Essentials of Computational Chemistry

300 8 DENSITY FUNCTIONAL THEORY
double-ζ basis set on C, H, and O, and a valence basis set of similar size for I and the metals
combined with relativistic effective core potentials. Happily, from a simplicity standpoint,
all species are predicted to be ground-state singlets by large margins, so a restricted DFT
formalism can be employed. In this instance, some experimental data are available for
species involved in the reductive elimination step, so the adequacy of the theoretical level
can be evaluated.
The authors begin by characterizing the relative energies of all possible stereoisomers in
the octahedral complexes. For acetyltriiododicarbonyl metal complexes, there are mer,trans,
mer,cis, andfac,cis possibilities (mer implies two iodides to be trans to one another while
fac implies all I–M–I bond angles to be about 90 ◦ ; trans and cis refer to whether the
central iodine atom in the mer arrangement is opposite the acetyl group or adjacent to it)
as well as acetyl rotamers to consider. For both rhodium and iridium, a single mer,trans
geometry is predicted to be lower than all other possibilities by at least 2.7 kcal mol −1 .
Experimental IR and NMR data for the Rh system are in accord with this prediction, while
IR data for the Ir system suggest the presence of fac,cis, which is the next lowest energy
species predicted from the computations. Kinnunen and Laasonen suggest that weak IR
bands for the mer,trans isomer may make it difficult to detect experimentally, and infer
that it is possible that both may be present experimentally.
Of course, while the intermediate energies are of interest, so long as interconversion
between stereoisomers takes place at lower energy than reductive elimination, the latter
process may potentially go through any stereoisomer on the way to the lowest energy TS
structure for the reaction (the Curtin–Hammett principle). For the Rh system the lowest
energy TS structure, which follows from a mer,cis reactant, has an associated 298 K free
energy of activation of 20.1 kcal mol −1 , which compares well with an experimental value
of about 18. In the case of Ir, a fac,cis TS structure is computed to be slightly lower
than the mer,cis structure, and the overall free energy of activation is about 8 kcal mol −1
higher than was the case for Rh. In both cases, iodide dissociation is predicted to proceed
with a lower barrier than reductive elimination, so stereoisomer scrambling via elimination/addition
should be possible prior to reductive elimination.
Kinnunen and Laasonen carry out a similarly thorough analysis for the diiodotricarbonyliridium
case. Consideration of all possibilities is complicated (and will depend experimentally
on the iodide ion concentration and carbon monoxide pressure) but in essence ‘all’
stationary points corresponding to stereoisomeric minima and transition state structures for
dissociation/association and reductive elimination steps are found and characterized energetically
(‘all’ in quotes here because in such complicated systems it is essentially impossible
to be entirely certain that every stationary point has been found). This exhaustive mapping
of the PES provides insight into the catalytic process in a fashion typically not available
experimentally, and takes good advantage of DFT’s ability to handle transition metal
systems in an efficient manner.
Bibliography and Suggested Additional Reading
Adamo, C. and Barone, V. 1998. ‘Exchange Functionals with Improved Long-range Behavior and
Adiabatic Connection Methods Without Adjustable Parameters: The mPW and mPW1PW Models’,
J. Chem. Phys., 108, 664.