BDEs of Transition Metal Compounds and Main Group Complexes

207

4.IRON CARBONYL COMPLEXES

A large number of iron carbonyl complexes with different ligands L in the axial or equatorial positions has been investigated [58]. Table 7.5 lists the theoretically predicted relative energies of the isomers and the Fe–L BDEs at the B3LYP/II and CCSD(T)/II levels of theory. Experimental values of these bond energies are not known. The complexes, where L is a group-13 diyl ligand, are presented separately.

The data compiled in Table 7.5 show that the relative energies of the axial and equatorial isomers yielded by the B3LYP/II level of theory are very similar to their CCSD(T) counterparts. The B3LYP/II BDEs are always larger than the CCSD(T)/II results, but the trends predicted by the two methods for different ligands are the same. It is worth noting

208

Chapter 7

that the B3LYP/II and CCSD(T)/II levels of theory agree on ethylene in being more strongly bonded than acetylene.

BDEs of Transition Metal Compounds and Main Group Complexes

209

5. GROUP-10 CARBONYL COMPLEXES

Table 7.6 lists the theoretical BDEs of the M–L bonds in the group- and complexes calculated at the MP2/II and CCSD(T)/II levels of theory [49, 50]. The only experimental value known for those compounds is an estimate of ca. 10 kcal/mol obtained for the bond energy at 298 K [59]. This estimate is based on kinetic measurements of nitrogen extrusion from the complex. Thermal corrections to the CCSD(T)/II value of kcal/mol yield a theoretical prediction of 6.7 kcal/mol, which is in a reasonable agreement with experiment [49]. The MP2/II BDEs listed in

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Chapter 7

Table 7.6 are significantly larger than their CCSD(T)/II counterparts. is much more strongly bonded at the MP2/II level of theory ( kcal/mol) than at the CCSD(T)/II level. The stronger metal–ligand bonds at the MP2/II level of theory lead to energy minima for the complexes that are predicted by the CCSD(T)/II calculations to be unstable with respect to the M–L dissociation. For example, both and are minima on the MP2/II potential energy hypersurfaces, while the calculations at the CCSD(T)/II level of theory produce negative dissociation energies [42]. The M–L BDEs of the group-10 carbonyl complexes are clearly lower than those of their group-6 analogues They are also lower than the Fe–L BDEs of

6.GROUP-6 CARBONYL COMPLEXES WITH PHOSPHANE LIGANDS

The structure and bonding of group-6 TM carbonyl complexes with phosphane ligands and have been the subjects of another theoretical study [60]. Table 7.7 lists the BDEs calculated at the BP86 level of theory in conjunction with our standard basis set II and the larger TZ(2)P Slater basis set, which

has one set of f-type	polarization functions on the transition metals and
two sets of polarization functions on the other atoms.
Except for the	complexes, the BP86/II and BP86/TZ(2)P

BDEs values are very similar. The BP86/II BDE estimates for these complexes are 4 - 6 kcal/mol higher than their BP86/TZ(2)P counter-

parts. Both levels of theory predict the trend in the	BDEs for
the different phosphane ligands being

7.NOBLE GAS COMPLEXES

A special type of TM ligands are the noble gas atoms argon, krypton, and xenon [61]. Although they are weak Lewis bases, TM complexes with and and Xe have been experimentally investigated in the gas phase as well as in the liquid phase and in supercritical The M–Ng BDEs were estimated with