Lewis acid complexes theoretical studies

The first chapter on these carbonyl-Lewis acid complexes uses information on (1) theoretical study, (2) NMR study, and (3) X-ray crystallographic study. For the rest, the subjects of chelate complexes and bidentate Lewis acid complexes, mainly featuring recent advances, are discussed. 

Theoretical Study of Carbonyl-Lewis Acid Complexes... 

Theoretical Study of Carhonyl-Lewis Acid Complexes 5... 

The nature of the donor-acceptor interactions in main group complexes has also been studied theoretically [88-91], The calculated BDEs of main-group complexes predicted at the MP2/II level of theory are in very good agreement with experimental results [88], This is an important difference from the performance of the MP2/II level of theory for TM complexes, where the computed BDEs are always too high. Table 7.18 lists the calculated BDEs for complexes of group-13 Lewis acids EX3 with various Lewis bases. 

Diacyl dications have been examined in both experimental and theoretical studies. Attempts to directly observe the oxalyl dication (174) were not successful as ionization of oxalyl chloride (170) with SbFs leads to the chlorocarbonyl cation (172, Scheme 4).65 The initially formed chloroxa-lyl cation 171 immediately decarbonylates under the reaction conditions. Interestingly, complexation of the chloroxalyl cation 171 with excess Lewis acid is expected to generate superelectrophile 173. Theoretical calculations (MP2/6-31G level) indicate that the oxalyl dication (174) is at a minimum on the potential energy surface.66 Calculated bond lengths for... 

Most of the early theoretical studies of Lewis acid catbonyl interactions focused on alkali metal cations such as Li and Na, partly due to the significance of such interactions in biological systems and partly due to computational limitations. In 1973 some of the first calculations on the formaldehyde/Li system were performed. Using both ab initio (SCF-LCAO-MO) and semiempirical (CNDO/2) methods the researchers found that in the ground state the complex possesses Civ symmetry in which Li lies on the axis of the carbonyl C—O bond with an O— Li distance of 1.77 A. 

These points prompt a few words of caution regarding the results discussed in this section and some comments on theoretical results in general. In particular, besides considerations of the size and effect of basis sets, one needs to examine the energy dependence on electron correlation. For donor-acceptor complexes, such as Lewis acid-carbonyls, electron correlation seems to be crucial. It is also instructive to consider the dipole moments of various structures in theoretical studies. Assumption of a gas phase environment may overestimate destabilization due to a large dipole, as dipoles can often be stabilized quite effectively in solution. At times the interpretation of theoretical results are the source of trouble and confusion. Thus two different groups propound apparently conflicting and opposite views on the effect of Lewis acids on the carbonyl dipole. 

---