M(L)₈ complexes (M = Ca, Sr, Ba; L = PH₃, PF₃, N₂, CO): Act of an alkaline-earth metal as a conventional transition metal

Alkaline-earth elements have usually been treated as classical main group elements, with the occasional exception in the case of the heaviest element, Ba, which brings the suggestion with renaming it as “honorary transition metal.” However, the conventional transition metal-like behavior of Ca and Sr and the relevance of 18-electron rule to decide the overall structure and stability are counter-intuitive. We, through a series of studies, showed that alkaline-earth metals act as conventional transition metal where the metal-ligand bonding involves dominant interplay of d orbitals of M. Our previous studies on M(CO)₈ and M(N2)₈ in triplet electronic ground state and M(Bz)3 (M=Ca, Sr, Ba; Bz=benzene) in singlet electronic ground state showed that the complexes are stable with respect to single ligand dissociation and they satisfy the 18-electrons rule like transition metal complex. For L=CO and N₂, M in an excited triplet state with ns0(n−1)d2 valence electronic configuration, and for L=Bz, M in an excited singlet state with ns⁰(n−1)d² valence electronic configuration interacts with L predominantly via M(d)→(L)₈ π-backdonation. Moreover, herein we also show that this behavior is not only exclusive to these ligands but also can be extended to PH3 and PF3 ligands. These ligands (i.e., L=PH₃, PF₃, and N₂), which have somewhat lower π-accepting ability than CO, can also compensate the high excitation energy needed for the transition, ns2→ns0(n−1)d2, inducing enough stability in the title complexes to be viable.