Dr. Beaker
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an inorganic chemistry mystery
Hi all,
I've tried looking for a textbook like mechanism to the following reactions:
1. MCl4 + 2Li+Me2N(CH2)2NMe- -----> 2LiCl+ M(Me2N(CH2)2NMe)2Cl2
M = group IV trans. metal
2. M(Me2N(CH2)2NMe)2Cl2 +2MeLi*LiBr -----> 2LiCl+2LiBr+M(Me2N(CH2)2NMe)2Me2
I think it's salt metathesis but it's like a magic word since I don't know how it works...
I've saw the sigma bond metathesis:
http://www.ilpi.com/organomet/sigmabond.html
but I think it's different here.
also note that in the 1st reaction the metal goes from tetrahedral to octahedral, and that we only get the cis isomer (90 deg. between 2 Cl, and the
same between 2 Me ligands.
any suggestion and prefferably answers will be greatly apreciated.
[Edited on 25-10-2006 by Dr. Beaker]
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Dr. Beaker
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well for the 1st one its association and then dissociation of Cl-. the 2nd one is still unknown.
I wonder if anyone here really knows chemistry, or just all stuck in the chemistry children'd kit.
(wow, it's so cool to mix sugar and KClO3, or to freez a frog with liquid nitrogen, or to waste 3 days to make some silly solvents you can buy by the
gallons for few dollars.)
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The_Davster
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Some thoughts,
The first one, the Li reduces the MCl4 to MCl2, which leaves more room for that ligand(no idea what its name is) to bond to the metal centre.
Second one, the methyllithium essentially acts as a CH3- which is a stronger nucleophile than Cl-, so it can easily boot out the chloride
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Dr. Beaker
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thanx for the answer , however:
in the 1st one it's a Li salt of trimethyl ethylene diamine, so no redox chem. there.
1. in the 2nd one you need to make MeLi dissociate to Li+ and Me- which is unlikely in ether which has low dialectric const. on the other hand, I read
that Li salt chem. is governed by association dissociation...
2. if you consider the inorganic equivalent of SN2 ( SN1 is unlikely to the early electrophilic trans. metals) then you need a vacant orbital for the
4 centered concerted sigma bond metathesis. but what atomic orbital is available in an octahedral field for that?
see my dilema?
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guy
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I dont see why it cant be an SN1 equivalent. Cl- are constantly in equilibrium, moving in and out of bonding. When it is off, the methyl groups
replace it since it is more stable.
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Dr. Beaker
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if you want something to be ionized you need the ions to be stabilized in solution.
ether, being a lewis base does not do a good job in solvating anions. and also, group IV metals at the IV oxidation state are HIGHLY electrophilic.
ie:
TiCl4+Cl- -----> TiCl5-
TiCl4+Et2O -----> TiCl4*Et2O (etherate)
but TiCl4 ----> TiCl3+ + Cl- is unlikely
(equivalent to HCl ---> H- + Cl+)
or in other words - TiCl4 is a too strong lewis acid to give away a cloride. (which is a bare ion in ether!)
[Edited on 29-10-2006 by Dr. Beaker]
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The_Davster
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| Quote: | Originally posted by Dr. Beaker
in the 1st one it's a Li salt of trimethyl ethylene diamine, so no redox chem. there.
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"2Li+Me2N(CH2)2NMe-"
I am not sure how your statement is compatible with the reaction, Could you elaborate a bit/draw this out? Or is one of the reactants 'Li+', and not
lithium metal, I read it as Li(0) + the ethylenediamine stuff. So it is just MCl4 + LiMeNC2H4NMe2? If I just read the reaction badly, couldent it
simply be a metathesis driven by the insolubility of one of the products in the given solvent?
For 2, assuming suitable solvent, the chloride could dissociate from the metal centre allowing Me- to jump in. But the given solvent ether is bad for
this. I do not see why >octahedral geometry could not be formed here, is an empty orbital really necessary? In regular SN2, there is no empty
orbital around.
Ah, ethylenediamine derived ligand, the (CH2)2 threw me off and I was thinking all sorts of weird stuff. I also did not notice the negative charge
initially..
[Edited on 29-10-2006 by The_Davster]
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guy
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| Quote: | Originally posted by Dr. Beaker
if you want something to be ionized you need the ions to be stabilized in solution.
ether, being a lewis base does not do a good job in solvating anions. and also, group IV metals at the IV oxidation state are HIGHLY electrophilic.
ie:
TiCl4+Cl- -----> TiCl5-
TiCl4+Et2O -----> TiCl4*Et2O (etherate)
but TiCl4 ----> TiCl3+ + Cl- is unlikely
(equivalent to HCl ---> H- + Cl+)
or in other words - TiCl4 is a too strong lewis acid to give away a cloride. (which is a bare ion in ether!)
[Edited on 29-10-2006 by Dr. Beaker] |
Don't the other ligands stabilize the cation making SN1 more likely?
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Dr. Beaker
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Davster,
yup, it's just like LiNH2 but replace the 2 Hydrogens with a methyl and a Me2NCH2CH2 moieties.
I'm sticking to "salt metathesis since in all the papers I read about the replacement of a labile (say Cl-) ligand with less labile ligands they say
something like " the synthesis of complex ML2Me2 was achived through salt metathesis of ML2Cl42 and 2 equiv. of MeLi*LiBr". so I must assume it to be
the mechanisem and not a uni or bi mulecular substitutions. but through what mechanism this occur?
if L is a bidentate ligand and ML2X2 is of octahedral geometry then the only vacant low energy orbitals are the nonbonding dz2 and dx2+y2 that are 45
deg to the ZXY axes. the TS of sigma bond metathesis is something like:
M--------------Cl
-גגגגגגגגגגגגגגג-
-גגגגגגגגגגגגגגג-
-גגגגגגגגגגגגגגג-
Me-------------Li
(sorry for the bad drawing capabilities)
i.e bonds break and created simultaniously and ther MUST be a vacant orbital 90 deg ("cis") to the M-Cl to be replaced (where the L with it's
electrons come).
and you're right in that the insolubility of the Li halide pushes the reaction to the dimethyl complex.
Guy,
there could be more than 1 path to a reaction, but possibly the ionization is too energetically unfavoured in compaire to metathesis.
[Edited on 29-10-2006 by Dr. Beaker]
[Edited on 29-10-2006 by Dr. Beaker]
[Edited on 29-10-2006 by Dr. Beaker]
[Edited on 29-10-2006 by Dr. Beaker]
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