High-spin face-capped deltahedra in divanadadicarbaboranes are very different from the singlet structures of chromium analogues

Abstract

Using density functional theory (DFT), divanadadicarbaboranes Cp₂V₂C₂B_n−4H_n−2 are found to have very different low-energy structures than the corresponding dichromadicarbaboranes Cp₂Cr₂C₂B_n−4H_n−2. Thus, the low-energy divanadadicarbaborane structures with n vertices have triplet or quintet states rather than singlet spin states, frequently based on an (n − 1)-vertex VC₂B_n−4 deltahedron having a face capped by the second vanadium atom bearing most of the spin density. Such structures are analogous to the low-energy structures of dimanganaboranes Cp₂Mn₂B_n−2H_n−2, even though the vanadium and manganese systems are not isoelectronic with each other. Most of the low-energy 8-vertex Cp₂V₂C₂B₄H₆ structures are based on the hexagonal bipyramid, whereas most of the low-energy 9- and 10-vertex structures are based on the 9-vertex isocloso deltahedron. The bicapped square antiprism capped by a high-spin vanadium vertex is characteristic of the low-energy 11-vertex Cp₂V₂C₂B₇H₉ structures. Similarly, an 11-vertex closo deltahedron capped by a high-spin vanadium vertex is the lowest energy Cp₂V₂C₂B₈H₁₀ structure by a substantial margin of ∼18 kcal mol⁻¹.