Transition metals coordination geometry diversity

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). 

Coordination complexes are a remarkably diverse group of molecules that form from virtually all transition metals In a variety of oxidation states. These compounds involve an extensive array of ligands, and they adopt several molecular geometries. 

NO binds covalently to transition metals M forming a diversity of structural types comprising mononuclear, NO bridging, and cluster compounds.8-10 Within the first class, main coordination numbers are 6,5, and 4, as sketched in Table 7.1, revealing an important structural result, namely, the adoption of linear or bent geometries of the MNO fragments. 

Pincer ligands, that is, tridentate Hgands that enforce meridional geometry upon complexation to transition metals, result in pincer complexes which possess a unique balance of stability versus reactivity [3]. Transition-metal complexes of bulky, electron-rich pincer ligands have found important appHcations in synthesis, bond activation, and catalysis [4, 5]. Among these, pincer complexes of Pr-PNP (2,6-bis-(di-iso-propylphosphinomethyl)pyridine), Bu-PNP (2,6-bis-(di-terPbutyl-phosphinomethyl)pyridine), and PNN ((2-(di-tert-butylphosphinomethyl)-6-diethyl-aminomethyl)pyridine), PNN-BPy (6-di-tert-butylphosphinomethyl-2,2 -bipyridine) ligands exhibit diverse reactivity [6-8]. These bulky, electron-rich pincer ligands can stabilize coordinatively unsaturated complexes and participate in unusual bond activation and catalytic processes. 

The geometries and reactivities found for transition metal complexes are very diverse. This can partly be attributed to the fact that transition metals tolerate a variety of oxidation states and coordination numbers. Furthermore, the electronic situation of the participating ligands can change dramatically upon coordination to the metal ion. It is therefore not surprising that many unusual molecules are found within this class of compounds. 

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