Kinetics and Mechanism of Nucleophilic Substitutions on Coordinated Polyenes and Polyenyls. 3. 1 Activation of η 5 -Cyclopentadienyl Ligands toward Nucleophilic Attack through η 5 → η 3 Ring Slippage and a Comparison with Reaction at C 5 H 4 O in [Ru(η 5 -C 5 H 5 )(η 4 -C 5 H 4