Covalent Bonding in Octahedral (MnCl6)4− and Tetrahedral (MnCl4)2− Complexes

Abstract

Chlorine ligand hyperfine structure has been observed in the EPR spectrum of the octahedral complex ion (MnCl₆)⁴⁻¹, in dilute solution in a single crystal of K₄CdCl₆ at room temperature. From the isotropic and anisotropic components of the spectrum, it is estimated that $f_s\text{\hspace{0.17em}=\hspace{0.17em}0.40\%}$ , and ${\mathrm{(f}}_{\sigma }{\mathrm{\hspace{0.17em}-\hspace{0.17em}f}}_{\pi }\text{)\hspace{0.17em}=\hspace{0.17em}3.9\%}$ , respectively. No chlorine ligand hyperfine structure is observed in the tetrahedral complex ion (MnCl₄)²⁻ introduced as an impurity in either [N(CH₃)₄]₂ZnCl₄ or [(C₆H₅)₃CH₃As]₂ZnCl₄. A substantial reduction in the manganese nuclear hyperfine coupling constant $A_s(\mathrm{Mn})$ is observed in both cases, however, $A_s(\mathrm{Mn})$ having the value − 79.50 ± 0.50 G for [N(CH₃)₄]₂ZnCl₄, and − 79.30 ± 0.50 G for [(C₆H₅)₃CH₃AS]₂ZnCl₄ compared to − 86.40 ± 0.40 G found for MnCl₆⁴⁻ in K₄CdCl₆. The failure to observe ligand hyperfine interactions in the tetrahedral ion is discussed in terms of the shorter bond distance in the four coordinated compound and the increased importance of $\pi$ bonding.