Stable five-coordinated (16-electron) half-sandwich iridium(III) and ruthenium(II) complexes are rarely reported, and their biological evaluations have not been considered to date. Herein, in an experiment designed to synthesize six-coordinated half-sandwich iridium(III) and ruthenium(II) complexes containing N,N-chelated α-keto-β-diimine ligands, we observed the serendipitous formation of half-sandwich aminoimine iridium(III) and ruthenium(II) complexes via solvent-involved rearrangement reaction. These unsaturated 16-electron complexes had sufficient stability in DMSO-water solution. Moreover, no reaction with two-electron donors (CO and PPh3) and nucleobase (9-MeA and 9-EtG) was observed. Most of the complexes show good anticancer activities toward A549, HeLa, and HepG2 cancer cells, which are higher than the clinical drug cisplatin. The investigation of mechanism by flow cytometry showed that the complexes exert their anticancer efficacy by inducing apoptosis or necrosis, and increasing the intracellular ROS level. In addition, fluorescence property of these complexes makes it possible to investigate the microscopic mechanism by confocal microscopy. Notably, the complexes Ir3 and Ru1 enter A549 cancer cells through an energy-independent pathway, and they are mainly located in mitochondria and lysosomes.