Bioctahedral dimers

The reddish brown pentachloride, uranium pentachloride [13470-21 -8], UCl, has been prepared in a similar fashion to UCl [10026-10-5] by reduction—chlorination of UO [1344-58-7] under flowing CCl, but at a lower temperature. Another synthetic approach which has been used is the oxidation of UCl by CI2. The pentachloride has been stmcturaHy characterized and consists of an edge-sharing bioctahedral dimer, U2CI2Q. The pentachloride decomposes in H2O and acid, is soluble in anhydrous alcohols, and insoluble in benzene and ethers. 

Metal aUcoxide complexes with related alcohols are apparently edge-sharing bioctahedral dimers, on the evidence of the X-ray stmcture of the isopropyl complex. Some stable mixed alkoxides of the type Q[M3(OR)9] (Q = Li, Na, K, NH4, Ca/2) have been reported they distill in vacuo without decomposition. The X-ray stmctures of several bimetallic alkoxides obtained by Caulton,QZr2[0(/-Pr)]9,Q = Li[HO(i-Pr)], K(DME), Ba[0(i-Pr)], show a similar triangular stmcture with two 743 and three /r-OR bridges. The thallium salt of composition Tl2Zr((/R)6 obtained by the reaction shown in equation (18) has a distorted octahedral stmcture stabilized by six T1 F contacts. ... 

The extension of the theory of the double exchange to systems containing orbitally degenerate metal ions with unquenched orbital angular momenta is given in [109-111]. As an example we will consider the corner-shared bioctahedral dimer... 

Just as the confacial bioctahedral dimer [TijXj] " provides the structural unit that is reproduced to form the j6-TiClj polymer chain, so [TijCl ] " and [TijCl,]" are the building blocks for the [TijClj j] and [TijCl j g] " complex anions prepared from redox reactions involving Pt(PPhj)j and TiCl Physical properties of these mixed Ti(III)/Ti(IV) anions suggest triple chloride bridges. The possibility of metal-metal interactions exists, but structures are not available. 

Na/Hg. These neutral confacial bioctahedral dimers with metal-metal double bonds serve as reagents for preparation of additional metal-metal bonded complexes described below. 

M(OR)5 compounds are known to be monomeric or dimeric. Examples of the latter include [Nb(OMe)5]2203 and [U(OBut)s]22(M which have edge-shared octahedral M2O10 units (Figure 1). An interesting example of a mixed valence compound of U4+ and U5+, U2(OBu )9, has been found to have a confacial bioctahedral U209 moiety.204 In both [U(OBu )5]2 and U2(OBu )9, the long U---TJ distances, ca. 3.5 A, preclude any direct metal-metal bonding. 1... 

Anionic halide dimers of Nb(III) are prepared in quartz ampules at 600-700°C from CsX and NbsXs (X = Cl, Br, I)T The [Nb2Br9] ion displays a confacial bioctahedral geometry (Nb-Nb 277 pm) and is paramagnetic in accord with a double-bond formulation in which only two electrons occupy degenerate 7t-bonding orbitals. No description of analogous Ta(III) anions is available. 

Conproportionation of ReCls and ReCls in a sealed tube at 300-600°C yields jS-ReCU containing dimeric confacial bioctahedral M2X9, units that share terminal chlorides as indicated by [Re2(/i-Cl3)Cl4]Cl2/2. Other routes to ReCU include heating Reds above 350 C under an N2 stream and hydrolyzing ReCU to Re02 prior to refluxing in thionyl chloride . In contrast to ReCU, the TcCU structure excludes metal-metal bonding. ... 

Although [Re2(NCS)s] forms from substitution of [Re2CU] with thiocyanate in acidic MeOH, when [SCN] is added to [Re2Cl8] in acetone oxidation to [RefNCS) ] and [Re2(NCS)io] occurs. The mixed-valence Refill, IV) dimer has a conlateral bioctahedral geometry with two N-bridging thiocyanate ligands and an Re-Re distance of 261 pm. 

The X-ray crystal structure of U2(O-/-Pr)10 has confirmed the edged-shared bioctahedral geometry (152) in the solid state (Fig. 8), although H NMR studies in solution (6) revealed a monomer-dimer equilibrium. 

Fig. 10 The electronic lability of the trans ligands in the D4j,-like dimer is transferred and spread out over all four coordination sites for the edge-sharing bioctahedral structure...

The dimeric molecules [W2(0)2(At-0R)2(0R)e] (R = Me, c-CeHn) have terminal 0X0 ligands trans to the bridging alkoxo gronps in an edge-shared bioctahedral structure." ... 

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