Ligand-bridged binuclear complexes

In recent years, we have encountered and undertaken experimental studies 14-18) on two families of ligand-bridged binuclear complexes, which undergo the one-step, two-electron transfer shown in equation 4. 

Fe(bpe)2(NCS)2 MeOH, bpe = tra x-l,2-bis(4-pyridyl)ethene, is a supramolecular coordinationpoly-catenane, consisting of two interlocked 2D networks. Bis-pyridylethane and bis-pyiidylethene also appear elsewhere as bridging ligands in binuclear complexes (e.g., pentacyanoferrates (Seetion 5.4.2.2)). 

The chemistry of dppm has been recently reviewed by Puddephatt1228,1433 and by Balch.1227 The ligand will form either four-membered ring chelate complexes or bridged binuclear complexes. 

The dihydroxo-bridged binuclear complexes are among the most common, structurally well-characterized polynuclear complexes. For symmetrical bidentate nonbridging ligands, LL, the cation (LL)2M(OH)2M(LL)2"+ may exist in three isomeric forms a meso isomer (A,A) with configuration A and A, respectively, at the two metal centers, and a racemic pair of isomers (A,A and A, A) with configuration A or A at each of the two metal centers (cf. Fig. 2). If the bidentate... 

Following the well-recognized role of carboxylato anions as bridging ligands in binuclear complexes with metal-metal bonds, Cotton and coworkers94"97 have explored the use of the isostructural and isoelectronic amidinato (NR )RC(NR") anions (31). 

A simpler situation from the point of view of a theoretical treatment -although more difficult to study experimentally - is electron exchange between ions which constitute two halves of the same redox couple, e.g. MnO /MnO -, Co(NH3) +/Co(NH3)6+ etc. Two distinct types of mechanism have been postulated. In the outer-sphere mechanism, the coordination spheres of both oxidant and reductant remain intact as electrons are transferred, and the oxidation numbers of the central atoms change. The inner-sphere mechanism describes a situation where a bridged binuclear complex is formed as an intermediate, and the bridging ligand - which may be Cl-, OH etc. or an ambidentate ligand like NCS" - provides a pathway for electron transfer. 

Oxidative additions both to bridged, binuclear complexes lacking metal-metal bonds and to mononuclear complexes with sterically small ligands can result in metal-metal bonds. 

Complex palladium azides are obtained by dissolving palladium salts in excess azide, e.g., palladium nitrate and sodium azide (1 4) yield a triazido anion, [Pd (N3)3]" [167]. Similarly, a tetraazido anion, [Pd (N3)4] , is obtained from K2 [PdCU] by ligand exchange [162]. Another all-azido anion is the azide-bridged, binuclear complex [Pd2(N3)6] [168]. All of these have been isolated as salts of large organic cations. 

Of particular interest are the cases where, by a suitable design of the complex, the factors (a) to (d) above can be eliminated so that the departure of the electrochemical response from the purely statistical behaviour arises solely from cause (e). Two examples of such complexes which have been well studied are binuclear ruthenium complexes bridged by 4,4 -bipyridyl type ligands, and binuclear complexes where L, the binucleating macrocyclic ligand, is a symmetric... 

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