Electronic transitions and bonding properties in a series of five-coordinate “16-electron” complexes [Mn(CO)3(L2)]− (L2 = chelating redox-active π-donor ligand)

In this article we present for the first time accurate density functional theory (DFT) and time-dependent (TD) DFT data for a series of electronically unsaturated five-coordinate complexes [Mn(CO)3(L2)]−, where L2 stands for a chelating strong π-donor ligand represented by catecholate, dithiolate, amidothiolate, reduced α-diimine (1,4-dialkyl-1,4-diazabutadiene (R-DAB), 2,2′-bipyridine) and reduced 2,2′-biphosphinine types. The single-crystal X-ray structure of the unusual compound [Na(BPY)][Mn(CO)3(BPY)]·Et2O and the electronic absorption spectrum of the anion [Mn(CO)3(BPY)]− are new in the literature. The nature of the bidentate ligand determines the bonding in the complexes, which varies between two limiting forms: from completely π-delocalized diamagnetic {(CO)3Mn–L2}− for L2 = α-diimine or biphosphinine, to largely valence-trapped {(CO)3MnI–L22−}− for L22− = catecholate, where the formal oxidation states of Mn and L2 can be assigned. The variable degree of the π-delocalization in the Mn(L2) chelate ring is indicated by experimental resonance Raman spectra of [Mn(CO)3(L2)]− (L2 = 3,5-di-tBu-catecholate and iPr-DAB), where accurate assignments of the diagnostically important Raman bands have been aided by vibrational analysis. The L2 = catecholate type of complexes is known to react with Lewis bases (CO substitution, formation of six-coordinate adducts) while the strongly π-delocalized complexes are inert. The five-coordinate complexes adopt usually a distorted square pyramidal geometry in the solid state, even though transitions to a trigonal bipyramid are also not rare. The experimental structural data and the corresponding DFT-computed values of bond lengths and angles are in a very good agreement. TD-DFT calculations of electronic absorption spectra of the studied Mn complexes and the strongly π-delocalized reference compound [Fe(CO)3(Me-DAB)] have reproduced qualitatively well the experimental spectra. Analyses of the computed electronic transitions in the visible spectroscopic region show that the lowest-energy absorption band always contains a dominant (in some cases almost exclusive) contribution from a π(HOMO) → π*(LUMO) transition within the MnL2 metallacycle. The character of this optical excitation depends strongly on the composition of the frontier orbitals, varying from a partial L2 → Mn charge transfer (LMCT) through a fully delocalized π(MnL2) → π*(MnL2) situation to a mixed (CO)Mn → L2 charge transfer (LLCT/MLCT). The latter character is most apparent in the case of the reference complex [Fe(CO)3(Me-DAB)]. The higher-lying, usually strongly mixed electronic transitions in the visible absorption region originate in the three lower-lying occupied orbitals, HOMO − 1 to HOMO − 3, with significant metal-d contributions. Assignment of these optical excitations to electronic transitions of a specific type is difficult. A partial LLCT/MLCT character is encountered most frequently. The electronic absorption spectra become more complex when the chelating ligand L2, such as 2,2′-bipyridine, features two or more closely spaced low-lying empty π* orbitals.