Heretofore, η2-bonding of pyrazolato ligands to d-block metals has been extremely rare. The tantalum−pyrazolato complexes 1 and 2 exhibit η2-pyrazolato bonding and have several unusual features. The homoleptic pyrazolato complex 1 contains three η2-pyrazolato ligands and two η1-pyrazolato ligands and is the first complex to contain both types of bonding modes. Complex 2 contains “slipped” η2-pyrazolato ligands, where one tantalum−nitrogen bond in each pyrazolato ligand is 0.087−0.170 Å longer than the other. This type of pyrazolato coordination is unprecedented.
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Guzei et al. (1997) studied this question.
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