Photochemistry of pentacarbonylmethylmanganese and acetylpenta-carbonylmanganese in frozen gas matrices at 12 K. Infrared spectroscopic evidence for the formation of methyl- and acetyl-tetracarbonyl-manganese species

Abstract

Infrared spectroscopic evidence is presented to show that on photolysis of [Mn(CO)₅R](RCH₃ or CH₃CO) complexes at high dilutions in Ar, CH₄, and CO matrices at 12 K new species [Mn(CO)₄R] are produced. Carbon- 13 labelling of CO and energy-factored force-field fitting was carried out for [Mn(CO)₅(CH₃)] and [Mn(CO)₄(CH₃)]. For [Mn(CO)₄(CH₃)] the CH₃ ligand was shown to occupy an equatorial position in a trigonal-bipyramidal structure with C_2v symmetry. The observations of CO ejection, a new ketonic CO stretching band, and the formation of a species with a common [Mn(CO)₄] fragment from both [Mn(CO)₅(CH₃CO)] and [Mn(CO)₅(CH₃¹³CO)], were interpreted as confirming the existence of the co-ordinatively unsaturated species [Mn(CO)₄(CH₃CO)] as the intermediate in the decarbonylation reaction. Formation of trans-[Mn(¹²CO)₄(¹³CO)(CH₃)] on reverse photolysis in experiments starting with cis-[Mn(¹²CO)₄(¹³CO)(CH₃)] was interpreted as demonstrating that the [Mn(CO)₄(CH₃)] species is fluxional. The implications of fluxionality for the mechanism of the carbonylation reaction are discussed.