Pentanuclear complexes

Pentanuclear complexes are represented by 2 major structural types closo- clusters with a (i5-0-ligand in the center (heteroleptic complexes) and by combinations of 2 [M2M ] triangular aggregates, having 1 common metal atom (heterometallic ones as this M central metal atom should have higher coordination number and be rather electropositive to permit the existence of such an aggregate). 

The aggregates of this type are very stable they are present in solutions and in the gas phase and display low reactivity, in particular, toward the complex formation with the alkoxides of other metals (see Chapter 8). 

Mov2Mov,Ta2Og(OPri)10 provides the first example of a molecule, where all the 5 metal atoms are involved in anonplanar cycle (incorporating aMo=Mo bond in addition to (t-O and (t-OR ones) [847]. 

Like the trinuclear complexes discussed in the preceding section, the main redox processes exhibited by the actual pentanuclear complexes in dichloromethane solution are constituted by a first, nearly reversible, ferrocene-centered oxidation followed by a second Group 6 metal-centered irreversible oxidation [49]. The relevant redox potentials are reported in Table 7-36. 

In this case too, complexation of bis(diphenylphosphino)ferrocene with metal fragments makes the electron removal step notably difficult (by about 0.3 V). 

Unfortunately, in spite of the fact that electrochemistry allows access to redox congeners of many heterometallic complexes that could be isolated, we were unable to find examples of solid-state structures of redox congeners. So the main goal of judging quantitatively the effects of electron-transfer processes on these types of complexes could not be fulfilled. 

We must note that in many cases the electrochemical recognition has been limited to the simple evaluation of the redox potentials of the electron transfer steps, together with a superficial examination of the chemical stability of the complexes in various oxidation states. This is of little help to synthetic chemists, who would have to prepare redox congeners for complete chemical, physico-chemical, and structural characterization. Controlled potential coulometry and macroelectrolysis tests must be routinarily performed, both to define the number of electrons involved in each redox change and to obtain redox congeners, even if in low quantity, at least for a preliminary determination of their stability and of their spectroscopic properties. 

A significant number of important papers has appeared after submission of the manuscript. A critical selection is given below [208—239]. The numbers in round brackets indicate both the section that the reference refers to and its content according to the following classification  

All the Co atoms are co-linear and are separated from each other by metal-metal like bond distances (really, theoretical calculation assigns Co-Co bond order of 0.541a). At variance with the trinuclear 

The quite similar series [Ni5(tpda)4(X)2] (X = C1, N3, CN, NCS) has been structurally characterized 40a,41a Although also in this case 

We finally mention that [Cr5(tpda)4(Cl)2] has been also structurally characterized 42a,b The five Ni11 atoms are assumed to form alternating Ni-Ni bonds, or Ni-Ni- -Ni-Ni- -Ni.42c Such a bonding situation is even more evident in the monocation [Cr5(tpda)4(F)2]+ 42b Also in this case the oxidation process [Cr5(tpda)4(X)2]0/+ has not been electro-chemically followed. 

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The X-ray structure of zinc naphthalocyanate has been determined with Zn—N bond lengths of 1.983(4) A.829 Pentanuclear complexes with a zinc phthalocyanine core and four ruthenium subunits linked via a terpyridyl ligand demonstrate interaction between the photoactive and the redox active components of the molecule. The absorbance and fluorescence spectra showed considerable variation with the ruthenium subunits in place.830 Tetra-t-butylphthalocyaninato zinc coordinated by nitroxide radicals form excited-state phthalocyanine complexes and have been studied by time-resolved electron paramagnetic resonance.831... 

Pentanuclear complexes are represented by the pairs of triangles with a common vertex (MM 4(OR)lg type). This type is displayed by the structure of BaCu thdMOR ROH), R = C2H4OMe [150]. 

Interaction of phosphacymantrene (L) with [Rh(COD)Cl]2 in CH2C12 at room temperature leads to tetra- and pentanuclear complexes [L3RhCl] and [L4Rh]+(BF4) [487]. 

Reactions of 279 with H /H+ (=H2) gave 280 in which hydrogen has added both to the cluster and to the central carbons of the C4 unit. In contrast, the reaction of 279 with molecular dihydrogen resulted in cleavage of an Ru(CO)3 fragment and formation of the pentanuclear complex 281.474... 

Pentanuclear Complexes. In [Cu5(/j,-SBu )6] there is tbp Cu1 with /X.-SR and this structure is also found in the aryl species (see later), MgCu4Ph6, [Cu5Ph6], and [Li2Cu3Ph6]. The phenyl groups bridge metal atoms that may be Cu only, Cu + Li, or Cu + Mg. 

However, when the transmetalation was carried out at reOux temperature in wet solvent with access to air, the pentanuclear complexes [Cu5(L )2(dmt)2][C10J3 (70),and[Cuj(L )2(dmt)2][C104]3 (71), (dmt = 3,5-dimethyl-l,2,4-triazolate anion) were obtained. The structures of these complexes were confirmed by an X-ray diffraction analysis of complex 71 (Fig. 11) [44], Separate experiments established... 

Combination of Metal Ions in Pentanuclear Complexes [M(tmphen)2]3[M (CN)6]2 ... 

An interesting case of spin transition behavior was discovered in our laboratories for the pentanuclear complex [Co(tmphen)2]3[Fe(CN)6]2 (175), which adopts a TBP structure (175, 176) (Fig. 34). Strictly speaking, this compound does not undergo spin crossover, but we consider this compound in the current section because its properties are unique among discrete cyanide-bridged clusters. As established by a combination of X-ray crystallography, Mossbauer spectroscopy. 

Shown in Figure 6.3 is the structure of Ln5(p,5-0)( X3-OPr )4(li-OPr )4(OPr )5, prepared by Hg(II)-catalyzed alcoholysis of lanthanide metals [16]. The pentanuclear complex is comprised of a square-pyramid of Ln atoms, each terminally coordinated to one OPr ligand. Upon each triangular face of the square-pyramid is a p-s-OPr, while bridging each edge of the square base is a p-OPr. An oxo ligand is in the interior of the structure, interacting with all five metal atoms. This structure is shared by a number of rare earth complexes (Ln = Eu, Nd, Gd, Er, Yb). 

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