POTENTIALLY STABLE CARBENES - A THEORETICAL-STUDY OF PYRAZOL-YLIDENES

We report ab initio and density functional calculations for pyrazol-3- ylidene (1) and pyrazol-4-ylidene (5) using polarized split-valence ba sis sets. MCSCF calculations predict that the singlet-triplet separati ons of these species are 1.72 eV (1) and 1.11 eV (5). The correspondin g proton affinities, calculated at the MP2 level, are 280.3 and 294.0 kcal mol(-1), respectively. Finally, non-local density functional theo ry was employed to estimate the relative metal-ion affinities of these species towards (CO)(5)Cr by calculating the energetics of the reacti on C(Cr-5-L'+L-->(CO)(5)Cr-L+L' where L represents species 1 or 5, an d L' is the related imidazol-2-ylidene 2. The nature of the bonding in the triplet species is discussed, as is the nature of the metal-ligan d bond in the complexes with (CO)(5)Cr.