Subject: Summary: criteria for agostic interactions

From: [email-masked] (Eilert Ofstead)
Date: Thu, 13 Jul 1995 09:49:17 -0500

Dear netters;

Recently I posted the following question:

>Are there criteria which define/verify the presence of an agostic H---metal
>interaction?
>
>I am mainly interested in organometallics where the H is not on C-alpha.

I have enclosed the replies to this question at the end of this note.  My
thanks to all who responded!  I was mainly interested in looking at this
issue for structures generated in modeling studies, where experimental data
are not available.

Well-defined agostic interactions appear to be easily recognized from a
simple examination of the geometry of the structure in most cases.
Unfortunately, I suspect that I am dealing with systems where this effect
is very marginal, yet may be important.  There's the rub.  How do you
quantify these weak interactions?

I also came across a method which is based on second-order perturbation energy
estimates (Kaufmann et al, Organometallics, v7, 1597, 1988).  This method
seems to allow one to obtain quantitative estimates of these non-bonded
interactions.  Any comments on this technique?  More importantly, can this
method be invoked using Spartan software, for example? (forgive my novice
state of ignorance)

REPLIES =====================================

>
>there are several diagnostic spectroscopic criteria:
>
>1) 13C-NMR: Low C-H coupling constant may be for instance lowered to
>70 Hz for a sp2-C-H carbon atom
>
>2) IR: C-H stretching frequency in the range of 2600-2800 cm-1
>
>also: selective Deuteration of the particular C-H bond helps to establish
>2) and also allows to distinguish between an agostic interaction and an
>equilibrium between the C-H elimimation product (gives a metal hydride and
>an olefin in the case of a beta-hydrogen atom) and the metal alkyl complex
>(equilibrium isotope pertubation). For details of the latter check
>the following review by M. Brookhart, M.L.H. Green, Progress on Inorganic
>Chemistry (1988?) - I don't have the exact reference handy but I can find
>it for you if necessary.
>
>Further methods are:
>
>X-ray and Neutron diffraction: Grounds: M-H distance and M-C-H (alpha) and
>C-C-H (beta) angle arguments. Care should be taken however in the
>interpretation of X-ray crystal structures due to the inherent problems of
>this method to locate the "true" proton position.
>
>Alpha vs Beta vs .. There are quite a number of examples for both - lately
>even also Gamma-H agostic interactions have been discussed in the context
>of early transition metal Ziegler-Natta polymerizations with metallocene
>based catalysts. Also alpha (evidenced) and beta agostic interactions may
>play a role in this catalytic system.
>You'll find a pretty good discussion on agostic interactions in
>Bob Crabtree's monograph: The Organometallic Chemistry of the Transition
>Metals, 2. ed, Wiley Interscience, (1994/3). Hope this helps, let me know
>if I can be of further assistance
>
>Peter
>_________________________________
>Peter Burger
>Anorg.-Chem. Institut
>Universitaet Zuerich
>Winterthurerstr. 190
>8057 Zuerich
>Switzerland
>PHONE:+41 1 257 4692 and 4677 (work)
>      +41 1 252 1793 (home)
>FAX:  +41 1 364 0191
>E-mail:[email-masked]

*********************************************************

>
>A few years ago, we described (Mealli and Proserpio, Comments Inorganic
>Chemistry, 9, 37, 1989) how the agostic interactions can play a role for the
>stability of certain Re3 clusters. The arguments were based on some careful
>considerations of the stereochemistry of the non-hydrogen substituents
>at the alkyl groups suspected to give rise to the agostic interactions.
>Further support to the hypothesis came from the simplest MO theory arguments
>which allowed us to see some clear-cut differences between models with and
>without the agostic interactions at work.
>You may wish to take a look at that paper.
>Best regards,
>Dr. Carlo Mealli
>ISSECC, CNR
>Via J.NARDI 39, 50132 Firenze, ITALY
>e-mail: [email-masked]
>Tel. 0039-55-2346653 (243990) - Fax 2478366 - Home 4492709

*********************************************************

>
>        We have looked at agostic metal complexes and have tried several
>approaches to 'quantifying' agostic interactions.
>        One simple approach is to look at the CH bond length versus
>non-agostic models.  This is often a tough comparison since the CH bonds
>lengthen very little when participating in an agostic  interaction, which seems
>to be consistent with exptl evidence from neutron diffraction.  Likewise, if
>you can calc the IR spectrum you should see a lowering of the CH stretching
>freq for an agostic CH bond.  With GAMESS we can decompose the IR spectrum into
>normal mode contributions so you can look at the force constant of the agostic
>CH bond and compare it to non-agostic CH bonds.
>        Other things to look for are bond angles involving agostic CH bonds
>which deviate from 'normal' (i.e., VSEPPR) values.  It is not clear if this
>will be as evident when looking at carbons other than C-alpha.
>        You should also be able to plot the MOs and see if there is a
>multi-centered interaction among the agostic MHC unit.  Similarly, plotting
>the total density in the MHC plane may yield some clues.
>        Finally, in our work we have used a metric parameter defined by
>Crabtree
>and co-workers in the following paper.
>
>        RH Crabtree et al. Inorg. Chem. 1985, 24, 1986.
>
>Essentially, you need the calcd MH, MC, and CH distances, which you can plug
>into the equation given for rbp.  As rbp decreases, agostic intrxn should
>increase.  There are some exptlly determined systems to provide calibration.
>        None of these approaches are unambiguous, but together they should
>provide some insight into whether or not a substantial agostic interaction
>is present.
>
>Sincerely,
>
>Tom Cundari
>+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+==
>=
>Thomas R. Cundari                           Address from 3/15/95 -> ca. August:
>Asst. Professor of Chemistry                Prof. Tom Cundari
>Computational Inorganic Chemistry Lab       Visiting Scientist
>University of Memphis                       Cornell Theory Center
>Memphis, TN 38152                           520 Engineering & Theory Center
>Bldg
>phone: 901-678-2629                         Cornell University
>fax:   901-678-3447                         Ithaca, New York 14853-3801
>e-mail: [email-masked]             phone: (607)254-8686
>                                            office: (607)254-8621
>http://www.memst.edu/chemistry/umchem.html  fax: (607)254-8888
>                                            e-mail: same as always
>+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+==

*********************************************************

>        Reply to:   RE>CCL:verification of agostic inter
>This would be an obvious application of the "Theory of
>Atoms in Molecules" (AIM), by Richard Bader and coworkers.
>
>See the book, or the large literature on the subject.  You
>need an 'ab initio' waverfunction and the AIM programs.
>
>Richard Bone
>__________________________________________________________
>
>Richard G. A. Bone, PhD.
>Computational Chemist
>Terrapin Technologies, Inc.
>South San Francisco
>USA
>
>E-mail  [email-masked]

*********************************************************

>1/r is probably pretty good but still bad compared with explicit
>water.
>
>arne
>
>--------------------------------------------------------
>               From: Arne Elofsson
>         Email: [email-masked]
>  WWW:  http://www.doe-mbi.ucla.edu/arne/main.html

*********************************************************

Once again, thanks all.

-

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  Eilert Ofstead, Sr R&D Assoc         Research Division
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