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203 |
2.HOMOLEPTIC CARBONYL COMPLEXES
The number of TM carbonyl complexes for which experimental BDEs are known is relatively large. Theoretical studies of neutral 18valence electron carbonyl complexes of groups 6, 8 and 10 have been reported [37-42]. Table 7.1 lists the calculated and experimental values of 
and 
In most cases, the computed values of 
are in a very good agreement with experiment. Although the calculated BDEs of 
and 
are higher than their experimental counterparts, a closer study of the latter suggests that the reported values may be too low. Inspec-
tion of Table 7.1 leads to the conclusion that the performance of the BP86/II and B3LYP/II levels of theory is very good as the DFT BDEs are close to their CCSD(T) counterparts. The MP2/II data are always too high but the trends in the calculated BDEs are correct. Note that the MP2/II level of theory yields particularly high BDEs for 
and . These erroneously high values are representative of the problems that are often encountered when first TM row compounds are treated at the MP2 level of theory.
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Chapter 7 |
Table 7.2 lists the calculated and experimental BDEs of two series of charged carbonyl complexes [39, 40, 47]. The first series comprises positively and negatively charged hexacarbonyls 
that are isoelectronic with 
There are no experimental bond energies available for 
The B3LYP/II and BP86/II results are very similar to their CCSD(T)/II counterparts while the MP2/II level of theory always yields values of BDEs that are too high. The second set of data consists of theoretical and experimental results for the positively charged group11 carbonyls 
(
Ag, Au; 
). The valence basis sets used for the latter metals were much larger than those employed for the hexacarbonyls. Another difference is that the group-11 cations have a completely filled 
shell. The bonding in these compounds has only a negligible 
while backdonation is important in the hexacarbonyls. This different bonding situation leads to an altered performance of theoretical methods. As expected, the CCSD(T) values are in good agreement with experiment. The MP2 values are also quite
BDEs of Transition Metal Compounds and Main Group Complexes |
205 |
accurate. The BP86 and B3LYP functionals yield bond energies for the monocarbonyls 
that are much higher than their MP2 and CCSD(T) counterparts. Even more troublesome is the fact that the DFT methods sometimes predict a wrong trend for the BDEs of monoand dicarbonyls. The BP86 and B3LYP functionals predict the BDE of 
to be lower than that of 
while MP2 and CCSD(T) levels of theory yield the opposite result, in agreement with experiment.
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BP86 also fails to predict the relative BDEs of 
and 
The B3LYP approach produces a higher bond energy of 
as compared with 
but this difference is much smaller than that predicted within the CCSD(T) approximation.
3.GROUP-6 CARBONYL COMPLEXES (M = Cr, Mo, W)
Singly substituted species 
have been investigated [4951, 54, 55]. Table 7.3 lists the calculated BDEs of the group-6 complexes of the type 
with various ligands L, whereas Table 7.4 contains the BDEs for the W–L and the W–CO bonds in 
complexes. The latter values are given for the least bonded carbonyl ligand. Note that BDEs for other 
complexes with certain particular classes of ligands are discussed in other sections of this chapter.
Table 7.3 lists only few experimental data that can be used to estimate the accuracy of the theoretical results. The CCSD(T)/II values agree quite well with experiment (note, however, rather large error bars for the measured BDEs of the thiocarbonyl complexes). The MP2/II values are always larger than their CCSD(T)/II counterparts. The latter values show that the tungsten complexes always have the strongest M–L bond while, in most cases, the molybdenum species have the lowest BDEs.
Table 7.4 lists BDEs calculated with both ab initio and DFT methods. The B3LYP/II values for the 
and 
bond energies are in very good agreement with their CCSD(T)/II counterparts. However, the results yielded by the two methods differ in the relative bond strengths of acetylene and ethylene in the 
complexes. The B3LYP/II level of theory predicts ethylene to be less bonded than acetylene while CCSD(T)/II and MP2/II levels yield the opposite result. The unstable 
and 
species have been detected experimentally by IR spectroscopy [56]. The decrease in the C–O stretching frequency of the trans CO ligand was found to be significantly larger for the former complex. This observation indicates that the tungsten–acetylene interactions are stronger than the tungsten– ethylene ones, which is in agreement with the larger BDE of 
predicted at the ab initio levels of theory. The only experimental BDE given in Table 7.4 is for the weakly bonded complex 
. It agrees well with the CCSD(T)/II result.