Electronic configuration and structure of paramagnetic iron dinitrosyl complexes

The electronic and geometric structures of paramagnetic iron dinitrosyl complexes have been investigated using electron spin resonance, infrared spectroscopy, and X-ray crystallography. It is concluded that these compounds are best described as 17 electron complexes with a d9 configuration rather than the d7 configuration assumed by most previous investigators. The anisotropy of the g values, determined from the electron spin resonance spectra of frozen solutions, varies considerably from complex to complex. The results are consistent with the supposition that all of the complexes have a distorted tetrahedral geometry, but the nature of the distortion changes as the ligands are varied. As a result of this variation there are changes in the nature of the spin-containing d orbital. Ligands containing hard, nonpolarizable donor atoms such as oxygen or fluorine produce a distortion towards a planar geometry, placing the odd electron in a predominantly [Formula: see text] orbital, while those containing softer donor atoms such as phosphorus or sulfur give complexes with a different type of distortion, leading to placement of the odd electron in a predominantly [Formula: see text] orbital. Nitrogen and halide donor ligands produce smaller distortions, leading to spin-containing molecular orbitals with contributions from a mixture of d orbitals. In accordance with this model, the crystal structure of [Fe(NO)2I2]− has been found to be only slightly distorted from regular tetrahedral coordination about the iron atom.