The chelating ligand (2,2′-C12H 8O2)P(CH2CH2O)4P(2,2 ′-O2C12H8) reacts with both Ru(bpy)2(O3SCF3)2 and Mo(CO) 4(C7H8) to form the new metallacrown ether complexes cis-[Ru(bpy)2((2,2′-C12H 8O2)P(CH2CH2O)4P(2,2 ′-O2C12H8))](O 3SCF3)2 (1) and cis-[Mo(CO)4((2, 2′-C12H8O2)P(CH 2CH2O)4P(2,2′-O 2C12H8))] (2). The X-ray crystal structures for both complexes indicate that the metallacrown ether ring oxygen atoms point their lone pairs towards the cavity in the cis-metallacrown ether ring. The effect of interactions of the metallacrown ether complex with selected monoatomic cations (Li+, Na+ and Hg2+) on the emissive properties of the Ru(bpy)22+ chromophore were studied by steady-state and time-dependent emission spectroscopy. Our results suggest that complex 1 interacts weakly with Li+, Na+ and Hg2+, and subtle changes are observed in its photophysical properties. These changes are discussed in terms of excited-state parameters obtained from a Franck-Condon analysis of the emission spectra. © 2004 Elsevier Ltd. All rights reserved.