Compounds, attaching ligands

The most common reaction exhibited by coordination compounds is ligand substitution. Part of this chapter has been devoted to describing these reactions and the factors that affect their rates. In the solid state, the most common reaction of a coordination compound occurs when the compound is heated and a volatile ligand is driven off. When this occurs, another electron pair donor attaches at the vacant site. The donor may be an anion from outside the coordination sphere or it may be some other ligand that changes bonding mode. When the reaction involves an anion entering the coordination sphere of the metal, the reaction is known as anation. One type of anation reaction that has been extensively studied is illustrated by the equation... 

The mass spectra of a considerable number of 77--bonded complexes of the group VIA metals have been reported, but in many cases mass spectrometry has only been used to determine the molecular weight, so that a detailed examination of the fragmentation processes involved has not been attempted, and only the molecular ion and perhaps a few other major peaks are reported. Within this section it is more convenient to discuss the compounds in terms of the attached ligands rather than in terms of the central metal atom. The classifications are (A) cyclopentadienyl compounds (B) arene compounds and (C) olefin, acetylene, and allyl compounds. 

Earlier in the chapter, we discussed the term complex ion, which referred to the ions formed by a metallic cation and an anion acting as a Lewis base. These Lewis bases that surround metal ions in a complex are known as ligands. Some common ligands are NH3, H20, and CN. The most common metals to form complexes are the transition metals because they have many empty valence orbitals that allow them to function as Lewis acids (accept electron pairs). In these compounds, the central metal ion and the attached ligands are written in brackets to distinguish the complex from the rest of the compound. For instance, the formula for the salt potassium hexacyanoferrate (II) is written ... 

Catalysis by Supported Metal-cluster Compounds. Further work has been reported recently on methods of chemically binding cluster compounds to supports and on the characterization of the resulting materials by various spectroscopic techniques. For example, the reaction of Rh6(CO)i6 with amine- and phosphine-modified silicas has been examined by infrared spectroscopy and has shown that cluster breakdown occurs giving L Rh(CO)2 and Lfn I (CO), where L comprizes the surface attached ligands. This behaviour is similar to that observed with Rh4(CO)i2 on unmodified silica where cluster breakdown occurs readily, particularly in the presence of traces of water and/or oxygen. ... 

X-Ray analyses of [Mn(TPP)] and the THF solvate of [Mn(TPP)(l-MeImH)] have been performed. " The X-ray data for the former compound suggests" that the Mn ion, with its relatively large radius, is positioned out of the N4-plane of the macrocycle. This result is in accordance with earlier theoretical predictions. The five-coordinate [Mn(TPP)(l-MeImH)] also has the Mn" displaced from the porphyrin plane in this case, the Mn" hes 0.56 A above the plane towards the 1-methylimidazole ligand. Calculations of the charge and spin distribution in the related Mn" porphyrin containing a coordinated water in the axial position indicate that the Mn" is strongly bound and that about 30% of the unpaired spin population S = 5/2) is delocalized away from the Mn" on to the attached ligands." ... 

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