Five METAL COORDINATION COMPOUNDS

Coordination Compounds with Transition Metals Coordination compounds formally do not count among the covalently bound structures. Still they shall be discussed in this chapter as, compared to other noncovalent variants, they are rather related to the covalently modified nanotubes. One can presume that nanotubes like fullerenes form coordination compounds with transition metals. Yet the tendency to do so is much less pronounced which is, among other reasons, due to the lack of five-membered rings. Hence, firstly, the delocalization of double bonds is less disturbed and, secondly, the energy gap between HOMO and LUMO is wider. The latter effect hinders an efficient backbonding and thus decreases the affinity of the nanotubes toward electron-rich metal systems. The cyclopenta-dienyl units as existent in fullerenes can stabilize the complexes once they come into being, whereas their absence in nanotubes hampers the formation of T -complexes. 

In the CeSI (115) and NdSBr (334) type of structure, bromine and iodine are coordinated to five metal ions (four of the same layer, and one of the opposite layer) and four halogen ions of the double layer. In the SmSI type (335), iodine is coordinated to three metal ions of a [LS] layer and three other iodine ions of the double layer. In the FeOCl type of compound, such as ErSCl (355) and LuSBr (85), the halogen is surrounded by a polyhedron formed by six sulfur and four halogenide ions. 

The most common coordination number of titanium is six (recognized for all oxidation states of the metal), although compounds are known in which the coordination number is four, five, seven or eight. The common oxidation states of titanium with the associated coordination numbers and stereochemistries are summarized in Table 3. The properties of these molecules will be discussed in the appropriate sections. In brief, however, titanium compounds in the +III or lower oxidation states are readily oxidized to the +IV state. Furthermore, titanium compounds can usually be hydrolyzed to compounds containing Ti—O linkages. 

As already concluded above, heterocyclic ligands with a ring size smaller than five or larger than six are rare, and few coordination compounds are known. In principle, however, ligands such as (31) and (32) could be expected to coordinate to metal ions in a chelating manner. 

The N,N-, N,0-, and 0,0-metal-cyclic structures can be found in these chelates. The first structures are characteristic for di- and triamine ligands (430a, 431), the second and third ones are formed in the complexes of all three types of ligand systems described above (429-431). In principle, the same situation as mentioned before (Sec. 2.2.4.2), for di- and triamine chelates, is observed for N,N-coordination. The 0,0-coordination is mostly present in structures formed only by carboxyl groups (compare with the data of Sec. 2.2.4.4). Here the most propagated structures in coordination compounds are 277, 279, and 280, for example, in complexonates of the triamine series [764], In the case of N,0-metal-binding, the most propagated structures are those with five (432) and six-member (433) metal-cycles [756-760,762-767] ... 

In a difference from the above, the formation of cis-planar structures takes place in case of chelates 868 (X = S, Se, M = Ni, Pd) [100,130,154-157], The stereochemical situation, close to that above, is mainly observed in coordination compounds of azomethinic ligands containing five-member metal-cycles [100,130]. 

A plausible reaction mechanism for such couplings is presented in Figure 13.7 for the specific example of the transformation of Figure 13.6. We do not specify the number n and the nature of the ligands L of the intermediate Ni complexes in Figure 13.7. Little is known about either one. Also, it is quite possible that more than one elementary reaction is involved in some of the steps 1-5. In any case, these five steps are certainly involved in the overall reaction. In step 1, the aryl bromide enters the coordination sphere of the Ni(0) compound as a 7r ligand. At least one metal coordination site must be vacated before an oxidative addition of the aryl bromide to the Ni atom may occur in step 2. The nickel inserts into the Csp2—Br bond, and its oxidation... 

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