Low-valent homobimetallic Rh complexes: influence of ligands on the structure and the intramolecular reactivity of Rh–H intermediates

Supporting two metal binding sites by a tailored polydentate trop-based (trop - 5H-dibenzo[a,d] cyclohepten-5-yl) ligand yields highly unsymmetric homobimetallic rhodium(I) complexes. Their reaction with hydrogen rapidly forms Rh hydrides that undergo an intramolecular semihydrogenation of two C≡C bonds of the trop ligand. This reaction is chemoselective and converts C≡C bonds to a bridging carbene and an olefinic ligand in the first and the second semihydrogenation steps, respectively. Stabilization by a bridging diphosphine ligand allows characterization of a Rh hydride species by advanced NMR techniques and may provide insight into possible elementary steps of H₂ activation by interfacial sites of heterogeneous Rh/C catalysts

New Data on the Isomorphism in Eudialyte-Group Minerals. 2. Crystal-Chemical Mechanisms of Blocky Isomorphism at the Key Sites

The review considers various complex mechanisms of isomorphism in the eudialyte-group minerals, involving both key positions of the heteropolyhedral framework and extra-framework components. In most cases, so-called blocky isomorphism is realized when one group of atoms and ions is replaced by another one, which is accompanied by a change in the valence state and/or coordination numbers of cations. The uniqueness of these minerals lies in the fact that they exhibit ability to blocky isomorphism at several sites of high-force-strength cations belonging to the framework and at numerous sites of extra-framework cations and anions

Hydrokenomicrolite, (□,H2O)2Ta2(O,OH)6(H2O), a new microlite-group mineral from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil

Hydrokenomicrolite, (□,H2O)2Ta2(O,OH)6(H2O) or ideally □2Ta2[O4(OH)2](H2O), is a new microlite-group mineral approved by the CNMNC (IMA 2011-103). It occurs as an accessory mineral in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. Associated minerals are: microcline, albite, quartz, muscovite, spodumene, “lepidolite”, cassiterite, tantalite-(Mn), monazite-(Ce), fluorite, “apatite”, beryl, “garnet”, epidote, magnetite, gahnite, zircon, “tourmaline”, bityite, and other microlite-group minerals under study. Hydrokenomicrolite occurs as euhedral octahedral crystals, occasionally modified by rhombododecahedra, untwinned, from 0.2 to 1.5 mm in size. The crystals are pinkish brown and translucent; the streak is white, and the luster is adamantine to resinous. It is non-fluorescent under ultraviolet light. Mohs hardness is 4½–5, tenacity is brittle. Cleavage is not observed; fracture is conchoidal. The calculated density is 6.666 g/cm3. The mineral is isotropic, ncalc = 2.055. The infrared spectrum contains bands of O-H stretching vibrations and H-O-H bending vibrations of H2O molecules. The chemical composition (n = 3) is [by wavelength-dispersive spectroscopy (WDS), H2O calculated from crystal-structure analysis, wt%]: CaO 0.12, MnO 0.27, SrO 4.88, BaO 8.63, PbO 0.52, La2O3 0.52, Ce2O3 0.49, Nd2O3 0.55, Bi2O3 0.57, UO2 4.54, TiO2 0.18, SnO2 2.60, Nb2O5 2.18, Ta2O5 66.33, SiO2 0.46, Cs2O 0.67, H2O 4.84, total 98.35. The empirical formula, based on 2 cations at the B site, is [□0.71(H2O)0.48Ba0.33Sr0.27U0.10Mn0.02Nd0.02Ce0.02La0.02Ca0.01 Bi0.01Pb0.01]Σ2.00 (Ta1.75Nb0.10Sn0.10Si0.04Ti0.01)Σ2.00[(O5.77(OH)0.23]Σ6.00[(H2O)0.97Cs0.03]Σ1.00. The strongest eight X‑ray powder-diffraction lines [d in Å(I)(hkl)] are: 6.112(86)(111), 3.191(52)(311), 3.052(100)(222), 2.642(28)(400), 2.035(11)(511)(333), 1.869(29)(440), 1.788(10)(531), and 1.594(24)(622). The crystal structure refinement (R1 = 0.0363) gave the following data: cubic, Fd3m, a = 10.454(1) Å, V = 1142.5(2) Å3, Z = 8. The Ta(O,OH)6 octahedra are linked through all vertices. The refinement results and the approximate empirical bond-valences sums for the positions A (1.0 v.u.) and Y′ (0.5 v.u.), compared to valence calculations from electron microprobe analysis (EMPA) and ranges expected for H2O molecules, confirm the presence of H2O at the A(16d) site and displaced from the Y(8b) to the Y′(32e) position. The mineral is characterized by H2O dominance at the Y site, vacancy dominance at the A site, and Ta dominance at the B site.FAPESP (08/04984-7, 09/09125-5, 11/22407-0)CNPqRFBR (11-05-12001-ofi-m-2011)

New Data on the Isomorphism in Eudialyte-Group Minerals. 1. Crystal Chemistry of Eudialyte-Group Members with Na Incorporated into the Framework as a Marker of Hyperagpaitic Conditions

A review of the crystal chemistry of Fe-deficient eudialyte-group minerals is given. Specific features of cation distribution over key sites in the crystal structure, including partial substitution of Fe2+ with Na, Mn and Zr at the M2 site are discussed. It is concluded that Na-dominant (at the M2 site) eudialyte-group members (M2Na-EGMs) are markers of specific kinds of specific peralkaline (hyperagpaitic) igneous rocks and pegmatites. New data are obtained on the chemical composition, IR spectra and crystal chemistry for two samples of M2Na-EGMs with disordered M1 cations, which are a potentially new mineral species with the simplified formula (Na,H2O)15Ca6Zr3[Na2(Fe,Zr)][Si26O72](OH)2Cl·nH2O