Expanded Inverse-Sandwich Complexes of Lanthanum Borides: LaB and LaB.

Inverse-sandwich structures have been observed recently for dilanthanide boride clusters, in which two Ln atoms sandwich a monocyclic B ring for = 7-9. An interesting question is if larger B rings are possible to form such inverse-sandwich clusters. Here we address this question by investigating LaB and LaB using photoelectron spectroscopy and quantum chemical calculations. Photoelectron spectra of LaB and LaB show complicated, but well-resolved, spectral features that are used to compare with theoretical calculations. We have found that global minimum structures of the two clusters are based on the octa-boron ring. The global minimum of LaB consists of two chiral enantiomers with symmetry, which can be viewed as adding a B unit off-plane to the B ring, whereas that of LaB can be viewed as adding a B unit in-plane to the B ring in a second coordination shell. Chemical bonding analyses reveal localized B-B bonds on the edge of the clusters and delocalized bonds in the expanded boron frameworks. The interactions between the La atoms and the boron frameworks include the unique (d-p)δ bonding, which was found to be the key for inverse-sandwich complexes with monocyclic boron rings. The current study confirms that the largest monocyclic boron ring to form the inverse-sandwich structures is B and provide insights into the structural evolutions of larger lanthanide boride clusters.