Carbon monoxide (CO) has recently been shown to impart beneficial effects in mammalian physiology and considerable research attention is now being directed toward metal-carbonyl complexes as a means of delivering CO to biological targets. Two ruthenium carbonyl complexes, namely trans-dicarbonyldichlorido(4,5-diazafluoren-9-one-κ(2)N,N')ruthenium(II), [RuCl2(C11H6N2O)(CO)2], (1), and fac-tricarbonyldichlorido(4,5-diazafluoren-9-one-κN)ruthenium(II), [RuCl2(C11H6N2O)(CO)3], (2), have been isolated and structurally characterized. In the case of complex (1), the trans-directing effect of the CO ligands allows bidentate coordination of the 4,5-diazafluoren-9-one (dafo) ligand despite a larger bite distance between the N-donor atoms. In complex (2), the cis disposition of two chloride ligands restricts the ability of the dafo molecule to bind ruthenium in a bidentate fashion. Both complexes exhibit well defined (1)H NMR spectra confirming the diamagnetic ground state of Ru(II) and display a strong absorption band around 300 nm in the UV.

A photo-active luminescent rhenium carbonyl complex namely, [Re(CO)₃(phen)(pyAl)](CF₃SO₃) was grafted on a biocompatible carboxymethyl chitosan (CMC) matrix through Schiff base condensation reaction. The light-induced CO delivery from ReCMC has been shown to eradicate human colorectal adenocarcinoma cells (HT-29) very efficiently in a dose-dependent fashion. The onset of CO-induced apoptosis was realized by caspase-3,-7 detection aided by fluorescence confocal microscopy. ReCMC represents a unique example of a biocompatible and biodegradable antineoplastic agent that could find its use in cancer photopharmacology.