, LiBH4 is a better reductant than NaBH4 because Li+ is a better Lewis acid. The Lewis acidity of a lithium cation increases the rate of borohydride reduction. However, if we were to try to estimate the Lewis acid strength of LiCl, BeCl2, BCl3, and CCl4, we could miss how their chemistry changes through this short series. LiCl is a Lewis acid because it forms a lithium cation. BCl3 is a Lewis acid because it can accept a pair of electrons. CCl4 is not a Lewis acid because it cannot readily ionize to form a similar carbocation.

If I were to compare BCl3 with PCl3, they share a similar difference as ZnCl2 and SnCl2. PCl3 and SnCl2 are in the next period and two groups over. Now I am going to really jump off the cliff. I think ZnCl2 is a Lewis acid because it forms Zn++. SnCl2, if it similarly formed a cation, it may be less acidic because it has an extra pair of electrons. We also know the affinity of SnCl2 for this extra pair of electrons is low as it is also a reductant. We may infer than ionization of SnCl2 to Sn++ may not have a high affinity for electrons from its willingness to donate a pair of electrons. We could expect that SnCl4 should be a much stronger Lewis acid than SnCl2. Since ZnCl2 and SnCl2 can both form divalent cations, we may infer the extra pair of electrons on tin can reduce its effective Lewis acidity.

BCl3 is a much better Lewis acid because it can accept a pair of electrons to achieve an octet. Phosphorous, if it accepts a pair of electrons, will exceed its octet. PCl3 reacts with Cl2 to form PCl5. I presume that PCl3 acts as a nucleophile to form PCl4+ and then accepts a pair of electrons to form PCl5. We can write an SN2-like mechanism for PCl3 with an alcohol or we can have it act for first form a pentavalent anion intermediate in an addition-elimination mechanism.

We probably know more about the chemistry of boron and phosphorous than tin and zinc. However, I am fearful of how little I know about how zinc or tin actually react. I could use a different mechanism to explain contrary results.