Heteroborospherene clusters Ni(n) ∈ B(40) (n = 1–4) and heteroborophene monolayers Ni(2) ∈ B(14) with planar heptacoordinate transition-metal centers in η(7)-B(7) heptagons

With inspirations from recent discoveries of the cage-like borospherene B(40) and perfectly planar Co ∈ B(18) (−) and based on extensive global minimum searches and first-principles theory calculations, we present herein the possibility of the novel planar Ni ∈ B(18) (1), cage-like heteroborospherenes Ni(n) ∈ B(40) (n = 1–4) (2–5), and planar heteroborophenes Ni(2) ∈ B(14) (6, 7) which all contain planar or quasi-planar heptacoordinate transition-metal (phTM) centers in η(7)-B(7) heptagons. The nearly degenerate Ni(2) ∈ B(14) (6) and Ni(2) ∈ B(14) (7) monolayers are predicted to be metallic in nature, with Ni(2) ∈ B(14) (6) composed of interwoven boron double chains with two phNi centers per unit cell being the precursor of cage-like Ni(n) ∈ B(40) (n = 1–4) (2–5). Detailed bonding analyses indicate that Ni(n) ∈ B(40) (n = 1–4) (2–5) and Ni(2) ∈ B(14) (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(z2) (2), 3d(zx) (2), and 3d(yz) (2)) and two effective nearly in-plane 8c-2e σ-coordination bonds between the remaining tangential Ni 3d orbitals (3d(x2−y2) and 3d(xy)) and the η(7)-B(7) heptagon around it. The IR, Raman, and UV-vis absorption spectra of 1–5 are computationally simulated to facilitate their experimental characterizations.