Isotope effects in (195)Pt NMR spectroscopy: unique (35/37)Cl- and (16/18)O-resolved "fingerprints" for all [PtCl6-n(OH)n](2-) (n = 1-5) anions in an alkaline solution and the implications of the trans influence.

Abstract

A detailed analysis of the intrinsic (1)Δ(195)Pt((37/35)Cl) and (1)Δ(195)Pt((18/16)O) isotope 128.8 MHz (195)Pt NMR profiles of the series of kinetically inert [PtCl6-n(OH)n](2-) (n = 1-5) anions generated in strongly alkaline aqueous solutions shows that each (195)Pt NMR resonance of the [Pt(35/37)Cl6-n((16/18)OH)n](2-) (n = 1-5) anions is resolved only into [(6 - n) + 1 for n = 1-5] (35/37)Cl isotopologues at 293 K. Evidently, the greater trans influence of the hydroxido ligand in the order OH(-) > Cl(-) > H2O in [PtCl6-n(OH)n](2-) (n = 1-5) complexes results in somewhat longer Pt-Cl bond displacements trans to the hydroxido ligands, resulting in the absence of isotopomer effects in the [PtCl6-n(OH)n](2-) (n = 1-5) anions in contrast to that observed in the corresponding [PtCl6-n(H2O)n]((2-n)-) (n = 1-5) complexes. In suitably (18)O-enriched sodium hydroxide solutions, additional intrinsic (1)Δ(195)Pt((18/16)O) isotope effects are remarkably well-resolved into unique isotopologue- and isotopomer-based (195)Pt NMR profiles, ascribable to the higher trans influence of the OH(-) ligand. The consequent significantly shorter Pt-OH bonds in these anions emphasize (16/18)O isotopomer effects in the (195)Pt NMR peaks of [Pt(35/37)Cl6-n((16/18)OH)n](2-) (n = 1-5) for magnetically nonequivalent (16/18)OH isotopomers statistically possible in some isotopologues. These (195)Pt NMR profiles constitute unique NMR "fingerprints", useful for the unambiguous assignment of the series of [PtCl6-n(OH)n](2-) anions including their possible cis/trans/fac/mer stereoisomers in such solutions, without a need for accurate chemical shift measurements.