Solvent nuclear quantum effects in redox electron transfer (ET) reactions between metal ions in aqueous solution are studied via a molecular dynamics simulation analysis. The impacts of the solute size and charge variations together with the solvent ligand effects are examined by comparing with our previous study on a moderate size donor-acceptor system that assumed typical organic fluorescer-quencher molecules [J. Chem. Phys. 106, 116 (1997)]. It is shown that the spectral density function of the solvent coupling to ET, and consequently the quantum ET rate and its energy gap law, are strongly dependent on these variations of the system parameters. Two kinds of decomposition analysis, one into spatial contributions from inner- and outer-sphere solvations, and the other into motional frequency contributions from solvent intramolecular vibrations and intermolecular collective modes, are presented. (C) 2001 American Institute of Physics.