Single crystals of ruthenocene doped with nickelocene show a near-infrared luminescence band with a maximum at 13 100 cm(-1) and a width of 1100 cm(-1) at 5 K in addition to the ruthenocene luminescence, The band is symmetric and attributed to nickelocene on the basis of a comparison of luminescence and absorption spectra. Energy transfer from the ruthenocene host enhances the nickelocene luminescence intensity at temperatures between 50 and 100 K. An activation barrier of 306(18) cm(-1) is determined from luminescence decay measurements. The Stokes shift of 2200 cm(-1) and the large change of 0.16 Angstrom in nickel-cyclopentadienyl distance in the emitting state confirm the assignment as (3)E(1g), arising from an electron configuration different from that of the ground state. The comparison of experimental and calculated spectra shows that only low-energy vibrational modes contribute to the luminescence band shape, in contrast to the case of cobaltocene doped in the same host lattice, where a distortion along a high-frequency, ligand-centered mode affects the luminescence band shape.