Transition-Metal Chemistry of the Heavier Alkaline Earth Atoms Ca, Sr, and Ba

Alkaline earth elements beryllium, magnesium, calcium, strontium, and barium with an ns(2) valence-shell configuration are usually classified as main-group elements that belong to the s-block atoms. For a long time, the elements were considered to be rather chemically uninteresting atomic species due to preconceived ideas about bonding, structure, and reactivity. They typically use the two ns valence electrons in forming ionic salt compounds with the metal in a formal oxidation state of +2. For the heavier alkaline earth atoms, calcium, strontium, and barium, their (n - 1)d atomic orbitals (AOs) are empty but lie close in energy to the valence np orbitals. Earlier theoretical investigations have already suggested that these elements can employ the (n - 1)d AOs to some extent to form polar bonds in divalent species in which the alkaline earth metal centers are sufficiently positively charged. The d orbital involvement increases from Ca to Sr and markedly in Ba. Thus, barium has been termed an honorary transition metal. Recently, molecular complexes of Ca, Sr, and Ba were prepared in the gas phase and in a low-temperature solid neon matrix and were detected by infrared spectroscopy. An analysis of the electronic structures of [Ba(CO)](+), [Ba(CO)](-), saturated coordinated octacarbonyls [M(CO)(8)] and [M(CO)(8)](+), isoelectronic dinitrogen complexes [M(N-2)(8)] and [M(N-2)(8)](+), and the tribenzene complexes [M(Bz)(3)] (M = Ca, Sr, Ba) revealed that the metal-ligand bonding can be straightforwardly discussed using the traditional Dewar-Chatt-Duncanson (DCD) model as in classical transition-metal complexes. The metal-ligand bonds can be explained with metal -> ligand pi back donation from occupied metal (n - 1)d AOs to vacant antibonding pi molecular orbitals of the ligands with concomitant sigma donation from occupied MOs of the ligands to vacant metal d orbitals of the alkaline earth atoms. In addition, heteronuclear Ca-Fe carbonyl cation complexes were also produced in the gas phase. Bonding analysis of the coordination saturated [CaFe(CO)(10)](+) complex implies that it can be described by the bonding interactions between a [Ca(CO)(6)](2+) fragment and an [Fe(CO)(4)](-) anion fragment in forming a Fe -> Ca d-d dative bond. The nature of metal-ligand and metal-metal bonding was quantitatively elucidated by the energy decomposition analysis in conjunction with the natural orbitals for the chemical valence (EDA-NOCV) method, which indicate that the (n - 1)d AOs of the alkaline earth metals are the dominant orbitals participating in the covalent interactions, just as typical transition metals. The results indicate that the heavier alkaline earth elements have a much richer covalent chemistry than previously thought. These findings, along with earlier studies, suggest that the heavier alkaline earth atoms Ca, Sr, and Ba should be classified as transition metals rather than main group atoms in the periodic table of the elements. This interesting structural chemistry, together with some recently reported examples of spectacular reactivity, establishes these elements as exciting and promising research targets in current research.

Automated summary

The alkaline earth elements, previously considered chemically uninteresting, are now recognized for their rich covalent chemistry and similar coordination structures to classic transition metals. Their properties and structures suggest a classification closer to transition metals rather than main group elements.