Triply Bonded Zero-Valent Heavy Alkaline Earth Metals (Ae = Ca, Sr, Ba) in Group 9 Carbonyl Anion Complexes

The involvement of (n – 1)d orbitals in covalent interactions with main-group elements has recently reshaped the fundamental understanding of bonding in heavy alkaline earth metals (Ae = Ca, Sr, Ba), particularly in terms of coordination modes, bond strength, and bond order. Herein, we report an unusual class of Group 9 carbonyl anion complexes, AeM(CO)3– (M = Co, Rh, Ir), featuring direct Ae-transition metal interactions with pronounced multiple-bond character. Comprehensive structural searches reveal that the global minimum structures of these complexes adopt C3v-symmetric geometries with a 1A1 ground electronic state, in which the Ae center occupies a terminal position and forms an exceptionally short Ae–M bond along the principal axis. Detailed bonding analyses indicate that the Ae–M interaction can be described as a formal triple bond, comprising a dominant σ component, supplemented by weaker π contributions involving donation from d orbitals of the transition metal into vacant orbitals of Ae. These interactions give rise to an unusual zero-valent description of the heavy alkaline earth metals and establish high-order Ae–transition-metal bonding, leading to remarkably short Ae–M distances within carbonyl frameworks.