Heteroborospherene clusters Ni ∈ B (n = 1-4) and heteroborophene monolayers Ni ∈ B with planar heptacoordinate transition-metal centers in η-B heptagons.

With inspirations from recent discoveries of the cage-like borospherene B and perfectly planar Co ∈ B and based on extensive global minimum searches and first-principles theory calculations, we present herein the possibility of the novel planar Ni ∈ B (1), cage-like heteroborospherenes Ni ∈ B (n = 1-4) (2-5), and planar heteroborophenes Ni ∈ B (6, 7) which all contain planar or quasi-planar heptacoordinate transition-metal (phTM) centers in η-B heptagons. The nearly degenerate Ni ∈ B (6) and Ni ∈ B (7) monolayers are predicted to be metallic in nature, with Ni ∈ B (6) composed of interwoven boron double chains with two phNi centers per unit cell being the precursor of cage-like Ni ∈ B (n = 1-4) (2-5). Detailed bonding analyses indicate that Ni ∈ B (n = 1-4) (2-5) and Ni ∈ B (6, 7) possess the universal bonding pattern of σ + π double delocalization on the boron frameworks, with each phNi forming three lone pairs in radial direction (3d, 3d, and 3d) and two effective nearly in-plane 8c-2e σ-coordination bonds between the remaining tangential Ni 3d orbitals (3d and 3d) and the η-B heptagon around it. The IR, Raman, and UV-vis absorption spectra of 1-5 are computationally simulated to facilitate their experimental characterizations.