Abstract
Chemical bonds are traditionally assigned as electron‐sharing or donor‐acceptor/dative. External criteria such as the nature of the dissociation process, energy partitioning schemes, or quantum chemical topology are invoked to assess the bonding situation. However, for systems with marked multi‐reference character, this binary categorization might not be precise enough to render the bonding properties. A third scenario can be foreseen: spin polarized bonds. To illustrate this, the case of a NaBH3 − cluster is presented. According to the analysis NaBH3 − exhibits a strong diradical character and cannot be classified as either electron‐sharing or a dative bond. Elaborated upon are the common problems of popular bonding descriptions. Additionally, a simple model, based on the bond order and local spin indicators, which discriminates between all three bonding situations, is provided.
Highlights
Chemical bonds are traditionally assigned as electron-sharing or donor-acceptor/dative
The increase of local spin is concomitant with polarization of the Fe=O unit stands behind debates over its the decrease of the covalent bond order due to spin polarelectronic structure, namely oxo-iron(IV) vs. oxyl-iron(III) ization
We have investigated the lowest singlet and triplet electronic states of NaBH3À at the CASPT2 level
Summary
Considering a simple two-electron single-determinant interaction than the donor-acceptor one. The increase of local spin is concomitant with polarization of the Fe=O unit stands behind debates over its the decrease of the covalent bond order due to spin polarelectronic structure, namely oxo-iron(IV) vs oxyl-iron(III) ization. Signatures of diradical character are a small singlet-triplet gap and a spin-polarized (broken-symmetry, BS) solution below the closed-shell (CS) description of single determi-. EDA is not designed to distinguish a classical electron-sharing from a spin-polarized interaction and, in the limiting case, from a diradical!. For the previously discussed A-B interaction, in the limiting case of having a perfect singlet diradical, one would expect the local spins to be hS2iA = hS2iB = 3/4 and the diatomic term to amount to hS2iAB = À3/4, indicating a perfect entanglement of the electrons.[10]. Relative to the CS state and Na-B equilibrium distance (Re in ) for NaBH3À. hS2i and diradical character (nrad).[a]
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