Highly labile metal centers

It is not insignificant that a large proportion of reported coordination polymers contains highly labile metal centers, in particular d10 centers Cu(I) [54], Ag(I) [55], Zn(II) [51] and Cd(II) [42], but also d9 Cu(II) [38] and d7 Co(II) [56] cations and increasingly with lanthanide cations, Ln(III) [7]. Examples of second or third row transition metal based coordination polymers, with the exception of Ag(I) and Cd(II), are extremely rare amongst reported examples. 

A large and diverse number of transition-metal coordination numbers and geometries can be used in the construction of coordination-driven assemblies, giving access to different topologies rather difficult to obtain with the classical synthetic methods. The synthetic process is under thermodynamic control when relatively labile metal centers are used (true supramolecular self-assembly), while it is under kinetic control with inert metals (unless high temperatures are used). 

The high solubility and lability of reduced metal centers should make metal ion release fast relative to preceding steps in many reductive dissolution reactions. 

The above value of k4 1 s for bpy loss from Rh(bpy)3 + may be compared with k4 - 3 s for bpy loss from the formally related Co(bpy)32+ (13,14) Recently obtained results indicate that the rate constant for addition of bpy to Rh(bpy)2(H2O)2 (k 4 s 0.2 x lO Ms"1) is greater than that for the comparable cobalt(II) reaction (13,14) The more-or-less comparable labilities of Rh(bpy)3 T and Co(bpy)3 + are not unexpected in light of data for rates of ammonia loss from the two metal centers which are also available ammonia loss from rhodium(II) is quite rapid (10 s 1 to 10 s l with loss from Rh(NH3)5 H20 + being much faster than from Rh(NH3)4 +, etc ) W t>ut somewhat slower than the comparable process for cobalt(II) (15) Of course, here the relative affinities of the two metals for NH3 are not known and so cannot be taken into account A further reason these comparisons lack great validity is that, although these Co(II) complexes contain 3d metal centers, Co(bpy)3 + and Co(NH3)n + are high-spin complexes i.e. the ground states are (t2g) (eg) whereas 4d species are expected to be low spin, (t2g) (eg)1. Furthermore, as will be seen shortly it is not clear that even "low spin 4d " is an adequate description of the... 

A special application of bimetallic ruthenium complexes was found in the olefin metathesis reaction vide infra) The two metal centers were closely attached to one another through /r-halide anions. The labile assembly was the key feature to the formation of highly active catalysts. 

A brief historical note on the structure of the iron-sulfur clusters in ferredoxins is relevant. After the first analytical results revealed the presence of (nearly) equimolar iron and acid-labile sulfur, it was clear that the metal center in ferredoxins did not resemble any previously characterized cofactor type. The early proposals for the Fe S center structure were based on a linear chain of iron atoms coordinated by bridging cysteines and inorganic sulfur (Blomstrom et al., 1964 Rabino-witz, 1971). While the later crystallographic analyses of HiPIP, PaFd, and model compounds (Herskovitz et al., 1972) demonstrated the cubane-type structure of the 4Fe 4S cluster, the original proposals have turned out to be somewhat prophetic. Linear chains of sulfide-linked irons are observed in 2Fe 2S ferredoxins and in the high-pH form of aconitase. Cysteines linked to several metal atoms are present in metallothionein. The chemistry of iron-sulfur clusters is rich and varied, and undoubtedly many other surprises await in the future. 

Thorimbert and coworkers have reported the use of complexes (TBA)sH2[al-RE(H20)4 P2W17O61] (RE = La, Sm, Eu, Yb TBA=tetrabutylammonium) as Lewis acid catalysts. These complexes are soluble in organic solvents, and the water molecules on the lanthanide ions are labile, thus providing the metal centers with available coordination sites for organic substrates. These catalysts show high chemoselectivity for the competition reactions between... 

As alluded to above, metal complexes of a number of lanthanide and actinide texaphyrin complexes have also been prepared. In the case of Dy(III) texaphyrin 9.74, as in the case of the bis-pyridine cadmium complex 9.61b, the metal center sits directly within the mean plane of the macrocycle (Figure 9.1.12). This result stands in direct contrast to the highly labile, typically sandwich-type 2 1 or 3 2 complexes observed for porphyrin complexes with these larger metal cations. ... 

Since the mid-1980s the chemistry of related cationic 16-electron Cp2M(R)(L) complexes (1) and base-free 14-electron Cp2M(R) complexes (2) (M = Ti, Zr) has been developed (/O). These complexes are considerably more reactive than their neutral counterparts as a result of the increased Lewis acidity of the cationic metal center, as well as the presence of the labile ligand L in 1 and the increased unsaturation of 2. These features promote coordination and activation of olefins, acetylenes, H, C— H bonds, and other substrates, and they open reaction pathways which are unavailable to neutral 16-electron analogs. Cationic complexes 1 and 2 are thus classified as highly electrophilic metal alkyls. 

Cu(II) and Cu(I) translocation processes within the bidimensional system 13 are much faster processes than those observed for Fe(III) and Fe(II) cations in the tridimensional system 11 and analogues. This may be referred to the higher simplicity of the two-dimensional ligand (which can allow a facing pages mechanism for the translocation of the oxidized/reduced metal center) and to the extremely high substitutional lability of Cu(II) and Cu(I) cations. 

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