Chameleon Complexes

The introduction and diversification of genetically encoded fluorescent proteins (FPs) [1] and the expansion of available biological fluorophores have propelled biomedical fluorescent imaging forward into new era of development [2], Particular excitement surrounds the advances in microscopy, for example, inexpensive time-correlated single photon counting (TCSPC) cards for desktop computers that do away with the need for expensive and complex racks of equipment and compact infrared femtosecond pulse length semiconductor lasers, like the Mai Tai, mode locked titanium sapphire laser from Spectra physics, or the similar Chameleon manufactured by Coherent, Inc., that enable multiphoton excitation. 

It is obvious from the table that the ionic radii of lanthanides are very similar to the ionic radius of the Ca2+ ion. Further, the ionic radii of lanthanides differ by about 0.3 A in changing the coordination environment from 6 to 12. The differences of 0.3 A in ionic radius between coordination number 6 and 12 makes the lanthanides(III) ions highly adaptable to many coordination environments. This has led to the development of a wide array of macrocyclic molecular complexes with exciting properties. From this point of view, lanthanides have been known as the chameleons of coordination chemistry [5]. 

The chameleon-Uke nature of Cr in biology is highlighted by the fact that it can be extremely toxic or nontoxic, depending on its chemical composition (2). Its chameleon-like nature in terms of its biological activity has been well camouflaged by its chemical complexity, which has led to conflicting reports of its mode of action and, indeed, whether it is active at aU as a dietary supplement (3-6). 

Chameleonic features of carbenes can be further amplified by complexation with transition metals (Figure 5.42). In complexes with low valent/low oxidation state late transition metals (Fischer carbenes), carbenes display electrophilic properties, and often behave similarly to a carbonyl compound. Such carbenes also often have p-donor substituents, such as-OR or-NR, on the carbene carbon and x-acceptor ligands at the metal. In contrast, carbene complexes with high valent/high oxidation state early transition metals (Schrock carbenes) are nucleophilic. The ability of metal in the Schrock carbenes is further enhanced by donor ligands. 

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