Lucky Number Seven For Metal Carbonyls

University of Georgia researchers have reported the first convincing experimental observation of transition-metal heptacarbonyls, M(CO)₇ ⁺, along with an interesting periodic trend for the highly coordinated metal species (J. Am. Chem. Soc., DOI: 10.1021/ja903983u). Transition metals can theoretically handle up to nine ligands by using all of the five d, three p, and one s orbitals in a valence shell. But overcrowding, available electrons, and other factors preclude most metals from taking on more than six ligands at a time. The group 5 metals vanadium, niobium, and tantalum are an exception: They should be able to form stable M(CO)₇ ⁺ if they utilize all their bonding capabilities, but chemists have never directly observed the heptacarbonyls until now. Georgia’s Michael A. Duncan and coworkers used laser vaporization to produce a range of multicarbonyl compounds and analyzed them by infrared spectroscopy. The researchers deduced that vanadium is limited to forming V(CO)₆ ⁺, whereas niobium forms Nb(CO)₆ ⁺ and Nb(CO)₇ ⁺, and tantalum forms only Ta(CO)₇ ⁺, a trend that they say points out steric and bond-energy limitations of the metals.