Subject: Quantifying Agostic H--M Interactions?

From: EILERT OFSTEAD <[email-masked]>
Date: Tue, 22 Dec 1998 08:46:07 -0500

I would be interested in the comments of CCL'ers regarding preferred strategies
for evaluating organometallic compounds which may have agostic interactions
present.  In particular, I want to compare the relative stabilities of isomers
where only one has a probable agostic interaction.  Both accuracy and
efficiency are of value here, so some compromise might be expected.  My
interest includes both alkali metal and transition metal compounds.

One would expect correlation effects to be important in these weak non-bonded
interactions, so post-HF methods are probably indicated (pain! avoid if
possible!), but since some of the DFT methods supposedly include correlation, I
have explored them tentatively as well.  Scatter!!! With Li complexes, the
highest Li-H interaction **appears** to be indicated when using MP2/HF, but
when I try the more efficient LMP2 method available in Jaguar, eg, a
substantially higher stabilization seems indicated (+1 kcal. Real??).  And DFT
methods consistently give somewhat lower apparent stabilization energies,
intermediate between MP2 and straight HF. What is a poor chemist to believe.

Maybe that's life in the modeling world (at least in my short existence
there).  So, recommendations, references, etc would be much appreciated.

thanks, and best wishes for the Holidays,

Eilert
[email-masked]
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