Octahedral complexes monodentate ligands

Both complexes have six monodentate ligands, so both are octahedral. 

Analogous to the dithiophosphate complexes, cis octahedral complexes are formed with bident-ate ligands like phenanthroline, 2,2 -bipyridine, or tmeda and trans octahedral complexes are formed with monodentate ligands like pyridine and its derivatives. Some examples are mentioned with linear bidentate ligands like 4,4 -bipyridine, which build up ID chains by bridging the [NiL2] molecules.926-933... 

General Structural Features. The general structure of halfsandwich ruthenium(II)-arene complexes is shown in Fig. 12. The structural, stereochemical and electronic features of metal-arene complexes have been discussed (63). A typical piano-stool geometry consists of an rj6-arene occupying three coordination sites of the pseudo-octahedral complex, leaving the three legs X, Y, and Z available for coordination. The sites X and Y can be taken up by two monodentate ligands, but are more commonly... 

The Co system is more reactive as well as much more selective than the Ni and Rh catalyst systems (Table XVII). The best systems allow almost 100% conversion with almost 100% yield of c -l,4-hexadiene. The best of the Ni and Rh systems known so far are still far from such amazing selectivity. The tremendous difference between the Ni system and the Co or Fe system must be linked to the difference in the nature of the coordination structures of the complexes, i.e., hexacoordinated (octahedral complexes) in the case of Co and Fe and tetra- or penta-coordinated (square planar or square pyramidal) complexes in the case of Ni. The larger number of coordination sites allows the Co and Fe complex to utilize chelating phosphines which are more effective than monodentate phosphines for controlling the selectivity discussed here. These same ligands are poison for the Ni (and Rh) catalyst system, as shown earlier. 

There have been a number of other reports of the use of largely unsaturated, N4 donor macrocycles to generate 1 1 manganese(II) complexes." The majority of these have been assigned octahedral geometries in which two monodentate ligands occupy the remaining positions in the coordination sphere. 

The diazines also form metal complexes. Thus, pyrazine forms tetrahedral and octahedral complexes with Co11 and other transition metals it functions as a monodentate and also as a bidentate bridging ligand to give polymeric complexes. The stability of these complexes is increased by back-bonding. 

It is, thus, important that the ruthenium(II) complexes that are to be used as building blocks of the future machines contain sterically hindering chelates so as to force the coordination sphere of the metal to be distorted from the perfect octahedral geometry. We will discuss the photochemical reactivity of rotaxanes and catenanes of this family as well as non-interlocking systems like scorpionates since the lability of bulky monodentate ligands could also lead to useful photosubstitution reactions. 

The reaction between Ph2SnCl2 and thiosemicarbazide using acetone-ethanol as a solvent mixture resulted in the formation of bis(acetone thiosemicarbazone-S)dichlorodiphenyltin. In the monomeric hexacoordinate complex, each of the two monodentate ligands coordinate to the tin atom through the sulfur atom to form a distorted-octahedral geometry (the S —Sn bond length is 2.712 A)1122. 

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