Infrared Spectroscopy and Structures of Manganese Carbonyl Cations, Mn(CO) n+ ( n = 1–9)

Abstract

Manganese carbonyl cations of the form Mn(CO)(n)(+) (n = 1-9) are produced in a molecular beam by laser vaporization in a pulsed nozzle source. Mass selected infrared photodissociation spectroscopy in the carbonyl stretching region is used to study these complexes and their "argon-tagged" analogues. The geometries and electronic states of these complexes are determined by comparing their infrared spectra to theoretical predictions. Mn(CO)(6)(+) has a completed coordination sphere, consistent with its predicted 18-electron stability. It has an octahedral structure in its singlet ground state, similar to its isoelectronic analogue Cr(CO)(6). Charge-induced reduction in pi back-bonding leads to a decreased red-shift in Mn(CO)(6)(+) (upsilon(CO) = 2106 cm(-1)) compared with Cr(CO)(6) (upsilon(CO) = 2003 cm(-1)). The spin multiplicity of Mn(+)(CO)(n) complexes gradually decreases with progressive ligand addition. MnCO(+) is observed as both a quintet and a septet, Mn(CO)(2)(+) is observed only as a quintet, while Mn(CO)(3,4)(+) are both observed as triplets. Mn(CO)(5)(+) and Mn(CO)(6)(+) are both singlets, as are all larger complexes.