Alkaline earth carbonyls break the rules
Complexes of Ca, Sr and Ba adopt a transition metal-like 18-electron configuration, rather than the octet more usually found in main group compounds
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.