Coordination compounds ligand substitution

Functionally substituted phosphines play an important role as ligands in a great variety of phosphorus coordination compounds. They have some interesting features that distinguish them from other phosphine ligands, namely (a) the presence of other heteroatoms bearing lone electron pairs in addition to phosphorus (b) the presence of functional groups able to form bonds with a metal with the participation of its valence electrons ... 

The enormous number of coordination compounds undergo many reactions, but a large number of reactions can be classified into a small number of reaction types. When one ligand replaces another, the reaction is called a substitution reaction. For example, when ammonia is added to an aqueous solution containing Cu2+, water molecules in the coordination sphere of the Cu2+ are replaced by molecules of NH3. Ligands are held to metal ions because they are electron pair donors (Lewis bases). Lewis bases are nucleophiles (see Chapter 9), so the substitution of one nucleophile for another is a nucleophilic substitution reaction. Such a reaction can be illustrated as... 

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... 

Abstract In this chapter, selected results obtained so far on Fe(II) spin crossover compounds of 1,2,4-triazole, isoxazole and tetrazole derivatives are summarized and analysed. These materials include the only compounds known to have Fe(II)N6 spin crossover chromophores consisting of six chemically identical heterocyclic ligands. Particular attention is paid to the coordination modes for substituted 1,2,4-triazole derivatives towards Fe(II) resulting in polynuclear and mononuclear compounds exhibiting Fe(II) spin transitions. Furthermore, the physical properties of mononuclear Fe(II) isoxazole and 1-alkyl-tetrazole compounds are discussed in relation to their structures. It will also be shown that the use of a,p- and a,C0-bis(tetrazol-l-yl)alkane type ligands allowed a novel strategy towards obtaining polynuclear Fe(II) spin crossover materials. 

A solution of the isolated platinum blue compound usually contains several chemical species described in the previous section. Such complicated behaviors had long been unexplored, but were gradually unveiled as a result of the detailed equilibrium and kinetic studies in recent years. The basic reactions can be classified into four categories (l)HH-HT isomerization (2) redox disproportionation reactions (3) ligand substitution reactions, especially at the axial coordination sites of both Pt(3.0+)2 and Pt(2.5+)4 and (4) redox reactions with coexisting solvents and atmosphere, such as water and 02. In this chapter, reactions 1-4 are summarized. 

Phosphorus trifluoride is a ligand that is used extensively in coordination chemistry. It substitutes readily into various metal carbonyl complexes using either thermal or photochemical techniques. As a ligand, it is unique in its similarity to carbon monoxide in lower-valent organometallic compounds. In its role as a model for CO, a number of studies are possible that cannot be done on the carbonyls themselves.1 The name normally used for PF3 in complexes is trifluorophosphine. 

Cationic complexes of Tl3+ with oxygen donors are known, and can be regarded as substitution products of [T1(C)H2)6]3+ (c/. Section 25.2.8.3.1). The DMSO product has been characterized with various anions,403 and there seems no reason to suppose that similar compounds cannot be prepared with donor ligands which are resistant to oxidation. The cation [Tl(pyNO)8]3+ (prepared as the perchlorate) may be an indication that eight-coordinate compounds are also accessible in thallium(III) chemistry.404... 

Other addition compounds of Hg2(N03)2 with aromatic amines97 contain two molecules of the N ligand and one molecule of Hg2(N03)2 with 4-fluoroaniline four molecules of the ligand are coordinated.97 Similar results have been obtained with Hg2(N03)2 or Hg2(C104)2 and mono-, di- and tri-substituted anilines, pyridines and l,4-diazabicyclo[2.2.2]octane.101,102 Coordination compounds are also formed with aminopyridine, trichloroacetaminopyridine and aminochloropyridine.103 The X-ray structure determination of the coordination compound between Hg2(C104)2 and 3-aminopyridinium perchlorate revealed the structure (12).103... 

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