Elongated dihydrogen complexes: what remains of the H-H bond?

Of the several hundred examples of transition metal dihydrogen complexes that have been reported to date, the vast majority have H-H distances of less than 1.0 Angstrom. A small number of complexes have been reported with distances in the range of 1.1 to 1.5 Angstrom. These complexes have been termed elongated dihydrogen complexes. In this review, experimental methods for structure determination of such complexes are summarized, along with computational approaches which have proven useful in understanding the structures of these molecules.     

Baryon pairs production ine + e − annihilation at\sqrt s = 2.4 GeV

Search for baryon pairs production ine ⁺ e ^? annihilation at \(\sqrt s = 2386 MeV\) is reported. The data relate to a luminosity of 161 nb^?1 collected by the DM2 experiment at DCI, the Orsay colliding ring. First measurements of directe ⁺ e ^? annihilation into \(\Lambda \bar \Lambda \) and ofe ⁺ e ^?→ \(p\bar p\) at this energy are presented. First observation of a goode ⁺ e ^?→ \(n\bar n\) candidate is reported and upper limits are given fore ⁺ e ^?→ \(n\bar n, \Lambda \bar \sum ^0 + c.c.\) and \(\Sigma ^0 \bar \Sigma ^0 \) .     

Dinuclear iron isonitrile complexes: models for the iron hydrogenase active site

Reaction of Fe(2)(mu-S(2)C(3)H(6))(CO)(6) (1) with 2 equiv of t-BuNC affords a disubstitued species Fe(2)(mu-S(2)C(3)H(6))(CN-t-Bu)(2)(CO)(4) (2). The structure of 2 has been determined by X-ray crystallography, which shows that in the solid state both isonitrile ligands are cis to sulfur. In solution, NMR and IR spectroscopy suggest that multiple isomers are present. Protonation of 2 occurs at the Fe-Fe bond to give a cationic complex 3 as four different isomeric species. Complex 3 does not react with deuterium gas (98 psi) in the absence of light. Irradiation of solutions of 3 with visible light under D(2) gas leads to formation of HD.     

Synthesis and properties of compressed dihydride complexes of iridium: theoretical and spectroscopic investigations

Reaction of [Cp*Ir(P-P)Cl][B(C6F5)4] (P-P = bisdimethydiphosphinomethane (dmpm), bisdiphenyldiphosphinomethane (dppm)) with [Et3Si][B(C6F5)4] in methylene chloride under 1 atm of hydrogen gas affords the dicationic compressed dihydride complexes [Cp*Ir(P-P)H2][B(C6F5)4]2. These dicationic complexes are highly acidic and are very readily deprotonated to the corresponding monohydride cations. When the preparative reaction is carried out under HD gas, the hydride resonance exhibits JHD = 7-9 Hz, depending upon the temperature of observation, with higher values of JHD observed at higher temperatures. A thermally labile rhodium analogue, [CpRh(dmpm)(H2)][B(C6F5)4]2, was prepared similarly. A sample prepared with HD gas gave JHD = 31 Hz and J(HRh) = 31 Hz, allowing the Rh complex to be identified as a dihydrogen complex. Quantum dynamics calculations on a density functional theory (DFT) potential energy surface have been used to explore the structure of the Ir complexes, with particular emphasis on the nature of the potential energy surface governing the interaction between the two hydride ligands and the Ir center.     

A carbonyl-rich bridging hydride complex relevant to the Fe-Fe hydrogenase active site

Protonation of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) with an acid derived from [SiEt(3)][B(C(6)F(5))(4)] and HCl affords a hydride-bridged dimer.     

Synthesis and investigation of [Cp(PMe(3))Rh(H)(H(2))](+) and its partially deuterated and tritiated isotopomers: evidence for a hydride/dihydrogen structure

Hydrogenolysis of [Cp(PMe(3))Rh(Me)(CH(2)Cl(2))](+)BAr'(4)(-) (4, Ar' = 3,5-C(6)H(3)(CF(3))(2)) in dichloromethane afforded the nonclassical polyhydride complex [Cp*PMe(3))Rh(H)(H(2))](+)BAr'(4)(-) (1), which exhibits a single hydride resonance at all accessible temperatures in the (1)H NMR spectrum. Exposure of solutions of 1 to D(2) or T(2) gas resulted in partial isotopic substitution in the hydride sites. Formulation of 1 as a hydride/dihydrogen complex was based upon T(1) (T(1)(min) = 23 ms at 150 K, 500 MHz), J(H-D) (ca. 10 Hz), and J(H-T) (ca. 70 Hz) measurements. The barrier (Delta G(++)) to exchange of hydride with dihydrogen sites was determined to be less than ca. 5 kcal/mol. Protonation of Cp(PMe(3))Rh(H)(2) (2) using H(OEt(2))(2)BAr'(4) resulted in binuclear species [(Cp(PMe(3))Rh(H))(2)(mu-H)](+)BAr'(4)(-) (3), which is formed in a reaction involving 1 as an intermediate. Complex 3 contains two terminal hydrides and one bridging hydride ligand which exchange with a barrier of 9.1 kcal/mol as observed by (1)H NMR spectroscopy. Additionally, the structures of 3 and 4, determined by X-ray diffraction, are reported.     

Determination of the temperature dependence of the H-D spin-spin coupling constant and the isotope effect on the proton chemical shift for the compressed dihydride complex [CpIr(P-P)H2]2+

Complex [CpIr(dmpm)H(2)](2+) (dmpm = bis(dimethylphosphino)methane) has been reported to display temperature-dependent spin-spin coupling constant ((1)J(HD)) and isotope effect on the (1)H NMR chemical shift (Deltadelta). A combined electronic structure density functional theory + quantum nuclear dynamics study is used to determine from first-principles the unusual temperature dependence of the spin-spin coupling constant. It is found that the potential energy surface describing the motion of the Ir-H(2) unit has a deeper minimum in the dihydride region and is characterized by important anharmonicities. These anomalies affect the nature of the vibrational states of the unit and are the main reason for the unusual temperature dependence of (1)J(HD) and Deltadelta. These results suggest experimental tests to identify compressed dihydride transition metal complexes.