Intramolecular electron transfer from pentacyanoferrate(II) to pentaamminecobalt(III) with 3,3'-dimethyl-4,4'-bipyridine, 4,4'-bipyridylacetylene, 1,4-bis(4-pyridyl)butadiyne, 2,7-diazapyrene, and 3,8-phenanthroline as bridging ligands: adiabaticity and the role of distance