Spectroscopic and Theoretical Studies of Iron Tricarbonyl 1,4-DimethyItetraazadiene and Related Complexes. Evidence for a Hückel Aromatic Metal-Nitrogen Ring

Complexes of the type Fe(CO)3(N4(R)2), Fe(CO)₂L(N4(CH3)2), Fe(CO)L2(N4(CH3)2), and Fe(P(OCH3)3)3(N4(CH3)2) [L = P(C6H5)3, P(CH3)3, P(OCH3)3; R = CH3, C6H5] exhibit two intense electronic transitions at 470-520 and 349-390 nm. These are attributed to the presence of a low-lying unoccupied metallocycle π* orbital. The d8 Fe(CO)3 fragment possessing two dT electrons can interact with the four p, electrons of the N4R2 ligand to create a six -electron (Hückel aromatic) cyclic system. Both Hückel MO and SCC-DV- calculations support the qualitative picture, and the latter study provides a quantitative account of the optical spectroscopic data. The vapor-phase He I photoelectron spectrum of Fe(CO)3(N4(CH3)2) has been measured. Ionizations in the 8-11-eV spectral region arise from orbitals containing dominant metal d character, in addition to N4R2 lone-pair and pT character. Orbital density plots for the five FeN4 orbitals are discussed. The theoretical data suggest that the N4R2 ligand rivals carbon monoxide as a -acceptor ligand, and vapor-phase IR spectra support this conclusion.