We have quantum chemically analyzed elementÀ element bonds of archetypal H n XÀ YH n molecules (X, Y = C, N, O, F, Si, P, S, Cl, Br, I), using density functional theory. One purpose is to obtain a set of consistent homolytic bond dissociation energies (BDE) for establishing accurate trends across the periodic table. The main objective is to elucidate the underlying physical factors behind these chemical bonding trends. On one hand, we confirm that, along a period (e. g., from CÀ C to CÀ F), bonds strengthen because the electronegativity difference across the bond increases. But, down a period, our findings constitute a paradigm shift. From CÀ F to CÀ I, for example, bonds do become weaker, however, not because of the decreasing electronegativity difference. Instead, we show that the effective atom size (via steric Pauli repulsion) is the causal factor behind bond weakening in this series, and behind the weakening in orbital interactions at the equilibrium distance. We discuss the actual bonding mechanism and the importance of analyzing this mechanism as a function of the bond distance.
The tuning of metal-metal interactions in multinuclear assemblies is a challenge. Selective P coordination of a redox-active PNO ligand to Au(I) followed by homoleptic metalation of the NO pocket with Ni(II) affords a unique trinuclear Au-Ni-Au complex. This species features two antiferromagnetically coupled ligand-centered radicals and a double intramolecular d(8)-d(10) interaction, as supported by spectroscopic, single-crystal X-ray diffraction, and computational data. A corresponding cationic dinuclear Au-Ni analogue with a stronger d(8)-d(10) interaction is also reported. Although both heterobimetallic structures display rich electrochemistry, only the trinuclear Au-Ni-Au complex facilitates electrocatalytic C-X bond activation of alkyl halides in its doubly reduced state. Hence, the presence of a redox-active ligand framework, an available coordination site at gold, and the nature of the nickel-gold interaction appear to be essential for this reactivity.
We have quantum chemically investigated how methyl substituents affect the stability of alkyl radicals Me m H 3À m C * and the corresponding Me m H 3À m CÀ X bonds (X = H, CH 3 , OH; m = 0 -3) using density functional theory at M06-2X/TZ2P. The state-ofthe-art in physical organic chemistry is that alkyl radicals are stabilized upon an increase in their degree of substitution from methyl < primary < secondary < tertiary, and that this is the underlying cause for the decrease in CÀ H bond strength along this series. Here, we provide evidence that falsifies this model and show that, on the contrary, the Me m H 3À m C * radical is destabilized with increasing substitution. The reason that the corresponding CÀ H bond nevertheless becomes weaker is that substitution destabilizes the sterically more congested Me m H 3À m CÀ H molecule even more.
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