Mg-Fe Bonding in the MgFe(CO) n + ( n = 4-9) Cation Complexes: An Infrared Photodissociation Spectroscopic and Theoretical Study.

Heteronuclear magnesium-iron carbonyl cation complexes MgFe(CO) n + ( n = 4-9) are prepared in the gas phase and are detected by mass-selected infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The geometric structures and the metal-metal bonding are discussed with the aid of quantum chemical calculations. The MgFe(CO) 9 + cation is a coordinatively saturated complex. Each complex is characterized to contain more than one isomer. The small complexes ( n = 4-6) possess the Mg-Fe bonded [(OC) n -4 Mg-Fe(CO) 4 ] + and/or [(OC) n -5 Mg-Fe(CO) 5 ] + structures with all the carbonyl ligands terminally bonded. For the larger complexes with n = 7-9, the [(OC) n -4 Mg-Fe(CO) 4 ] + structure is the major isomer experimentally observed. In addition, the [(OC) n -5 Mg-OC-Fe(CO) 4 ] + isomer involving a linear bridging carbonyl ligand is also characterized. Bonding analyses indicate that each [(OC) n -4 Mg-Fe(CO) 4 ] + complex contains a Mg-Fe electron-sharing σ bond. The metal-metal bond is described as a Mg(+I)-Fe(0) bond in MgFe(CO) 4 + and as a Mg(+II)-Fe(-I) bond in the larger n = 5-9 complexes.