Abstract: Trinuclear carbonyl cluster compounds of Fe, Ru and Os were vaporized, ionized and dissociated by pulsed laser beam. The positive and negative ions were observed and studied by in situ mass spectroscopy. After comparison of their fragments distribution, it can be found that the osmium clusters have the special structural stability. Not only do they have the strong metal metal bonds, but also very strong coordination bonds formed between the metal-atoms and the carbonyl groups. Therefore, when the cluster dissociates, it loses a pair of oxygen atoms at first and forms a C₂bridged compound, unlike the clusters of iron or ruthenium which loses the carbonyl groups and fragments into dinuclear or even single nuclear radicals. The spectra also revealed the significant difference between the positive ions and the negative ones. In the recorded mass spectra, the molecular ion peak of the positive ions can be observed, but only fragment ions of the negative ions can be observed.

Key words: Carbonyl cluster compounds, Laser dissociation, Fe, Ru, Os