The cationic gold π-allene complex {[P(t-Bu) o-biphenyl] Au[η -H C=C=C(CH ) ]} SbF was isolated in 98% yield from reaction of 3-methyl-1,2-butadiene with a mixture of [P(t-Bu) o-biphenyl]AuCl and AgSbF and was characterized by X-ray crystallography and variable-temperature NMR spectroscopy. These studies revealed preferential binding of gold to the less substituted C=C bond of the allene in both the solid state and solution and also revealed fluxional behavior consistent with π-face exchange of the allene ligand via an η -C2 allene intermediate or transition state. © 2010 American Chemical Society. 2 2 3 2 6 2 6 2 + - 1