Our recent results of the chemistry of group 9 Rh and Ir metal complexes bearing the ubiquitous pyridine, diimine (PDI) terdentate nitrogen donor are reviewed. Examples reflecting the special nature of the PDI ligand include a facile C–H activation process and the stabilization of a very rare late transition metal iridium nitrido compound, (PDI)Ir≡N, and its direct hydrogenation to the corresponding amido complex according to (PDI)Ir≡N + H2 → (PDI)Ir–NH2. The amount of electron transfer to the PDI ligands in (PDI)Rh,Ir–R,X complexes encompassing a variety of R,X ligands with weak/strong σ- and π-donors/acceptors was analyzed in terms of the innocence of the PDI ligand. With this regard, a combination of DFT methods, X-ray-crystallographic data, and, in particular, 13C NMR spectroscopy was employed for a series of square-planar Rh and Ir complexes including a large number of new representatives. Their syntheses, spectroscopic characterization, and X-ray crystal structures are reported.