The Reduction of the NbX₅/AlX₃ System with Aluminum in the Presence of Aromatic Hydrocarbons: An Approach to Niobium(II), Niobium(I), and Niobium(0) Organometallics

Abstract

The additional occupation of the face centers of the M, octahedron of a M,X,, cluster is observed for the first time in Th,Br,,H,; this type of occupation is already known for systems with condensed clusters. In the compounds ZrXH (X = C1, B I -) [ '~~ and LnXH,[I5] two H atoms assume positions in opposite faces of elongated trigonal antiprisms of M atoms (d(D-D) = 220 pm in the case of ZrBrD and 298 pm in the case of TbBrD,).The number of bonding orbitals in the Th,Br,,H, cluster is the same as in the hypothetical empty Th,Br,, cluster: bonding H-Th interactions merely lead to a lowering of individual orbital energies; the antibonding combinations lie above the HOMO-LUMO gap. Thus, the electron balance for Th,Br,,H, (6 x 4 -15 + 7 = 16) corresponds exactly to that for a complete occupation of all bonding cluster orbitals (for comparison Nb,F,, : 6 x 5 -15 = 15). The metal-metal bonding in the (empty) M,X,, cluster is best described in terms of closed three-center two-electron bonds in all eight faces of the M, octahedron['61 and provides a clear picture of the bonding contribution of the H atoms. Seven of these multicenter bonds are reinforced by the mixing of the 1 s function of the respective H atom in the center of the bond and by the additional electron of each H atom, so that finally there are seven four-center two-electron bonds and one three-center two-electron bond.Since the electron balance already varies in the case of empty clusters (e.g. Ta,Cl;Y with n = 2-4, corresponding to 16 to 14 electrons in M-M bonding states['71) and can be further decreased in clusters with interstitial atoms (e.g. Sc,CI,,B corresponding to 12 electrons'181), it is to be expected that the hydrogen content in Th,Br,,H, is variable. Further investigations are currently under way. Received: September 3, 1990 [Z 4165 IE] German version: Angew. Chem. 103 (1991) 79 CAS Registry numbers: Th,Br,,H,, 131 104-1 6-0; Th,Br,,D,, 1311 30-13-7.

Modern applications of low-valent early transition metals in synthesis and catalysis

“…The most common routes for generating low-valent early transition metal complexes involve the reduction of metal halides with strong alkali metals 27–32 ; reduction with other (often weaker) reducing metals, such as Mg (REFS 33,34 ), Mn (REF. 35 ), Al (REFS 36,37 ) or Zn (REF. 38 ) (providing more functional group tolerance); or p-H abstraction 39–43 from a metal alkyl (Fig.…”

Niobium and Tantalum

“…It was later found that a 30-50% yield of (r|-C 6 H 3 Me 3 ) 2 Nb can be isolated from the reaction of NbCl 5 , Al, A 1 X 3 , and mesitylene in DME at -15 °C, which is a rare example of a bis(arene) group 5 complex being accessible via solution methods rather than metal vapor synthesis techniques. 590 ' The bis(toluene) complex (r|-C 6 H 5 Me) 2 Nb, and its analogues originating from metal vapor syntheses, have provided a rich reaction chemistry. Thus, (r|-C 6 H 5 Me) 2 features o f these compounds is a coordinated C 6 R 6 ligand which shows (i) substantial folding along the C-l-C-4 axis (the dihedral angle can be as large as 34°); (ii) an interruption o f aromaticity within the C 6 R 6 ring which can be described as a 1,4-diene-type 7 i localization; and (iii) the close approaches o f C-l and C-4 to the metal which compare to the values o f Ta-C(sp 3 ) single bond lengths, as indicated in (53).…”

Bis(pyrene)metal complexes of vanadium, niobium and titanium: isolable homoleptic pyrene complexes of transition metals

“…2). The niobium species, (II), is of interest because only one other Nb 0 complex has been prepared by a conventional procedure, namely bis(mesitylene)niobium(0) (Calderazzo et al, 1991). Also, the titanate, (III), is a rare example of a compound containing titanium in an oxidation state of À1, precedented only by the related bis(arene)titanates(1À) with arene = benzene and toluene (Bandy et al, 1984), and biphenyl and 4,4 0 -di-tertbutylbiphenyl (Blackburn et al, 1992).…”