Hybrid type of NO donors

The distribution of electron density in the sulfur-containing ligands is modified to allow the thiones to function as two unsymmetrically opposed 4e donors in the complex of type 4. The compound has no metal-metal bond. Behrens and co-workers 30) as well as Seyferth et al. 15) prepared symmetrical complexes in which the ligands are bound similarly to the one drawn below the two iron atoms in 4. In the Behrens compounds, prepared from thioketenes, the a-thio carbon atom is sp hybridized in contrast to the sp hybridization which occurs when the two iron atoms are held together by the O-thioformate groups in Seyferth s compound. The latter group proposed two completely different, competing pathways for the formation of complexes of types 1 and 3. 

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