Polynuclear structures

The first polynuclear structures with gold(i) centers coordinated to isocyanide and alkynyl groups were prepared through a novel exchange of alkynyl ligands. This special route became necessary, because the coupling of certain complexes proved elusive (Equation (40)) AuCl complexes of isocyanides with terminal alkynyl groups are subject to decomposition upon treatment with a variety of auxiliary bases. 

The [Fe(diimine)2X2] system has been modified by replacing the diimines by unidentate nitrogen donors. [Fe(diimine)(py)2(NCS)2] is a crossover system when the diimine is 2,2 -bipyrimidine or phen [99] but [Fe(py)4(NCS)2] is purely high spin [100]. However, [Fe(py)4(NCS)2] systems containing substituted pyridine derivatives have been shown to exhibit thermal SCO [101], while 4,4 -bipyridine derivatives are able to bridge Fe(II) centres and form polynuclear structures containing SCO [Fe(py)4(NCS)2] centres [102]. SCO is maintained in certain instances when the diimines are replaced by an N4 quadridentate [103,104]. 

The stoichiometry (equation 2) requires eight electrons transferred from vanadium(II) ions to dinitrogen atoms (six electrons) and hydrogen atoms (two electrons) and this kinetic equation (3) was interpreted as evidence for a polynuclear structure of the transition state during the reduction of dinitrogen. An alternative mechanism was suggested.145... 

Complexes of adenine (10 Ade) have been isolated by mixing the ligand in 2-methoxy-ethanol with chromium(II) halides in butanol. The low magnetic moments (Table 16)104 indicate dinuclear species [Cr2X2(Ade)4+] (Section 35.3.5.5), or polynuclear structures with bidentate adenine. 

The complexes HgNi(NCS)4, HgNi(NCSe)4 and HgNi(NCS)2(NCSe)2 are supposed to contain six-coordinate nickel(II) in a polynuclear structure.1082 1089 All of the complexes with the general formula HgNi(NCX)4 behave as Lewis acids towards a number of bases such as alcohols, pyridine and substituted pyridines, PPh3, bipy, phen, DMSO, etc., giving in most cases polynuclear species.1084,1089... 

A chain polynuclear structure with 0,N-bridging groups was found in the series of complexes [Ni(N02)(en)2]Y (218) (Y = C104, BF4, I3)1648,1652 1663 and strong antiferromagnetic intrachain coupling was measured in the two complexes with Y = C104 and I3.1652... 

In the past ten years it is probably (ii) and (iii) above that have provided the major incentive in the synthesis of novel dimeric structures in copper(II) systems.30 Reference 10 includes a review of polynuclear copper(II) complexes in general and ref. 30 includes reference to a large range of biologically relevant dinuclear copper(II) complexes. The structural chemistry of binuclear copper(II) complexes was reviewed in 1977.586 In order to systematize the description of copper(II) polynuclear structures the following notation is introduced. 

The following sections (53.4.4.2-6) attempt to describe the electronic properties of simple mononuclear complexes of the copper(II) ion,47,48 to show how these are related to the different stereochemistries of the copper(II) ion and how these properties are modified by the formation of polynuclear complexes.17,30 Particular emphasis is placed on the appearance of the different types of electronic property and how they may be used to provide qualitative evidence for the different types of copper-copper interactions, and hence for possible polynuclear structure formation, particularly in the solid state. While the main emphasis will be on the electronic properties in the solid state, where X-ray evidence may be obtained for a single magnetic species,10 the measurement of the electronic properties in solution will also be described, although in solution a mixture of complex species may be present in equilibrium and complicate the interpretation of the electronic properties.584,816,817,824... 

In general, electronic spectra, IR, Raman and EXAFS spectra relate to the mononuclear CuL geometry and give no evidence for the presence of a polynuclear structure. 

Central atoms in noncaged polynuclear structures are numbered from one end along the path containing the greatest number of central atoms to the remote end. The number one terminal central atom is chosen first, by the priorities of the central atoms listed in Table 17, the highest priority... 

In the case of the sulfate anion SO, complexes are described in which the ligand performs a terminal (104) or chelating (105) function in mononuclear compounds, but the same ligand behaves as a bridge ligand system (106) in di- and polynuclear structures [(3,13,128-131] ... 

Atoms, simplest ions, and molecules di- and polychelating compounds take part as bridge-type ligands in these complexes. Di- and polynuclear structures of the simplest ligands, in terms of the number of cluster structures, are reported in the literature, cited in Sec. 1.2.2.4. 

To obtain di- and polynuclear structures, other types of azomethinic ligands have been used, for instance, types 642 [99] and 643 [100] ... 

A change of salt anions allows us to carry out controlled syntheses of definite types of coordination compounds. Thus, metal halides are widely used to prepare molecular (Sec. 3.1.1.1) and ti-complexes (Sec. 3.1.1.3). Metal acetates are mostly applied in the syntheses of metal chelates and, especially, inner-complex compounds (Sec. 3.1.1.2). Di- and polynuclear structures are formed under use of the anions mentioned above, which, in some cases, determine (see Sec. 3.1.1.4) a type and donor centers of bridge fragments. At the same time, the mentioned approach to choice of salts of metal complex-formers has many exceptions, although it is useful. 

Summarizing the material, we note that the synthesis of di- and polynuclear structures can also be carried out in aprotic solvents with low dielectric constant. The above described reaction (3.72), yielding complexes 637, confirms this fact. 

Such active metals as aluminum can participate in formation (3.261) of polynuclear structures, for example 823 [646] ... 

Solid luminescent materials, with identical emission kmax values, were also obtained on the reaction of 1,6-hexane dithiol with cupric perchlorate. The origins of the luminescence in both the film and solid appear to be the same, although this is not yet clearly understood. It is likely that they are associated with Cu-Cu interactions in what is essentially a polynuclear structure. 

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