Complexes of higher coordination number

Although examples exist, coordination numbers of 10 and above are relatively rare. Further, it seems that the concept of coordination geometry becomes less applicable. The reason is that, whilst idealized geometries can be identified, most real structures show distortions and there may be some arbitrariness about which of the ideal structures the distorted structure is derived from. Examples of idealised coordination geometries are given in Figs. 3.22 (coordination number 10), 3.23 (coordination number 11) and 3.24 (coordination number 12). The captions to these figures describe the construction of the polyhedra. 

In neither set is the metal involved in the a bonding in the cube, d z-yi is not involved either. 

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The forward steps of H2 oxidative addition, equations (a) and (e), are usually considered to be promoted by coordinatively unsaturated metal centers in initially low oxidation states (i.e., high metal basicity). Loss of electrons via the oxidation is compensated for by a gain of electrons through an increase in the coordination number . This also explains why the earlier transition metal systems (d -d ) tend to form complexes of higher coordination number than those of the later d -d systems (especially Group... 

Complexes of higher coordination number are often in equilibrium with the tetrahedral form and may be isolated by increasing the ligand concentration or by adding large counter ions, e.g. [M(NH3)6] +, [M(en)3]2+ or [M(bipy)3]2+. With acetylacetone, zinc achieves both 5- and... 

The coordination numbers exhibited by the tripositive lanthanide ions usually vary from six to ten. Bidentate ligands with smaller bite often yield complexes with higher coordination numbers. Coordination numbers less than six are rare for the lanthanides 279-281). Recently, Harman etal. 133) have shown that La(III) in La(C-18-crown-6)-(N03)3 attains a coordination number of 12. 

Addition of excess isocyanide ligand gives the ionic [Au(RNC)2]+ and there is some evidence for [Au(RNC)4]+ also, but more work is needed to characterize complexes with higher coordination number.402,403,405,408... 

Table 5. Bond Distances (pm) for Silicon Compounds of Higher Coordination Number (Ionic Complexes) ...

The latter relationship holds rather well for molecules AX3 (A111 = P to Bi X-1 = F to I). The expression for A is simple for CN = 38 (Equation (3)), but rather complex in the case of higher coordination numbers.5... 

For complexes with a ri2 coordination of the BELT ligand, a large number of structures are known with a large variety of coordination numbers and geometries trigonal-planar (TP), tetrahedral (TET), square-planar (SQP), trigonal-bipyramidal (TBP), square-pyramidal (SQPY) octahedral (OCT) and distorted octahedral (DOCT) (see Scheme 4). Nevertheless, some structures are more common than others. Only one example of both the SQPY [5] and DOCT [6] and two examples of OCT [7,8] have been reported. Linear structures M-q2 - BH4 (T2.1) and LM-(q2 - BH4 ) (T2.2) have only been computationally studied [9,10] and are not models of any experimental compound. Several computational studies have also been performed in order to study q2 complexes with higher coordination numbers [11-17]. 

In keeping with the hard or class a nature of the Fe " ion the most stable complexes are formed with the small non-polarizable F ion. Consecutive formation constants for some fluoro and chloro complexes in aqueous solution may be found in ref. 322. Bromo complexes are even less stable than chloro complexes while with the easily oxidizable 1 ion no stable simple iodides have been prepared although a very few complexes containing Fe "—I bonds are known in combination with other ligands. Consistent with the stability order F > CL > Br is the occurrence of higher coordination number Fe complexes with F than with Br . Thus, while F readily forms hexacoordinate [FeFfi] complex ions and no tetrafluoro complex ions, CL forms both [FeClgf and [FeCl4] while Br apparently does not form a stable hexabromo complex [FeBr ] ". 

These ligands are now well represented in complexes with the lanthanides, but were not investigated until 1962, when a study in aqueous solution using a Job s plot method, e.g. at 451 nm for Ho ", showed that a complex Ho(phen)2" was formed. Pr, Nd and Er tripositive ions were also studied, but no solid products were characterized. Indeed, stability constants in water are possibly too low for a solid complex to be isolated. A little later, there were several reports of the isolation of bipyridyl and 1,10-phenanthroline complexes from ethanolic solution. It is perhaps of interest that at this time, the lanthanides were believed not to form stable amine complexes and the role of higher coordination numbers in lanthanide complexes was not fully appreciated. Complexes isolated included the stoichiometries M(N03)s(bipy)2, where M = M(NCS)3(bipy)3, where M = La, Ce, Dy M(MeC02)3(bipy),... 

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