We present the first measurements of the excited-state relaxation dynamics of a bimetallic class III mixed-valence molecule. The 800 nm absorption of [Ru2TIEDCl4](+) (TIED = tetraiminoethylenedimacrocycle) relaxes in 250 and 1000 fs to at least two different intermediate states that can be followed with transient absorption spectroscopy. These states decay in 1.3 and 11.5 ps, and the absorption of the 1.3 ps intermediate displays a large amplitude, very low frequency, highly damped vibrational coherence that completely modulates the absorption. The coherence frequency is 20 +/- 5 cm(-1), and the dephasing times range from 360 to 730 fs over the wavelength range of the absorption band. The occurrence of a low-frequency coherence at room temperature, the nearly 100% modulation amplitude, and the phase properties as a function of wavelength are consistent with a nonradiative rate modulation rather than the typical impulsive mechanism that creates a coherent Franck-Condon modulation of the absorption. A nonradiative rate modulation can occur from a vibronic coupling mechanism that is created by breakdown of the Born-Oppenheimer approximation. This type of electronic state coupling likely occurs via nontotally symmetric vibrations, and this is the first time domain measure of a vibronic coupling frequency for inorganic complexes. The resonance Raman activity of the ground-state absorption is consistent with very small mode displacements for the optically connected ground and excited states, as expected for a class III molecule. Since similar nonradiative rates are measured for both the optically excited-state and intermediate-state decays, they both require similar energy gaps in the range of 5000-7000 cm(-1). With these energy gaps, we infer that vibronic coupling matrix elements from 4500 to 11 200 cm(-1) can explain the observed nonradiative decay time of 250 fs. These experiments show that class III molecules, and probably many other inorganic complexes, can have fast nonradiative decay channels from vibronic coupling when electronic states are available at lower energies. Therefore, applications with such molecules require careful molecular design to compete with or reduce rates of return to the ground state.