Structural, electronic, and magnetic properties of the golden cage doped with a transition-metal atom, MAu16- (M =Fe,Co,Ni), are investigated using trapped ion electron diffraction, photoelectron spectroscopy, and density-functional theory. The best agreement to experiment is obtained for endohedral M@Au16- structures but with considerable distortions to the parent Au16- cage. Fe@Au16- and Co@Au16- are found to have similar structures with C2 symmetry while a C1 structure is obtained for Ni@Au16-. The 4s electrons are observed to transfer to the Au16 cage, whereas atomiclike magnetism due to the unpaired 3d electrons is retained for all the doped clusters
