Observation of Confinement-Free Neutral Group Three Transition Metal Carbonyls Sc(CO)₇ and TM(CO)₈ (TM=Y, La)

Spectroscopic characterization of neutral highly-coordinated compounds is essential in fundamental and applied research, but has been proven to be a challenging experimental target because of the difficulty in mass selection. Here, we report the preparation and size-specific infrared-vacuum ultraviolet (IR-VUV) spectroscopic identification of group-3 transition metal carbonyls Sc(CO)₇ and TM(CO)₈ (TM=Y, La) in the gas phase, which are the first confinement-free neutral heptacarbonyl and octacarbonyl complexes. The results indicate that Sc(CO)₇ has a C_2v structure and TM(CO)₈ (TM=Y, La) have a D_4h structure. Theoretical calculations predict that the formation of Sc(CO)₇ and TM(CO)₈ (TM=Y, La) is both thermodynamically exothermic and kinetically facile in the gas phase. These highly-coordinated carbonyls are 17-electron complexes when only those valence electrons that occupy metal-CO bonding orbitals are considered, in which the ligand-only 4b_1u molecular orbital is ignored. This work opens new avenues toward the design and chemical control of a large variety of compounds with unique structures and properties.