Alkaline earth metals can form 18-electron complexes

It’s a helpful rule of thumb: Main group elements prefer forming bonds that give them eight valence electrons; transition metals go for 18 electrons. Now scientists have shown that main group elements calcium, strontium, and barium can form 18-electron complexes with carbon monoxide at temperatures near absolute zero thanks to contributions from the metals’ d orbitals. (Science 2018, DOI: 10.1126/science.aau0839).

Past research already suggested heavy alkaline earth metals like these might break the rule because their d orbitals are often involved in bonding, something usually limited to transition metals. But Pekka Pyykkö, a physical chemist at the University of Helsinki who was not involved in the work, says demonstrating the elements can form 18-electron complexes is a significant step in understanding the rules of alkaline earth metal bonding.

In the new study, Mingfei Zhou of Fudan University and Gernot Frenking of Nanjing Tech University and the University of Marburg reacted calcium, strontium, and barium with carbon monoxide in a solid neon matrix at 4 K and probed the resulting complexes with infrared spectroscopy.

For each resulting metal complex, they observed a single spectroscopic peak for the carbon-oxygen bond, indicating eight molecules coordinated symmetrically. The peak also was at a longer wavelength than free carbon monoxide. That suggests longer than normal carbon-oxygen bonds, which the authors attribute to backbonding from the d orbital just beneath the metals’ valence shells. Zhou explains that, for these three elements, the d orbital’s energy is only slightly higher than the valence s orbital’s, which allows the former to participate in bonding. Quantum chemical calculations confirmed this electronic structure for the three complexes.

Zhou says the d-orbital involvement means “heavy earth alkaline metals might have more chemistry than we previously thought.” One example of chemistry that this d-orbital involvement might explain is a 2017 finding that a calcium reagent can mediate nucleophilic substitutions on benzene.

The group is now exploring whether alkali metal anions like cesium can also form 18-electron complexes.