Remarkable spectator ligand effect on the rate constant of ligand substitution of (aqua)ruthenium(II) complexes

The influence of two different di(I-pyrazolyl)alkane ligands on the rate co nstant of aqua ligand substitution of ruthenium(II) complexes with the form ula [Ru(H2O)(L-2)(tpmm)](2+) (L-2 = di(I-pyrazolyl)methane (DPMet) or 2,2-d i(I-pyrazolyl)propane (DPPro)) was investigated. A 9.4 x 10(5)-fold increas e in the rate constant of ligand substitution at pH = 6.86 was observed whe n DPMet was replaced with DPPro. This remarkable increase was unexpected, c onsidering that these bidentate ligands appear quite similar. To help lend insight into this dramatic spectator ligand effect, the activation paramete rs for the ligand substitution reactions were determined, and single-crysta l X-ray data were collected on the structurally analogous (chloro)ruthenium (II) complexes, [Ru(Cl)(L-2)(tpmm)](+). These results are discussed in the context of a heteroscorpionate effect exerted by the DPPro ligand.