The temperature dependences of halogen and cobalt NQR frequencies in trans-dichlorobis(ethylenediamine)cobalt(III) chloride hydrochloride dihydrate and related compounds

Abstract

The temperature dependences of 59Co and ligand halogen NOR frequencies of trans-[Co(en) 2Cl 2]Cl·HCl·2H 2O( I), trans-[Co(en) 2Cl 2]NO 3, trans-[Co(en) 2Cl 2]ClO 4, trans-[Co(tn) 2Cl 2]Cl·HCl·2H 2O, trans-[Co(en) 2Br 2]Br·HBr·2H 2O, and trans[Rh(en) 2Cl 2]Cl·HCl·2H 2O are presented. While the 59Co coupling constants have substantial negative temperature coefficients, the 35Cl frequency of I is essentially temperature independent and the halogen frequencies of the other compounds have small temperature coefficients that are either positive or negative. An analysis of the effects of librational motion using thermal parameters from a neutron diffraction study of I accounts for less than 15% of the 59Co coupling-constant temperature dependence. Point charge model calculations indicate that the direct contributions to the 59Co and 35Cl field gradients from lattice charges are small and have only a minor contribution to the temperature dependences. On the basis of extended H6ckel molecular orbital calculations, hydrogen bonds involving the complex ions are postulated to give a positive contribution to the halogen temperature coefficient that competes with the negative contribution from motional averaging. The hydrogen bonding may also result in 59Co temperature coefficients more negative than expected, but this explanation does not seem completely consistent with the data.