Heteroborospherene clusters Nin ∈ B40 (n = 1–4) and heteroborophene monolayers Ni2 ∈ B14 with planar heptacoordinate transition-metal centers in η7-B7 heptagons

Abstract

With inspirations from recent discoveries of the cage-like borospherene B₄₀ and perfectly planar Co ∈ B₁₈⁻ 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 η⁷-B₇ 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 η⁷-B₇ heptagon around it. The IR, Raman, and UV-vis absorption spectra of 1–5 are computationally simulated to facilitate their experimental characterizations.