A stable and crystalline triarylgermyl radical: structure and EPR spectra.

A radical thing: After being obtained unexpectedly in low yields, the synthesis of the first stable triarylgermyl radical (*)Ge[3,5-tBu(2)-2,6-(EtO)(2)C(6)H](3) (1; C gray, O blue) was considerably optimized, and the product was investigated by X-ray analysis and EPR spectroscopy. The results were compared with DFT-MO studies for the model compound (*)Ge[2,6-(MeO)(2)C(6)H(3)].

Molecular Structures of M(2)(CO)(9) and M(3)(CO)(12) (M = Fe, Ru, Os): New Theoretical Insights.

A different number of bridging carbonyls is found in bi- or trinuclear clusters having the title formulas. Comparative calculations at the SCF, MP2, and DFT levels of theory show that only the latter is able to describe properly the energetics of various isomers of the whole triad. For the first-row transition metal, DFT gives excellent agreement with the experimental structures, whereas the MP2 approach fails completely. Conversely for the second- and third-row metals, the best agreement with the experiment is obtained by the MP2 optimizations. The quantitative computational results, associated with a qualitative MO analysis, allow one to conclude that the structural preferences are determined by a critical balance of metal-bridge bonding, metal-metal bonding, and intermetallic repulsion. Although the M-M bond order is expected to be 1 in all cases, the bridge-supported bond is experimentally and computationally shorter than the unsupported one. By contrast, the trend for the overlap population (OP) is reversed, with even negative values for the shorter bridge bonds. For the latter, only a weak attractive interaction stems from the almost pure t(2g) orbitals, taken as metal lone pairs or eventually responsible for back-donation (formation of metal-bridge sigma bonds). Thus, the negative OP values are consistent with a prevailing repulsion between the latter levels. In the iron systems, with more contracted metal orbitals, the direct metal-metal repulsion is relatively weak while the metal-bridge bonds are sufficiently strong. This is not equally true for the more diffuse ruthenium and osmium orbitals, so the alternative nonbridged structure is preferred.

Integration of electron density and molecular orbital techniques to reveal questionable bonds: the test case of the direct Fe-Fe bond in Fe2(CO)9.

The article illustrates the advantages of partitioning the total electron density rho(rb), its Laplacian (inverted Delta)2 rho(rb), and the energy density H(rb) in terms of orbital components. By calculating the contributions of the mathematically constructed molecular orbitals to the measurable electron density, it is possible to quantify the bonding or antibonding character of each MO. This strategy is exploited to review the controversial existence of direct Fe-Fe bonding in the triply bridged Fe2(CO)9 system. Although the bond is predicted by electron counting rules, the interaction between the two pseudo-octahedral metal centers can be repulsive because of their fully occupied t(2g) sets. Moreover, previous atoms in molecules (AIM) studies failed to show a Fe-Fe bond critical point (bcp). The present electron density orbital partitioning (EDOP) analysis shows that one sigma bonding combination of the t(2g) levels is not totally overcome by the corresponding sigma* MO, which is partially delocalized over the bridging carbonyls. This suggests the existence of some, albeit weak, direct Fe-Fe bonding.

The (P4HMes4)- anion: lability, fluxionality, and structural ambiguity (Mes = 2,4,6-Me3C6H2).

The synthesis and structural characterization of the tetramesityltetraphosphanide anion, (P4HMes4)- (1), is described. It is shown that 1 partially decomposes in solution and displays an unsymmetrical structure in which, depending on conditions, the proton may or may not fluctuate between the terminal P atoms of the P4 chain.

A counterintuitive structural effect of metal-metal bond protonation and its electronic underpinnings.

Protonation across the metal-metal bond in the complexes [(CO)(2)M(mu-dppm)(mu-PtBu(2))(mu-H)M(CO)(2)] (M=Fe or Ru, dppm=Ph(2)PCH(2)PPh(2)) induces M-M bond shortening of up to about 0.05 A. DFT calculations on simplified iron models reproduce this trend well. Conversely, the computations show that the M-M distance in the dimer [{Cp*Ir(CO)}(2)] lengthens with two consecutive protonations, but there are no crystal structure determinations to highlight the effects on the Ir-Ir bond. DFT calculations and the analogous cobalt system confirm that the transformation of a two-electron, two-center (2e-2c) bond into a 2e-3c bond is accompanied by the predicted elongation. An MO analysis indicated similar nature and evolution of the M-M bonding these cases. In particular, the HOMOs of the mono-hydrido cations [Cp(CO)M(mu-H)M(CO)Cp](+) (M=Ir, Co) have evident M-M bent-bond character, and hence subsequent protonation invariably causes a decrease in the bond index. The Fe(2) and Co(2) systems have also been analyzed with the quantum theory of atoms in molecules (QTAIM) method, but in no case was an M-M bond critical point located unless an artificially shorter M-M distance was imposed. However, the trends for the atoms-in-molecules (AIM) bond delocalization indexes delta(M-M) confirm the overall M-M bond weakening on protonation. In conclusion, all the computational results for the iron system indicate that the paradigm of a direct correlation between bond strength and distance is not always applicable. This is attributable to a very flat potential energy surface and various competing effects imposed by the bridging ligands.

Molecular structures and electronic transitions of 3,6-diphenyl-1,2-dithiin and its radical cation: a spectroelectrochemical and DFT study.

One-electron oxidation of 3,6-diphenyl-1,2-dithiin yields the corresponding radical cation. The product is stable at low temperatures and can be distinguished by a triplet EPR signal. Cyclic voltammetric, UV-vis spectroelectrochemical, and DFT studies were performed to elucidate its molecular structure and electronic properties. Time-dependent DFT calculations reproduce appreciably well the UV-vis spectral changes observed during the oxidation. The results reveal a moderately twisted structure of the 1,2-dithiin heterocycle in the radical cation.

Orbital contributions to the molecular charge and energy density distributions in Co2(CO)8.

For Co2(CO)8, the representative of a whole class of bridged cobalt complexes, the 18-electron rule predicts a direct metal-metal bond in addition to the metal-bridge bonds. By intuition, this bond should have bent-bond character. However, it is well-known from charge density analyses that no bond critical point exists in the corresponding spatial region. Otherwise, the energy density distribution points to a certain stabilizing contribution of this local area to the total molecular energy. It is shown that a partitioning of the total charge and energy densities into orbital contributions can lead to a deeper insight into complex bonding properties.

Energy density distribution in bridged cobalt complexes.

In the theory of atoms in molecules (AIM), the charge density is usually a suitable tool for bonding analyses. However, problems arise in some cases. So, no direct Co-Co bond is found in Co2(CO)8. It is shown that the energy density gives deeper insight into the bonding properties. This is demonstrated for Co2(CO)8, Co4(CO)12, and Co2(CO)6(InMe)2. The strategy is not restricted to transition metal compounds; it should be useful to identify any weak bonding or antibonding interactions.

Activation of molecular hydrogen over a binuclear complex with Rh2S2 core: DFT calculations and NMR mechanistic studies.

The dicationic complex [(triphos)Rh(mu-S)(2)Rh(triphos)](2+), 1 (modeled as 1c) [triphos = CH(3)C(CH(2)PPh(2))(3)], is known to activate two dihydrogen molecules and produce the bis(mu-hydrosulfido) product [(triphos)(H)Rh(mu-SH)(2)Rh(H)(triphos)](2+), 2 (modeled as 2b), from which 1 is reversibly obtained. The possible steps of the process have been investigated with DFT calculations. It has been found that each d(6) metal ion in 1c, with local square pyramidal geometry, is able to anchor one H(2) molecule in the side-on coordination. The step is followed by heterolytic splitting of the H-H bond over one adjacent and polarized Rh-S linkage. The process may be completed before the second H(2) molecule is added. Alternatively, both H(2) molecules are trapped by the Rh(2)S(2) core before being split in two distinct steps. Since the ambiguity could not be solved by calculations, (31)P and (1)H NMR experiments, including para-hydrogen techniques, have been performed to identify the actual pathway. In no case is there experimental evidence for any Rh-(eta(2)-H(2)) adduct, probably due to its very short lifetime. Conversely, (1)H NMR analysis of the hydride region indicates only one reaction intermediate which corresponds to the monohydride-mu-hydrosulfide complex [(triphos)Rh(H)(mu-SH)(mu-S)Rh(triphos)](2+) (3) (model 5a). This excludes the second hypothesized pathway. From an energetic viewpoint the computational results support the feasibility of the whole process. In fact, the highest energy for H(2) activation is 8.6 kcal mol(-1), while a larger but still surmountable barrier of 34.6 kcal mol(-1) is in line with the reversibility of the process.