Carbonyls in the s block: Conventional wisdom in chemistry distinguishes transition metals from other elements by their use of d orbitals in bonding. Wu et al. now report that alkaline earth metals can slide their electrons from s- to d-orbital bonding motifs as well (see the Perspective by Armentrout). Calcium, strontium, and barium all form coordination complexes with a cubic arrangement of eight carbonyl ligands and an 18-electron valence shell. The compounds were characterized in frozen neon matrices by vibrational spectroscopy and in gas phase by mass spectrometry. Science , this issue p. 912 ; see also p. 849

DFT calculations have been carried out for coordinatively saturated neutral and charged carbonyl complexes [M(CO)_n]^q where M is a metal atom of groups 2–10. The model compounds M(CO)₂ (M = Ca, Sr, Ba) and the experimentally observed [Ba(CO)]⁺ were also studied. The bonding situation has been analyzed with a variety of charge and energy partitioning approaches. It is shown that the Dewar–Chatt–Duncanson model in terms of M ← CO σ-donation and M → CO π-backdonation is a valid approach to explain the M–CO bonds and the trend of the CO stretching frequencies. The carbonyl ligands of the neutral complexes carry a negative charge, and the polarity of the M–CO bonds increases for the less electronegative metals, which is particularly strong for the group 4 and group 2 atoms. The NBO method delivers an unrealistic charge distribution in the carbonyl complexes, while the AIM approach gives physically reasonable partial charges that are consistent with the EDA-NOCV calculations and with the trend of the C–O stretching frequencies. The AdNDP method provides delocalized MOs which are very useful models for the carbonyl complexes. Deep insight into the nature of the metal–CO bonds and quantitative information about the strength of the [M] ← (CO)₈ σ-donation and [M(d)] → (CO)₈ π-backdonation visualized by the deformation densities are provided by the EDA-NOCV method. The large polarity of the M–CO π orbitals toward the CO end in the alkaline earth octacarbonyls M(CO)₈ (M = Ca, Sr, Ba) leads to small values for the delocalization indices δ(M–C) and δ(M···O) and significant overlap between adjacent CO groups, but the origin of the charge migration and the associated red-shift of the C–O stretching frequencies is the [M(d)] → (CO)₈ π-backdonation. The heavier alkaline earth metals calcium, strontium and barium use their s/d valence orbitals for covalent bonding. They are therefore to be assigned to the transition metals.

In this review, we present our recent studies on the MO + CH₄ and M + CH₃OH model reactions (M = transition metals) in order to provide quantitative information regarding the mechanisms for the catalytic methane-to-methanol conversion process. The reaction intermediates were trapped and probed by matrix isolation infrared absorption spectroscopy. Various important intermediates including OM(CH₄), M(CH₃OH), CH₃MOH, CH₃M(O)H and CH₃OMH are identified via isotopic substitution experiments in the MO + CH₄ and M + CH₃OH reactions for selected early and late transition metals. Based on the observed reaction intermediates, some unprecedented reaction pathways are proposed. Complementary quantum chemical calculations support the intermediate identification and help to gain insight into the reaction mechanisms and periodic trends.

We report the isolation and spectroscopic identification of the eight-coordinated alkaline earth metal–dinitrogen complexes M(N₂)₈ (M=Ca, Sr, Ba) possessing cubic (O_h) symmetry in a low-temperature neon matrix. The analysis of the electronic structure reveals that the metal-N₂ bonds are mainly due to [M(d_π)]→(N₂)₈ π backdonation, which explains the observed large red-shift in N-N stretching frequencies. The adducts M(N₂)₈ have a triplet (³A_1g) electronic ground state and exhibit typical bonding features of transition metal complexes obeying the 18-electron rule. We also report the isolation and bonding analysis of the charged dinitrogen complexes [M(N₂)₈]⁺ (M=Ca, Sr).