Heteroborospherene clusters 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 is an element of B-18(-) and based on extensive global minimum searches and first-principles theory calculations, we present herein the possibility of the novel planar Ni is an element of B-18 (1), cage-like heteroborospherenes Ni-n is an element of B-40 (n = 1-4) (2-5), and planar heteroborophenes Ni-2 is an element of B-14 (6, 7) which all contain planar or quasi-planar heptacoordinate transition-metal (phTM) centers in eta(7)-B-7 heptagons. The nearly degenerate Ni-2 is an element of B-14 (6) and Ni-2 is an element of B-14 (7) monolayers are predicted to be metallic in nature, with Ni-2 is an element of B-14 (6) composed of interwoven boron double chains with two phNi centers per unit cell being the precursor of cage-like Ni-n is an element of B-40 (n = 1-4) (2-5). Detailed bonding analyses indicate that Ni-n is an element of B-40(n = 1-4) (2-5) and Ni-2 is an element of B-14 (6, 7) possess the universal bonding pattern of sigma + pi 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 sigma-coordination bonds between the remaining tangential Ni 3d orbitals (3d(x2-y2) and 3dxy) and the eta(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.