The resolution of the contributions of CN and NH vibronic sidebands to the 77 K, transition metal-to-transition metal electron-transfer (MMCT) emission for a cyanide-bridged Ru(II)/Cr(III) coordination complex is reported. These vibronic sidebands were identified, and their reorganizational energy contributions evaluated by comparing the reorganizational energy profiles of the proteo (NH) and deutereo (ND) am(m)ine complexes in glassy solutions and in the microcrystalline solid. The reorganizational energy profiles were generated by subtracting a gaussian fit of the dominant, fundamental emission component (hv(0'0(max)) = 12 050 cm(-1) in glasses; 12 400 cm(-1) in the solid) from the high resolution, near-infrared emission spectrum, scaling the remainder amplitudes so that they correspond to the sum of reorganizational energy contributions at each emission energy and correcting for the effects of the significant component bandwidths. The reorganizational energy attributable to the NH-stretch in DMSO/water glasses, lambda(NH) congruent to 28 +/- 5 cm(-1), is much smaller than the values of lambda(x) = 200-400 cm(-1) attributable to the lower frequency vibrations (the dominant distortion modes; probably metal-ligand skeletal vibrations and C=N stretches). There also appears to be a similarly small contribution to the reorganizational energy from NH2 bending modes. The value of lambda(NH) is consistent with a tunneling pathway for the back electron-transfer and with the very large NH/ND isotope effect observed for this complex. The reorganizational energy contribution attributable to the CN stretching vibration of the bridging ligand is greater than 100 cm(-1). The molecular reorganizational contributions for the back electron-transfer are 30-40% smaller in butyronitrile glasses than in DMSO/water glasses or in the solid. This may be a consequence of configurational (or intervalence) mixing of the MMCT excited state with the ligand field E-2 excited state of the chromium center.