Calculation of Ligand Dissociation Energies in Large Transition-Metal Complexes

The
accurate calculation of ligand dissociation (or equivalently,
ligand binding) energies is crucial for computational coordination
chemistry. Despite its importance, obtaining accurate ab initio reference data is difficult, and density-functional methods of uncertain
reliability are chosen for feasibility reasons. Here, we consider
advanced coupled-cluster and multiconfigurational approaches to reinvestigate
our WCCR10 set of 10 gas-phase ligand dissociation energies [J. Chem. Theory Comput. 2014, 10, 3092]. We assess the potential multiconfigurational character of
all molecules involved in these reactions with a multireference diagnostic
[Mol. Phys. 2017, 115, 2110] in order to determine where single-reference coupled-cluster
approaches can be applied. For some reactions of the WCCR10 set, large
deviations of density-functional results including semiclassical dispersion
corrections from experimental reference data had been observed. This
puzzling observation deserves special attention here, and we tackle
the issue (i) by comparing to ab initio data that comprise dispersion
effects on a rigorous first-principles footing and (ii) by a comparison
of density-functional approaches that model dispersion interactions
in various ways. For two reactions, species exhibiting nonnegligible
static electron correlation were identified. These two reactions represent
hard problems for electronic structure methods and also for multireference
perturbation theories. However, most of the ligand dissociation reactions
in WCCR10 do not exhibit static electron correlation effects, and
hence, we may choose standard single-reference coupled-cluster approaches
to compare with density-functional methods. For WCCR10, the Minnesota
M06-L functional yielded the smallest mean absolute deviation of 13.2
kJ mol–1 out of all density functionals considered
(PBE, BP86, BLYP, TPSS, M06-L, PBE0, B3LYP, TPSSh, and M06-2X) without
additional dispersion corrections in comparison to the coupled-cluster
results, and the PBE0-D3 functional produced the overall smallest
mean absolute deviation of 4.3 kJ mol–1. The agreement
of density-functional results with coupled-cluster data increases
significantly upon inclusion of any type of dispersion correction.
It is important to emphasize that different density-functional schemes
available for this purpose perform equally well. The coupled-cluster
dissociation energies, however, deviate from experimental results
on average by 30.3 kJ mol–1. Possible reasons for
these deviations are discussed.