The heavier group 2 elements appear to involve their empty d-orbitals more in bonding than previously suspected. Calcium, strontium and barium can all form 18-electron octacarbonyl complexes with carbon monoxide, more akin to those formed by transition metals than other main group elements. The new results challenge previous notions of coordination chemistry and expand the limits of a widely accepted rule.

Elements are classified into main group elements, which include the s- and p-blocks of the periodic table, transition metals (d-block) and lanthanides and actinides (f-block). As main group elements, Ca, Sr and Ba normally form bonds using their s and p valence orbitals, following the ‘octet rule’, which states that atoms tend to combine in such a way that they have eight electrons in their valence shell. Transition metals bring in a further five d orbitals that must be filled to achieve such a stable configuration, resulting in a similar 18-electron rule for these elements.

Gernot Frenking form the University of Marburg, Germany, and colleagues in China have now demonstrated that the 18-electron principle is not limited to transition metals. ’The observation of the alkaline earth complexes M(CO) 8 indicates that the century-old octet rule for main group elements, which is taught in all chemistry textbooks, is not valid for some atoms,’ Frenking says. In a recent collaboration with Mingfei Zhou of Fudan University, he analysed the vibrational spectra of Ba(CO)+ and Ba(CO)– and was puzzled by the results.2 ‘The signal for the C–O stretching mode of the cation caught my attention because it was shifted to much lower wavenumbers with respect to free CO,’ Frenking says. ‘I have been working on such systems for 20 years and immediately recognised that something strange was going on.’ A theoretical analysis confirmed his assumption that the Ba atom was using d orbitals – instead of s or p orbitals – in the bond. Similar behaviour had been reported for Ba before, but only in exotic systems.3