“Direct”, “Inverted” and “Superdirect” Sigma Bonds: Substituent Angles and Bond Energy. The Case of the CC Bonds in Hydrocarbons.

The A-A dissociation energy with respect to geometry frozen fragments (BE) of has been calculated for AHn-AHn models (C2H6, Si2H6, Ge2H6 and N2H4) as a function of  = H-A-A angles. Following a sigmoidal variation, BE decreases rapidly when  decreases to yield “inverted bonds” for  < 90° and finally nearly vanishes. On the contrary BE increases when  increases with respect to the equilibrium value; we propose the term of “superdirect” to qualify such bonds. This behaviour has been qualitatively interpreted in the case of C2H6 by the variation of the overlap of both s+p hybrids. The BE of one C-H bond in CH3 behaves similarly as function of its H-C-H angle with the other three hydrogen atoms. The concept of inverted/direct/superdirect bond is generalized to any CC sigma bond in hydrocarbons and can be characterized by the mean angle value <> of this bond with substituents (multiple-bonded substituents are considered as several substituents). This applies as well to formal single bonds as to sigma bonds in a formally multiple bond. <br>

“Direct”, “Inverted” and “Superdirect” Sigma Bonds: Substituent Angles and Bond Energy. The Case of the CC Bonds in Hydrocarbons.

The A-A dissociation energy with respect to geometry frozen fragments (BE) of has been calculated for AHn-AHn models (C2H6, Si2H6, Ge2H6 and N2H4) as a function of  = H-A-A angles. Following a sigmoidal variation, BE decreases rapidly when  decreases to yield “inverted bonds” for  < 90° and finally nearly vanishes. On the contrary BE increases when  increases with respect to the equilibrium value; we propose the term of “superdirect” to qualify such bonds. This behaviour has been qualitatively interpreted in the case of C2H6 by the variation of the overlap of both s+p hybrids. The BE of one C-H bond in CH3 behaves similarly as function of its H-C-H angle with the other three hydrogen atoms. The concept of inverted/direct/superdirect bond is generalized to any CC sigma bond in hydrocarbons and can be characterized by the mean angle value <> of this bond with substituents (multiple-bonded substituents are considered as several substituents). This applies as well to formal single bonds as to sigma bonds in a formally multiple bond. <br>

Synthesis and Reactivity Studies of Cationic Ir(III) Alkylidines. α-Hydride Abstraction Reactions

The chemistry of late transition metal alkylidenes [M=CR₂], where R is H a hydrocarbyl group, have attracted widespread attention although mainly with reference to complexes of metals in low oxidation state. We focus in this paper on reactions based on electrophilic attacks by Ph₃C⁺ that allow either isolation of stable cationic Ir(III) alkylidenes, considerably more attractive than well-known Ir(I) counterparts, or the generation of very reactive variants that experience fast migratory insertion into existing Ir-C and Ir-H sigma bonds. The present studies are based on (η⁵-C₅Me₅)Ir(III) complexes that bear a cyclometalated PMeXyl₂ ligand (Xyl = 2,6-Me₂C₆H₃). The contribution of different monoanionic ligands (chloride, alkyl or hydride) to either stabilize the Ir=CR₂ linkage or provide facile reactivity routes has been investigated, including the use of various deuterium isotopologues of the iridium complex precursors.

Neutral nickel(ii) phthalocyanine as a stable catalyst for visible-light-driven hydrogen evolution from water

A neutral nickel(ii) phthalocyanine complex with two Ni–N sigma bonds was reported to be a highly stable catalyst for visible-light-driven H2 production from water in a homogeneous system when paired with an iridium(iii) photosensitizer and triethanolamine as a sacrificial electron donor.