Chapter 7

Theoretical Prediction of Bond Dissociation Energies for Transition Metal Compounds

and Main Group Complexes

with Standard Quantum-Chemical Methods

Nikolaus Fröhlich and Gernot Frenking

Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein Strasse,

D-35032, Marburg, Germany

1. INTRODUCTION

In the last decade, quantum-chemical investigations have become an integral part of modern chemical research. The appearance of chemistry as a purely experimental discipline has been changed by the development of electronic structure methods that are now widely used. This change became possible because contemporary quantum-chemical programs provide reliable data and important information about structures and reactivities of molecules and solids that complement results of experimental studies. Theoretical methods are now available for compounds of all elements of the periodic table, including heavy metals, as reliable procedures for the calculation of relativistic effects and efficient treatments of many-electron systems have been developed [1, 2] For transition metal (TM) compounds, accurate calculations of thermodynamic properties are of particularly great usefulness due to the sparsity of experimental data.

For many years, our group has been using quantum-chemical methods to calculate properties of TM compounds. During the last decade, we focused on theoretical analysis of chemical bonds in TM compounds [3-7] and on elucidation of mechanisms of TM-mediated reactions [8].

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J. Cioslowski (ed.), Quantum-Mechanical Prediction of Thermochemical Data, 199–233. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.

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Initially, the level of theory that provides accurate geometries and bond energies of TM compounds, yet allows calculations on medium-sized molecules to be performed with reasonable time and CPU resources, had to be determined. Systematic investigations of effective core potentials (ECPs) with different valence basis sets led us to propose a standard level of theory for calculations on TM elements, namely ECPs with valence basis sets of a DZP quality [9, 10]. The small-core ECPs by Hay and Wadt [11] has been chosen, where the original valence basis sets (55/5/N) were decontracted to (441/2111/N- l l ) with and 3, for the first-, second-, and third-row TM elements, respectively. The ECPs of the second and third TM rows include scalar relativistic effects while the first-row ECPs are nonrelativistic [11]. For main-group elements, either 6-31G(d) [12-16] all electron basis set or, for the heavier elements, ECPs with equivalent (31/31/1) valence basis sets [17] have been employed. This combination has become our standard basis set II, which is used in a majority of our calculations [18].

Early theoretical work in our group was based on ab initio methods at the Hartree-Fock and correlated levels of theory. Geometries were optimized at the HF/II and MP2/II levels of theory while energies were calculated at the MP2/II and CCSD(T)/II levels. Systematic investigations showed that the HF/II level of theory yields accurate geometries for TM compounds in high oxidation states, whereas its MP2/II counterpart provides reliable geometries of donor-acceptor complexes with metals in low oxidation states [18]. Bond energies calculated at the CCSD(T)/II // HF/II and CCSD(T)/II // MP2/II levels of theory were found to be in excellent agreement with experiment. Bond energies predicted at the MP2/II level are systematically too high but the trends are usually correct. It was shown that the MP2/II level of theory can produce accurate energies when isostructural reactions are employed, leading to error cancellation in the intrinsic deviations of the MP2 results from the CCSD(T) ones [19]. However, it was also found that MP2 calculations on compounds of the first TM row elements, which have partially filled 3d shells, frequently yield erroneous results [18].

In the mid-1990s we began to use DFT methods for the calculation of TM compounds. BP86 [20] and B3LYP [21] functionals proved to yield the most accurate results in terms of both geometries and energies. A systematic comparison with our previous ab initio work showed that in many cases BP86 and B3LYP provide bond energies and geometries that are at least as good as those obtained with the MP2 approximation. Compared to the MP2 approach, the DFT methods are significantly more robust for calculations on compounds containing first TM row elements and require much less computer time. This finding

BDEs of Transition Metal Compounds and Main Group Complexes

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allowed us to replace MP2/II with BP86/II and B3LYP/II as our standard levels of theory for calculations on TM compounds. Fortunately, our standard basis set II proved to yield accurate results in conjunction with the DFT methods as well. However, we do not recommend an indiscriminate use of DFT methods, in particular when bond energies are to be calculated without a critical examination of the reliability of the predictions. Like any approximate theoretical method, DFT can produce erroneous results. The drawback of DFT is that it is difficult to predict for which species excessive errors might be anticipated. Unlike ab initio methods, for which the reason of failure can be analyzed in terms of basis set insufficiency and/or inadequate correlation treatment, and where systematic improvement of the theoretical level is possible, DFT methods are much more a black-box tool. The results which are given in this chapter show that sometimes even the trends predicted by the DFT are wrong. For this reason, calculations on a few reference compounds have been carried out at the CCSD(T) level of theory in order to provide reliable data for calibrating the DFT results in cases where no experimental data are available.

In the last decade, more than one hundred theoretical studies of TM compounds have been carried out, addressing questions concerning the geometries, stabilities, reaction mechanisms, chemical and physical properties, and the bonding situations in a wide variety of molecules [1-7, 18, 22]. Many publications reported bond dissociation energies (BDEs) of TM–ligand bonds. As mentioned above, very few reliable experimental BDEs of TM compounds are available in the literature [18, 22]. Because the theoretical predictions for BDEs at the CCSD(T)/II level of theory were found to be very accurate, they have been used to calibrate the data obtained at the MP2/II, BP86/II and B3LYP/II levels. Therefore, these BDEs can be regarded as a valuable source of thermodynamic data on TM compounds. Since the theoretically predicted BDEs have not been previously summarized, in this chapter we provide a compilation of the calculated data which should be helpful as a reference. Although the majority of our work focused on TM compounds, we also calculated BDEs of main group complexes of the group-13 Lewis acids and BeO with various Lewis bases. A list of these theoretically predicted values is included in this work.

As mentioned above, most calculations were carried out with our standard basis set II. The compilation of results in the present form makes it possible to compare a great variety of fairly large compounds studied at the same level of theory. We are well aware of the fact that the apparently high accuracy of the theoretical values is partly due to a fortuitous error cancellation. Thus, when it was necessary to obtain more

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accurate data for a given project, we employed the Stuttgart ECPs that are associated with larger basis sets and include scalar relativistic effects also for first TM row elements [17, 23]. This is particularly important for copper, because relativistic effects have a significant impact on the calculated geometries and BDEs of its compounds [24-28].

We want to emphasize that other groups have also published accurate quantum-chemical calculations of TM–ligand bond energies. The fact that we do not include their results in the present compilation does not imply that we consider them less accurate than ours. Theoretical studies that predict BDEs of TM compounds at a much higher level of theory than those employed in our work have been published but the molecules studied have been smaller than the species listed in this chapter. Here we intend to compare data calculated at a level of theory that can be applied to larger molecules and that has been used for a large number of different classes of compounds. A uniform level of theory makes it possible to directly compare the strengths of bonds between different ligands and different metals. We are planning to create a data bank which contains all of the calculated BDEs. This continuously updated data bank will be available online.

The presentation of the theoretical BDEs is organized as follows.

We list the	calculated	values and their zero-point energy corrected
counterparts	(denoted by	) for TM compounds that belong to different

classes of molecules. This leads in some cases to double presentation, as some species belong to more than one class. However, we believe that the ordering chosen here facilitates comparison between compounds as well as between methods in a balanced way.

Most calculations were carried out with the Gaussian 94 [29], Gaussian 98 [30], Turbomole [31], ACES II [32] and MOLPRO [33] program packages. We also used the DFT program ADF [34], which is different from the other electronic structure software as it uses Slater-type orbitals rather than Gaussian functions. The calculations of the TM compounds with ADF were carried out at the BP86 level of theory with a numerical TZP-quality basis set. Relativistic effects were calculated either with the Pauli Hamiltonian [35] or, recently, with the more reliable ZORA approximation [36]. Details about the computational procedures can be found in the original publications.