Discrete trinuclear complexes of niobium and tantalum related to the local structure in niobium chloride, Nb3Cl8

Abstract

decreasing -donor and increasing -acceptor strengths of the trans L group.2 Although some pXa values are too close (those for 4-pic and py and those for isn, 4-acpy, and pz) to be used to make a very definite distinction in the and acidity of the trans ligands, the approximate ligand acidity order is NH3 < 4-pic 5 py < isn 5 4-acpy =* pz < pzH+ in these complexes.Reduction Potentials. Formal reduction potentials of trans-Ru(NH3)4(L)L'3+/2+ determined by cyclic voltammetry are listed in Table IV. The CV values of each complex fitted most of the criteria for a reversible couple.12 A CV of the reversible Ru-(NH3)5py3+/2+ couple was run under the same conditions and its values were used for comparison. Peak to peak separations increased (from 57 to 72 mV) with increasing scan rate (in different ranges, from 10 to 500 mV), which might be due to cell resistance. Some differences in the values of formal reduction potentials appeared in the literature (Table IV), and in some cases the differences are attributed to different experimental conditions.3•13 Previous observations showed that -unsaturated ligands, such as pyridines (py-X), lead to substantially more positive reduction potentials for the Ru(NH3)53+/2+ couples than when L is H20 or NH3 and that electron-withdrawing substituents (X) increase Ef, that is, the -accepting abilities of the ligands increase Et values.14 It was also observed13 that the substitution of a second ammonia by another ligand to give a Ru(NH3)4L23+/2+ complex yields more positive Ef values than for the corresponding Ru-(NH3)5L3+/2+ complex. Again, when an ammonia of Ru-(NH3)5(L)3+/2+ is substituted by another ligand U to yield Ru-(NH3)4(L,)(L)3+/2+, more positive E( values are also obtained.

Metal clusters on supports: synthesis, structure, reactivity, and catalytic properties

“…In contrast to the easily reduced and aggregated group-8 metals, more oxophilic metals offer opportunities for synthesis of stable metal clusters with multiple metal-metal bonds and metals in high oxidation states. The rich cluster chemistry of Ta, [69][70][71][72][73] Nb, 69,[71][72][73] and Re 74,75 is the foundation for research in this direction, which is still in its infancy.…”

“…82 The importance of the support in affecting the properties and structures of clusters of oxophilic metals is exemplified by the formation of large and nonuniform clusters on MgO after a treatment in H 2 of a sample prepared from H 3 Re 3 (CO) 12 ; the Re-Re bond lengths were found by EXAFS spectroscopy to be characteristic of metallic rhenium, consistent with the presence of significantly aggregated clusters, indicated by the Re-Re coordination number of 6.3. 83 The group-5 metals tantalum and niobium form families of clusters with strong multiple metal-metal bonds and a variety of ligands that often include oxygen; the metal atoms in these clusters are typically cationic; examples are Ta 3 Cl 10 (PEt 3 ) 3 -(HPEt 3 ), 71 69 Clusters that are at least roughly analogous to these, on supports such as SiO 2 , g-Al 2 O 3 , and mixed oxides SiO 2 -Al 2 O 3 , were formed by adsorption of Ta(benzyl) 5 followed by treatment in H 2 . The tantalum clusters formed on SiO 2 -Al 2 O 3 are represented as monolayer rafts, as indicated by characterization data summarized in a following section.…”

Rearrangement of van der Waals stacking and formation of a singlet state at T = 90 K in a cluster magnet

“…As for Mo 3 O 13 clusters in LiZn 2 Mo 3 O 8 10 , a formal electron count yields one S e f f = 1/2 magnetic electron per Nb 3 Cl 13 cluster, which are arranged on a two-dimensional triangular lattice. Nb 3 Cl 8 and various stacking variations [16][17][18][19][20] have been previously studied. The α-Nb 3 Cl 8 polymorph, with -AB-AB-stacking, is known to undergo a hysteretic magnetic transition 21 with a change in the magnetic signal at temperatures below T ≈ 90 K. Here, this transition is studied via detailed structural and physical property investigations of both powder and single crystals, revealing a dramatic trigonal to monoclinic phase transition at T = 90 K that quenches the magnetic response but without magnetic order, i.e.…”

Silica-supported tantalum clusters: catalysts for conversion of methane with n-butane to give ethane, propane, and pentanes

“…Changes in metal-metal bond lengths such as that reported here are indicative of changes in the oxidation state of early transition metal clusters, with longer metal-metal bonds indicating more highly oxidized clusters [20,21,22]. Thus, we suggest that the elongation of the Ta-Ta bond from 2.93 to 3.10 Å resulting from contacting of the freshly treated samples with the reactants represents an oxidation of the clusters-which was accompanied by a near total loss of activity for methane conversion after 22 h. Because the samples demonstrated the highest activity when the tantalum clusters were in the most highly reduced state (represented by a Ta-Ta bond length of 2.93 Å ), we suggest that maintaining the clusters in a reduced state might mitigate the loss in catalytic activity.…”

“…Thus, we suggest a relationship between the metal-metal bonding and metal-ligand bonding. Cotton's work gives insight into how metal-metal bonding might affect ligands bonded to group five metal clusters [20]. Cotton's group performed calculations based on theory to characterize related trinuclear clusters of Nb 3 (l 3 )Cl)(l )Cl) 3 Cl 6 (PH 3 ) 3 ) and Nb 3 (l 3 )Cl)(l)Cl) 3 C1 3 (PH 3 ) 6 , demonstrating that the molecular orbitals of metalligand bonds are of mixed character-that is, they incorporate some character of metal-metal bonding orbitals (up to approximately 15% metal-metal bonding character), as well as metal-ligand orbitals.…”