The Structure of [Fe(CO)4 ]-An Important New Chapter in a Long-Running Story.

The structure of the singlet state (1 A1 ) of [Fe(CO)4 ] in the gas phase has been determined by a combination of laser photochemistry of [Fe(CO)5 ] and electron diffraction imaging. The ground state of [Fe(CO)4 ] is known to be a triplet species (3 B2 ), and this is the species detected in picosecond time-resolved IR experiments with [Fe(CO)5 ] in solution. This is an appropriate moment to survey the state of knowledge on [Fe(CO)4 ], beginning from the first low-temperature matrix experiments.