The reaction of [LAlH2 ] (L=HC(CMeNAr)2 , Ar=2,6-iPr2 C6 H3 ) with MeOTf (Tf=SO2 CF3 ) resulted in the formation of [LAlH(OTf)] (1) in high yield. The triflate substituent in 1 increases the positive charge at the aluminum center, which implies that 1 has a strong Lewis acidic character. The excellent catalytic activity of 1 for the hydroboration of organic compounds with carbonyl groups was investigated. Furthermore, it was shown that 1 effectively initiates the addition reaction of trimethylsilyl cyanide (TMSCN) to both aldehydes and ketones. Quantum mechanical calculations were carried out to explore the reaction mechanism.

Scheme 1. Synthesis of aluminummonohydride complex 1. Ar = 2,6-iPr 2 C 6 H 3 ,T f= SO 2 CF 3 .

The ring contraction of six-membered metallabenzynes to five-membered metal-carbene complexes in comparison with the corresponding rearrangement of benzyne to cyclopentadienylidene carbene have been studied by computational quantum mechanical calculations. The rearrangement of benzyne to a cyclopentadienylidene carbene is endothermic by 27.8 kcal mol(-1). The instability of the cyclopentadienylidene carbene is attributed to the electron deficient singlet carbene carbon atom. However, the isolobal replacement of a sp-hybridized carbon atom in benzyne by a 14 valence electron transition metal fragment (M(PH3)2Cl2, M = Fe, Ru and Os) makes the corresponding rearrangement feasible. The first row transition metal iron shows thermodynamic (exothermic by 19.1 kcal mol(-1)) and kinetic preference (energy barrier of 1.8 kcal mol(-1)) towards the carbene complex. However, the preference reduces down the group. The coordination number of the metal as well as the low-lying in-plane M-C π*-MO play a crucial role for this rearrangement. The conversion of metallabenzynes to metal-carbene complexes is also associated with a ring plane rotation to attain an effective overlap between the metal d-orbital and the p-orbital on the carbene carbon atom.

The existence of a metal‐metal bond in organometallic and coordination complexes is a very important aspect. Metal‐carbonyl, carbene‐metal‐carbonyl and metal‐carbene complexes were studied for having this feature. Herein, an air stable dark green color dicobalt coordination complex [Co(III)2(hep)3(N3)3] (1) [hepH = 2‐(2‐ethylhydroxy)pyridine] with three μ‐alkoxide bridges has been synthesized and characterized by X‐ray single crystal diffraction, NMR and UV/vis spectroscopy. Complex 1 has a short Co⋅⋅⋅Co distance (2.595(6) Å) and thus it has been studied by theoretical calculations. QTAIM (quantum theory of atoms in molecules) as well as EDA‐NOCV analysis (energy decomposition analysis ‐ natural orbitals for chemical valence) do not indicate any significant metal‐metal interaction. The bonding in 1 can be best represented by the interaction of two alkokxy bridged valence electrons fragment Co(III)(hep)2N3 (3d6) with Co(III)(hep)(N3)2 (3d6) where the donation of the lone pair of electrons from three bridging Ohep‐atoms stabilizes the dinuclear Co(III) complex. Additionally, thermolysis of 1 at 550 oC led to the formation sponge like Co3O4 oxide.

A detailed quantum mechanical study on the role of transition metal fragments towards the epoxidation reaction of metallabenzynes (1M, M = Fe, Ru, Os) to give metallabenzyne epoxide or metallabenzooxirene (2M), followed by the Wolff type 1,2-rearrangement to give metal carbonyls (4M), has been carried out at the M06/def2-TZVPP//BP86/def2-SVP level of theory. The epoxide or oxirene product (2A) of linear alkynes like acetylene is unstable and converts to a ketene (4A) by highly exothermic (-78.7 kcal mol) Wolff rearrangement via an oxocarbene (3A) intermediate. The epoxide product of cyclic alkynes like benzyne (2C) is comparatively more stable than benzooxocarbene (3C), yet it undergoes rearrangement to cyclopentadienylketene (4C) through Wolff type ring contraction reactions (-65.3 kcal mol). The replacement of one of the carbyne carbons in benzyne by 14 VE metal fragment M(PH)Cl, M = Fe, Ru and Os enhances the stability of the 18 VE metallabenzyne epoxide product (2M, -88.1 kcal mol for 2Fe, -87.2 kcal mol for 2Ru and -82.8 kcal mol for 2Os) as compared to the 16 VE metallacyclohexadienone (3M). The ring contracted 18 VE product of 2M is a carbonyl bridged metallacyclopentadienyl complex (4M, M = Fe, Ru and Os). Even though this interconversion is thermodynamically favourable, it involves a high-energy barrier (19.3, 28.1 and 27.9 kcal mol for 2Fe, 2Ru and 2Os, respectively). However, the hepta-coordinated (5M) terminal metallacyclopentadienyl carbonyl analogues of ketene (4A/4C) are not minima on the potential energy surface.