Uracil and thiouracil complexes of dicyclopentadienyl molybdenum and tungsten: preparation and electrochemistry. The structures of [M(η5-C5H5)2(2-SN
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Cationic complexes [MoCp2(U)][PF6] (1), [MCp2(2TU)][PF6] (2, 2′), [MCp2(4TU)][PF6] (3, 3′), [MoCp2(2,4-DTU)]Cl (4), [MCp2(2MTU)][PF6] (5, 5′) and [MCp2(4MTU)][PF6] (6, 6′) (M = Mo, W; Cp = η5-C5H5; U = uracilato; 2TU = 2-thio-uracilato; 4TU = 4-thiouracilato; 2,4-DTU = 2,4-dithiouracilato; 2MTU = 2-methylthiouracilato and 4MTU = 4-methylthiouracilato) have been prepared from the reaction of [MCp2Cl2] with the corresponding uracil in the presence of triethylamine. All the uracilato derivatives act as bidentate ligands and experimental evidence (X-ray and IR) shows the following preference for binding atoms in this system: N(3) > N(1), = S > = O, O(2) > O(4); although there is no clear evidence it is also likely that S(2) > S(4). Electrochemical studies, by cyclic voltammetry carried out in acetonitrile or dimethylformamide, showed that compounds where ligands coordinate through N,S atoms are more stable upon oxidation than compounds where ligands coordinate through N,O donor atoms. The molecular structures of [MoCp2(2TU)][PF6](2), [WCp2(2TU)][PF6](2′), [MoCp2(4TU)][PF6](3), [MoCp2(2MTU)][PF6](5), [WCp2(2MTU)][PF6](5′) and [MoCp2(4MTU)][PF6](6) have been determined by X-ray crystallography.