Weak carbon-hydrogen-nitrogen interactions affect the heterocyclic ligand bonding modes in barium complexes containing eta2-tetrazolato and eta2-pentazolato ligands.

Treatment of Ba[N(SiMe3)2]2(THF)2 with 2 equiv of dimethylaminotetrazole or diisopropylaminotetrazole and 1 equiv of 18-crown-6 afforded Ba[CN4(NMe2)]2(18-crown-6) (87%) and Ba[CN4(NiPr2)]2(18-crown-6) (79%) as colorless crystalline solids. Ba[CN4(NMe2)]2(18-crown-6) contains two 1,2-eta2-tetrazolato ligands and one eta6-18-crown-6 ligand. The molecular structure of Ba[CN4(NiPr2)]2(18-crown-6) is similar to that of Ba[CN4(NMe2)]2(18-crown-6), except that the tetrazolato ligands exhibit the isomeric 2,3-eta2-coordination mode and the tetrazolato ligand CN4 cores are bent significantly toward the 18-crown-6 ligands. Molecular orbital calculations were carried out on the model complexes Ba(azolate)2(18-crown-6) (azolate = 1,2-eta2-CHN4, 2,3-eta2-CHN4, and eta2-N5) and demonstrate that the ligand coordination modes are influenced by intramolecular interactions between filled nitrogen orbitals on the azolato ligands and empty C-H sigma* orbitals on the 18-crown-6 ligands.