First Detection of Methylgermylene in the Gas Phase and Time-Resolved Study of Some of Its Reactions

Time-resolved, Gas-phase Kinetic and Quantum Chemical Studies of the Reaction of Germylene with Hydrogen Chloride

Time-resolved studies of germylene, GeH₂ , generated by laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene at 193 nm and monitored by laser absorption, have been carried out to obtain rate constants for its bimolecular reaction with HCl. The reaction was studied in the gas phase, mainly at a total pressure of 10 Torr (in SF₆ bath gas) at five temperatures in the range 295-558 K. Experiments at other pressures showed that these rate constants were unaffected by pressure. The second-order rate constants at 10 Torr (SF₆ bath gas) fitted the Arrhenius equation: log(k/cm³  molecule^-1  s^-1 )=(-12.06±0.14)+(2.58±1.03 kJ mol^-1 )/RTln10 where the uncertainties are single standard deviations. Quantum chemical calculations at G4 level support a mechanism in which an initial weakly bound donor-acceptor complex is formed. This can then rearrange and decompose to give H₂ and HGeCl (chlorogermylene). The enthalpy barrier (36 kJ mol^-1 ) is too high to allow rearrangement of the complex to GeH₃ Cl (chlorogermane).

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Gas Phase Formation of Methylgermylene (HGeCH3)

The methylgermylene species (HGeCH₃ ; X¹ A') has been synthesized via the bimolecular gas phase reaction of ground state methylidyne radicals (CH) with germane (GeH₄ ) under single collision conditions in crossed molecular beams experiments. Augmented by electronic structure calculations, this elementary reaction was found to proceed through barrierless insertion of the methylidyne radical in one of the four germanium-hydrogen bonds on the doublet potential energy surface yielding the germylmethyl (CH₂ GeH₃ ; X² A') collision complex. This insertion is followed by a hydrogen shift from germanium to carbon and unimolecular decomposition of the methylgermyl (GeH₂ CH₃ ; X² A') intermediate by atomic hydrogen elimination leading to singlet methylgermylene (HGeCH₃ ; X¹ A'). Our investigation provides a glimpse at the largely unknown reaction dynamics and isomerization processes of the carbon-germanium system, which are quite distinct from those of the isovalent carbon system thus providing insights into the intriguing chemical bonding of organo germanium species on the most fundamental, microscopic level.

Thermochemistry of germanium and organogermanium compounds

This article reviews the current state of thermochemistry (enthalpies of formation) of germanium and organogermanium compounds.

Excited-State Photolytic Mechanism of Cyclopentene Containing a Group 14 Element: An MP2-CAS//CASSCF Study

The potential energy surfaces corresponding to the photolytic reactions of 1,2-dimethyl-cyclopentene, 3,4-dimethyl-silacyclopent-3-ene, and 3,4-dimethyl-germacyclopent-3-ene were investigated by employing the CAS(6,6)/6-311G(d) and MP2-CAS-(6,6)/6-311++G(3df,3pd)//CAS(6,6)/6-311G(d) methods. Also, six kinds of substituted germacyclopent-3-ene were used as model reactants by way of the CASSCF and MP2-CAS methods to study their photolytic mechanisms. The theoretical findings indicate that the photolysis of the above reactants all adopt the same reaction path as follows: reactant → Franck-Condon region → conical intersection → germylene and 1,3-butadiene. However, the theoretical results demonstrate that no photolysis ((1)(π →π*)) can be observed in the 1,2-dimethyl-cyclopentene system. Above all, the theoretical investigations strongly suggest that both steric effects, originating from the bulky substituents, and the atomic radius of the group 14 element (C, Si, and Ge) play a crucial role in determining the cis/trans selectivity of the conformation of 1,3-butadiene during their photolytic reactions.

What have we learnt about heavy carbenes through laser flash photolysis studies?

Time resolved gas-phase kinetic studies have contributed a great deal of fundamental information about the reactions and reactivity of heavy carbenes (silylenes, germylenes and stannylenes) during the past two decades. In this article we trace the development of our understanding through the mechanistic themes of intermediate complexes, third body assisted associations, catalysed reactions, non-observed reactions and substituent effects. Ab initio (quantum chemical) calculations have substantially assisted mechanistic interpretation and are discussed where appropriate. Trends in reactivity are identified and some signposts to future studies are indicated. This review, although detailed, is not comprehensive.

A comparison of the reactivity of germylene and dimethylgermylene with some methylgermanes. Direct kinetic and quantum chemical studies

Time-resolved studies of germylene, GeH(2), and dimethygermylene, GeMe(2), generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to try to obtain rate coefficients for their bimolecular reactions with dimethylgermane, Me(2)GeH(2), in the gas-phase. GeH(2) + Me(2)GeH(2) was studied over the pressure range 1-100 Torr with SF(6) as bath gas and at five temperatures in the range 296-553 K. Only slight pressure dependences were found (at 386, 447 and 553 K). RRKM modelling was carried out to fit these pressure dependences. The high pressure rate coefficients gave the Arrhenius parameters: log(A/cm(3) molecule(-1) s(-1)) = -10.99 +/- 0.07 and E(a) =-(7.35 +/- 0.48) kJ mol(-1). No reaction could be found between GeMe(2) + Me(2)GeH(2) at any temperature up to 549 K, and upper limits of ca. 10(-14) cm(3) molecule(-1) s(-1) were set for the rate coefficients. A rate coefficient of (1.33 +/- 0.04) x 10(-10) cm(3) molecule(-1) s(-1) was also obtained for GeH(2) + MeGeH(3) at 296 K. No reaction was found between GeMe(2) and MeGeH(3). Rate coefficient comparisons showed, inter alia, that in the substrate germane Me-for-H substitution increased the magnitudes of rate coefficients significantly, while in the germylene Me-for-H substitution decreased the magnitudes of rate coefficients by at least four orders of magnitude. Quantum chemical calculations (G2(MP2,SVP)//B3LYP level) supported these findings and showed that the lack of reactivity of GeMe(2) is caused by a positive energy barrier for rearrangement of the initially formed complexes. Full details of the structures of intermediate complexes and the discussion of their stabilities are given in the paper.