DNA-mediated long-range electron transfer from photoexcited 1,10-phenanthroline complexes of ruthenium, Ru(phen)2(3)+, to isostructural complexes of cobalt(III), rhodium(III), and chromium(III) bound along the helical strand. The efficiency of transfer depended upon binding mode and driving force. For a given donor-acceptor pair, surface-bound complexes showed greater rate enhancements than those that were intercalatively bound. Even in rigid glycerol at 253 K, the rates for donor-acceptor pairs bound to DNA remained enhanced. For the series of acceptors, the greatest enhancement in electron-transfer rate was found with chromium, the acceptor of intermediate driving force. The DNA polymer appears to provide an efficient intervening medium to couple donor and acceptor metal complexes for electron transfer.