Polynuclear Bridged Complexes

Okamura, E. Miki, K. Mizumachi, and T. Ishimori, Chem. Letters, 1974, 103. 

The first example of a sulphito bridge is present in [Co2(S03)(OH)2(NH3)6]2+, formed from sulphur dioxide and [Co2(OH)3(NH3)6]3+ and isolated with I2, Br2, (N03)2, or S202- anions. The SOJ bridge is oxygen bonded.417 The reactions of p-peroxodicobalt(m) complexes with S02 in the gas and liquid phases and in aqueous solution have been examined in detail, and those with NO briefly 418 

Cobalt(iv).—In addition to the oxocobaltates(iv) (q.v), cobalt(iv) is present in the trisdithiocarbamates [Co(S2NR2)3]BF4 (R = Me, Et, Pr, or Cy) formed by oxidation of the appropriate cobalt(m) complex in toluene or benzene with BF3. All the complexes are diamagnetic, which is thought to be because of spin-pairing (d5, CoIV) due to formation of associated species.421 

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Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

Figure 11.13 Structures of polynuclear nitrosyl complexes (a) ((Cr( j -C5H5)(NO))2(M2-NH2)(M2-NO) showing linear-terminal and doubly bridging NO and (b) (Mn3( j -C5H5)3( 3-NO)3( 3-NO)] showing double-and triply-bridging NO the molecule has virtual symmetry and the average Mn-Mn distance is 250 pm (range 247-257 pm).

Byers, P.K, Carr, N. and Stone, F.G.A. (1990) Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 106. Synthesis and reactions of the alkylidyne complexes [M ( CR)(CO)2 (C6F5)AuC(pz)3 j (M = W or Mo, R — alkyl or aryl, pz — pyrazol-l-yl) crystal structure of pjC PtAu(C6F5)( l3-CMe)(CO)2(PMe2Ph)2 (C6F5)AuC(pz)3 ]. Journal of the Chemical Society, Dalton Transactions, (12), 3701—3708. 

Balch, A.L., Fung, E.Y. and Olmstead, M.M. (1990) Polynuclear ((diphenylphosphmo)methyl) phenylarsine bridged complexes of gold(I). Bent chains of gold(I) and a role for Au(I)—Au(I) interactions in guiding a reaction. Journal of the American Chemical Society, 112, 5181-5186. 

Balch et al.560 prepared the cluster [Pd3(/u-dppm)3(//3-I)(//3-PF3)]PF6 [dppm = bis-(diphenylphos-phino)methane] (shown in Figure 115), which contains a PF3 molecule triply bridging a central triangular Pd3 core. However, exceptions are not limited to PF3, an atypical phosphine often considered closer to carbon monoxide. For further examples, see the section on polynuclear Pd1 complexes. 

Only 45 ml. of ethylenediamine is needed if the solution contains no zinc, f Formation of polynuclear coordination complexes by means of hydroxyl groups as bridging ligands. 

Of note is the apparent lack of chloride in the first coordination shell of Mn in either the S or the S2 state as revealed by EXAFS studies of Mn (35). This observation is of particular interest because chloride is required for optimal O2 evolution rates (39) and has been proposed to act as a bridging ligand in a polynuclear Mn complex (40). Recent EPR studies, however, also suggest that chloride is not bound to Mn in the S or S2 state (41). 

Cyano-bridged complexes include both di- and polynuclear systems. Spectroelectrochemical studies of the oxidized and reduced forms of [(bpy)2(CN)Ru(/r-CN)Ru(C (bpy)2] have been carried out. This cation and its reduced form exhibit UV-vis spectra that are consistent with the chromophores being identical. Experimental evidence indicates that the one-electron oxidation... 

The next part of this section focuses on di- and polynuclear Ru"/Os" systems (organized according to the bridging ligands), and pertinent reviews covering multicomponent molecular arrays should be consulted. The cyano-bridged complexes [(bpy)(tpy)Ru(//-CN)Ru" (NH3)5]"+ ( = 3 or 4) have been studied. Picosecond excitation of [(bpy)(tpy)Ru-(//-CN)Ru(NH3)5] results in the observation of a transient intermediate which decays by an... 

Formation of hydroxo-bridged complexes by hydrolysis in aqueous solution is, not surprisingly, the most common preparative method. As a rule, such reactions give quite complex product mixtures containing species with different nuclearities, each of which may be present in many isomeric forms. The fact that most of the preparative procedures employed lead to the isolation of one single and pure isomer probably more often reflects favorable solubility properties rather than stereospecificity. In some cases ion-exchange chromatography has been used to isolate the polynuclear species, but systematic analysis of hydrolysis mixtures by this technique has been reported for only a few systems. 

The yields of the dihydroxo-bridged complexes vary and are often relatively low, probably owing to the formation of different isomers and higher polynuclear complexes and to hydrolysis of the ligand L. For many of the complexes listed above, the preparative procedures involved the preparation of the mononuclear species in situ. 

Analysis of the products of these cleavage reactions has often served as proof of the structures of the polynuclear species. Cleavage of hydroxo-bridged complexes of nuclearity higher than two will in most cases yield at least two different mononuclear species. Identification of these species and determination of the relative ratio in which they are formed reduce the number of possible bridged skeletons greatly, and the studies of polynuclear ammine and amine chromium(III) made by Andersen et al. (mentioned in Section IV) provide many examples of this, one of which is shown in Eq. (48) above (see also Section II,A). 

The subject of this Section is photochemical processes in which polynuclear tetrapyrrole complexes with metal-metal or metal-bridge-metal bonds are formed from or decomposed to mononuclear tetrapyrrole ligand containing... 

A multitude of related polynuclear gold complexes have been prepared utilizing a range of bridging ligands, and their spectroscopic properties have been examined [71]. An example involving 16 individual Au(I) ions deserves mention because of its extreme complexity [72],... 

Diphenyl ditellurium and bis[cyclopentadicnyltricarbonyl molybdenum] formed cyclopen-tadienyltricarbonyl(benzenetellurolato)molybdenum in benzene at 25" when irradiated with light from a 250 W infrared lamp. At higher temperatures, dinuclear and polynuclear benzentellurolato bridged complexes were produced3. 

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