Transition Metal Nitrido and Phosphido Complexes

The nitrido and phosphido complexes of TMs have been the subjects of intensive experimental studies in the recent years. Of particular interest has been the issue of Lewis basicity of the nitrogen and phosphorus atoms in the TM=N and TM=P groups. Table 7.17 lists the BDEs calculated at the MP2/II, B3LYP/B and CCSD(T)/B levels of theory for LnMN-X and LnMP-X, where X is a group-13 Lewis acid or a chalcogen atom [86, 87]. 

These results demonstrate that it is difficult to make a general statement about the accuracy of the MP2 or B3LYP approaches vis-a-vis that of CCSD(T). For example, the BDEs of the N-EH3 donor-acceptor bonds (E = B, Al, Ga) are very similar at the MP2/II and B3LYP/II levels of theory. They also agree with the CCSD(T)/II value for the N-BH3 bond. However, for the N-BCI3 and N-BBr3 bonds, the B3LYP/H 

NH3 and W-NH3 donor-acceptor bonds appear to be more reliable as they agree quite well with the CCSD(T)/II estimates. 

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. 

A comparison of the calculated and experimental data indicates that the MP2 values obtained with the basis sets II do not differ significantly from those afforded by the larger basis set TZP. The theoretical values agree very well with the experimental data. The only larger deviation from experimental data has been found for Cl3B-NMe3 (Table 7.18). However, a critical examination of the experimental value of 30.5 kcal/mol led us to suggest that it is probably too low [88], 

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