Spectroscopic and theoretical studies of [Fe(CO)3(1,4-diaza-1,3-butadiene)] complexes; X-ray structure and magnetic circular dichroism and resonance-Raman spectra of [1,2-bis(2′,6′-di-isopropylphenylimino)ethane-NN′]tricarbonyliron

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Abstract
The X-ray structure of the title compound has been determined by the heavy-atom method and refined by means of block-diagonal least-squares calculations from 5 537 independent reflections. The crystals are monoclinic, space group P21/n, with unit-cell dimensions a= 18.422(4), b= 16.155(3), c= 9.966(3)Å, β= 90.40(3)°, and Z= 4. The final R value was 0.044. The compound has a distorted square pyramidal structure with Fe–N bond lengths of 1.929(2) and 1.926(3)Å. Molecular-orbital (m.o.) calculations reveal a low-lying lowest unoccupied molecular orbital which has substantial π*(α-di-imine) character. Most low-lying transitions are directed to this orbital. M.c.d., electronic absorption, and resonance-Raman (r.R.) spectra, obtained by excitation into the lowest electronic absorption bands, are reported. They give evidence for several electronic transitions in the low-lying absorption band. Two of these transitions are pseudo-degenerate, leading to an A term in the m.c.d. spectrum. The r.R. spectra are very weak which means that the bonds of the complex are hardly affected by the charge-transfer transitions. The spectra show that the complex relaxes to another conformation in the excited state. Differences between the m.o. calculations and the r.R. spectra are discussed.

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