Heteroborospherene Nin∈B40(n=1-4) and heteroborophene monolayers Ni2∈B14 with planar heptacoordinate transition-metal centers in η7-B7 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 ∈ B₁₈  (1), cage-like heteroborospherenes Ni_n ∈ B₄₀ (n = 1–4) (2–5), and planar heteroborophenes Ni₂ ∈ B₁₄  (6, 7) which all contain planar or quasi-planar heptacoordinate transition-metal (phTM) centers in η 7 -B 7  heptagons. The nearly degenerate Ni₂ ∈ B₁₄ (6) and Ni₂ ∈ B₁₄  (7) monolayers are predicted to be metallic in nature, with Ni₂ ∈ B₁₄  (6) composed of

interwoven boron double chains with two phNi centers per unit cell being the precursor of cage-like Ni_n ∈ B₄₀ (n = 1–4) (2–5). Detailed bonding analyses indicate that Ni_n ∈ B₄₀ (n = 1–4) (2–5) and Ni₂ ∈ B₁₄ (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_z2² , 3d _zx² , and 3d_yz² ) 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 -B₇  heptagon around it. The IR, Raman, and UV-vis absorption spectra of 1–5 are computationally  simulated to facilitate their experimental characterizations