NMR INVESTIGATION OF NICKEL(II) AND COBALT(II) BONDING: FURTHER EVIDENCE FOR COORDINATION OF HYDROXY AND ETHEREAL OXYGEN ATOMS IN AQUEOUS SOLUTION

Abstract

Abstract The proton magnetic resonance spectra of a variety of nickel (II) and cobalt (II) complexes are reported. The spectra are interpreted to indicate a substantial amount of coordination by hydroxy and ethereal oxygens in all cases. Analysis of the spectral temperature dependence indicates that δ H for coordination of oxygen is very nearly zero in all but one nickel case. The driving force for coordination of these oxygens appears to be a gain in entropy. It was not possible to quantitatively measure the amount of coordination in the cobalt complexes. The ligands reported are: Methoxyethyliminodiacetic acid (MEIDA=CH3 OCH2 CH2 N(CH2 CO2)2), Hydroxyethyimino-diacetic acid (HEIDA=HOCH2 CH2N(CH2CO2H)2), Serine (SER=NH2CH(CH2OH)CO2H), 2(2-aminoethyl amino) ethanol (AEAE=NH2CH2[sbnd]CH2NHCH2CH2OH, N,N′-Bis-(2-Hydroxyethyl)ethylenediamine (BHEEN=HOCH2CH2-NHCH2 CH2NHCH2CH2OH). In all cases equilibration between coordinated and uncoordinated oxygens was rapid. The fraction of time that the oxygen is coordinated at 34°C was found to be 0.85 ± .18, 0.75 ± .17, 0.57 ± .18, and 0.62 ± .25 for the nickel (II) complexes of MEIDA, HEIDA, SER, AEAE, and BHEEN respectively. δ H was 0 in all cases except SER and BHEEN in which δH was - 800 cal/mole and + 1400 cal/mole respectively. The positive δ H for coordination of BHEEN oxygens was assigned to ring strain as δ H for oxygen coordination appears to be zero. The ring strain is found to be approximately 2.7 kcal/mole. The rapid exchange of cobalt (II) coordinated SER and uncoordinated SER is interpreted to indicate that the hydroxy oxygen is uncoordinated a substantial amount of time in the complex Co(SER)2, and that attack of an uncoordinated serine NH2 at the vacant (water occupied) site is the rate determining step in the reaction.