The reaction of the niobocene complex [Nb(η5-C 5H4SiMe3)2(H)(CO)] (1) with α,β-unsaturated acids, namely, fumaric and maleic acids, and methylfumarate ester yielded the corresponding carboxylato-containing niobocene derivatives [Nb(η5-C5H4SiMe 3)2(κ1-O-OOC-CH2-CH 2-R)(CO)] (R = COOH (2); R = COOMe (3)), resulting from the hydrogenation of the C=C bond and the consequent coordination of a carboxylato moiety on the Nb center. The process represents the first reported example of the hydrogenation of α,β-unsaturated acids mediated by a hydride niobocene complex. Spectroscopic and theoretical studies at the DFT level have allowed for proposing a plausible reaction mechanism. Initial coordination of the C=C bond to the Nb center is followed by the insertion of this bond into the Nb-H bond, and the reaction is completed by a proton transfer from the carboxylic moiety to the α-carbon of the alkyl moiety, giving rise to the final κ1-carboxylato ligand. © 2012 American Chemical Society.