Quadrupole Coupling and Anisotropic Chemical Shifts in Some Manganese Carbonyls

Single crystals of a number of manganese carbonyls were studied by broadline NMR at room temperature. In the pentacarbonylhalides, Mn(CO)₅X, the ⁵⁵Mn quadrupole coupling constants ${\mathrm{(e}}^2\mathrm{qQ\hspace{0.17em}/\hspace{0.17em}h)}$ are 13.86 MHz (X is Cl), 17.46 MHz (X is Br), and 19.85 MHz (X is I). The asymmetry parameters are only 0.024–0.043, indicating rather high symmetry of the molecules in the solid. The halogen NMR was also observed and analyzed to give values for the quadrupole coupling parameters; ³⁵Cl: $e^2\text{qQ\hspace{0.17em}/\hspace{0.17em}h\hspace{0.17em}=\hspace{0.17em}36.07}\mathrm{MHz}$ , ⁸¹Br: $e^2\text{qQ\hspace{0.17em}/\hspace{0.17em}h\hspace{0.17em}=\hspace{0.17em}226.7}\mathrm{MHz}$ , ¹²⁷I: $e^2\text{qQ\hspace{0.17em}/\hspace{0.17em}h\hspace{0.17em}=\hspace{0.17em}927}\mathrm{MHz}$ with $\eta$ values of 0.03–0.06. The $z$ axes of the field‐gradient tensor at the manganese sites were found to lie along the Mn–X bond. The anisotropic chemical shifts of ⁵⁵Mn in these compounds and (C₅H₅)Mn(CO)₃ were also determined. In the Mn(CO)₅X series the shift anisotropy decreases in the order Cl>Br>I ${\mathrm{(\sigma }}_{\mathrm{zz}}{\mathrm{\hspace{0.17em}-\hspace{0.17em}\sigma }}_{\mathrm{xx}}\text{\hspace{0.17em}=\hspace{0.17em}1.3‰, 1.2‰, 1.0‰)}$ . The $z$ component of the shift tensor was found to be constant within the series, but different from the isotropic shift in Mn(CO)₆⁺. The parameters were compared to the results of a recent MO calculation for this series.