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The estimate for the Ru(NH3)5pz2+ ion is modestly higher than the 0.20 electron (22% ligand character to the ground-state MO),21,22 which has been estimated from the ground-and excited-state pAfa's. Protonation is sure to alter the solvation in the resultant Ru(NH3)5(pzH)3+ complex, and this could lead to a small error in the estimate of the extent of back-donation from Ru(II) by the pKa method. 19 The results here are in good agreement with the estimates of Zwickel and Creutz based on Ru(NH3)4L22+ spectra.18 The result that CO and the coordinated acetylenic unit occurs with more significant charge transfer than the N-heterocycles is in keeping with prior studies.10,23'24 This effect has been attributed to the synergistic stabilization ofaccepting/a-donating with small molecules with triple bonds (e.g. CO, CN", R2C2, N2, NO+).23 Cook et al. estimated 0.8 ± 0.2 electron transferred from Pt(PPh3)2Pt(C2H4) and 1.8 ± 0.2 for 02 as the acceptor.1 Assuming less good -donation from a d6 Ru(II) center compared with a d10 Pt(0) center, one might anticipate that C2H4 or dmad would receive a somewhat smaller (21) Taube, H.