Subject: MM and the metals

From: [email-masked]
Date: Tue, 04 Jan 1994 13:02:02 -0600 (CST)

A nice and recent review on the application of molecular
mechanics to d- and f-block complexes can be found in the paper by
Ben Hay (Coord. Chem. Rev. 1993, 126, 177 - 236).  There are also refs to
earlier reviews and work.  Hay also gives good pointers on the development
and choice of a force field.

        One of the projects in my group is the use of MM to design of
specific chelates for f-block metal ions.  Based on our research we
have found the biggest hurdle is bond stretching parameters for
metal-ligand bonds.  In a preliminary work (Inorg. Chem. Acta 1993, 209, 101)
we used MACROMODEL and a simple electrostatic model to describe the metal-
ligand intrxn. This approach has the advantage that it is simple (i.e., cuts 
down on the number of parameters).  Also, since the bonding in lanthanides
is highly ionic it makes sense chemically.  We have switched to a "traditional"
approach (modeling M-L with a Hooke's law potential) similar to that in
Hay's paper (Inorg. Chem. 1991, 30, 2876.)

       Another area of concern is the numerous torsional potentials
involving the metal.  One approach that has been used is to see all
torsions about M-L bonds to zero; the geometry of the complex will then
be determined by that which gives the most favorable non-bonded interactions.
We use this approximation and can usually predict the tetrtiary structure of 
complexes quite well.   

        One final area of concern is ligand-metal-ligand angles.  As one
goes to very high coordinatyion numbers these increase rapidly.  This
is further complicated by the fact that higher coordination geoms. often have
inequivalent coordination sites (e.g., axial and equatorial in a
trigonal bipyramid) each of which needs a different equilibrium angle and
force constant.  One approach I have seen is to replace the L-M-L potential
for atoms with large coordination numbers with a potential that
describes the 1,3-non-bonded interactions between the ligands.  
To our way of thinking, such an approach is simiar to the excellent
 (and in my opinion, underappreciated) points-on-a-sphere model,
with which Kepert has done some seminal work on the coordination
chemistry of transition metals .

      Finally, we typically assume that parameters which only involve the
ligand  do not change upon coordination.  

	We are using the  MM2 force field, as modified in Chem 3D+, since it 
is  relatively quick, flexible (a true must), and provides a nice graphical user
interface on a Quadra.  These views are based on the specific needs of our 
MM/f-block research which is to  model complexes quickly and give the 
experimental folks in the group some insight on ligand modifications which we 
can target.  Any pointers,and experiences from other folks working on MM for 
metal complexes would be of interest to us.

				Tom Cundari
				Asst. Professor
				Dept. of Chemistry
				Computational Inorganic Chemistry Lab
				Memphis State University
				Memphis, TN 38152
				phone: 901-678-2629
				fax: 901-678-3447