A series of penta-coordinated nickel­(II) complexes bearing tetradentate pyridine-functionalized diNHC ligands with flexible propylene (1a–4a) and ortho-xylylene linkers (1b–4b) have been synthesized and fully characterized by various spectroscopic and spectrometric methods. In addition, single-crystal X-ray diffraction studies reveal that all of the nickel­(II) complexes exhibit a distorted trigonal-bipyramidal structure in the solid-state. Conductivity measurements have also been performed and provide further support that the penta-coordination of all complexes is robust and retained in solution.

Five palladium(II) complexes bearing potentially tetradentate pyridine-functionalized diNHC ligands with flexible linkers have been synthesized and characterized to study how coordination modes can be controlled by design and electronic factors. A combination of analytical techniques, including single-crystal X-ray diffraction and conductivity measurements, reveal that more strongly donating benzimidazole-derived NHCs induce tridentate coordination to form cationic N,C,C-pincer complexes, while more weakly donating 1,2,4-triazolinylidenes form neutral dibromido-dicarbene complexes with two pendant pyridyl groups. Increasing the flexibility between pyridyl and NHC units gives rise to a dicationic complex with a tetradentate ligand involving coordination of all four donors. The electronic influences from the different NHC donors can be rationalized by a Gutmann analysis.