Electron-pair donor ligand

When the structures for many ligands (e.g., H20, NH3, C032-, and C2042-) are drawn, there is no question as to which atom is the electron pair donor. Ligands such as CO and CN normally bond to metals by donation of an electron pair from the carbon atom. It is easy to see why this is so when the structures are drawn for these species and the formal charges are shown. 

The formation of a coordinate bond is the result of the donation and acceptance of a pair of electrons. This in itself suggests that if a specific electron donor interacts with a series of metal ions (electron acceptors) there will be some variation in the stability of the coordinate bonds depending on the acidity of the metal ion. Conversely, if a specific metal ion is considered, there will be a difference in stability of the complexes formed with a series of electron pair donors (ligands). In fact, there are several factors that affect the stability of complexes formed between metal ions and ligands, and some of them will now be described. 

Coordination compound or complex A compound containing dative bonds between electron pair donors (ligands) and a metal also known as Complex im. 

VSEPR theory, like all representations of the world, has its limitations. VSEPR theory works well for main group (non-transitional) compounds but is less useful for transition metal compounds. Lone pairs in transition metal complexes do not occupy directional orbits like those in the main group compounds. The shape of a complex ion can usually be deduced by counting the electron pair donors (ligands) and placing them as far apart from each other around the central metal ion. 

Quantitative Calculations The stoichiometry of complexation reactions is given by the conservation of electron pairs between the ligand, which is an electron-pair donor, and the metal, which is an electron-pair acceptor (see Section 2C) thus... 

Coordination compounds are also known as coordination complexes, complex compounds, or simply complexes. The essential feature of coordination compounds is that coordinate bonds form between electron pair donors, known as the ligands, and electron pair acceptors, the metal atoms or ions. The number of electron pairs donated to the metal is known as its coordination number. Although many complexes exist in which the coordination numbers are 3, 5, 7, or 8, the majority of complexes exhibit coordination numbers of 2, 4, or 6. 

Some ligands contain more than one atom that can function as an electron pair donor. For example, SCN is known to bond to some metal ions through the nitrogen atom but to others through the sulfur atom. In some instances, this situation is indicated in the name as thiocyanato-N- and... 

Many studies have been carried out on this reaction. Some of the unusual features of this reaction will be described in Chapter 20. Note that in writing the formulas for linkage isomers, it is customary to write the ligand with the atom that functions as the electron pair donor closest to the metal ion. Special consideration will be given in Chapter 20 to the behavior of cyanide complexes because CN-is also an ambidentate ligand. 

On the other hand, in the complex [Ni(CO)2cht] the cht ligand is a four-electron donor, so two double bonds are functioning as electron pair donors and the structure is... 

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... 

Ligands are generally electron pair donors (Lewis bases). Important ligands are NHs, CN, and OLT. Ligands bond to a central atom that is usually the positive ion of a transition metal, forming complexions and coordination compounds. On the AP exam, the number of ligands attached to a central metal ion is often twice the oxidation number of the central metal ion. 

In 1923 the American chemist G.N. Lewis provided a broad definition of acids and bases, which covered acid-base reactions not involving the traditional proton transfer an acid is an electron-pair acceptor (Lewis acid) and a base is an electron-pair donor (Lewis base). The concept was extended to metal-ligand interactions with the ligand acting as donor, or Lewis base, and the metal ion as acceptor, or Lewis acid. 

In reactions involving coordination compounds, the metal acts as the Lewis acid (electron-pair acceptor), while the ligand acts as a Lewis base (electron-pair donor). In the reaction above, the ammonia ligand displaced the water ligand from the chromium complex because nitrogen is a better electron-pair donor (less electronegative) than oxygen. 

A comparison of the metal-carbon bond lengths, ionic radii and formal coordination numbers of these compounds is summarized in Table 2. The formalism used in estabhshing coordination number assumes that a -cyclooctatetraene ligand is a 5 electron-pair donor. The ionic radii have been adjusted for both the charge of the central metal and coordination number (50). When the ionic radius for a given coordination number is not available, it has been estimated by interpolation from radii of other coordination numbers. It will be seen that the differ-... 

However, many ligands do not behave as donors of a single electron pair. Some ligands donate two or more electron pairs to the same central atom from different donor atoms. Such ligands are said to be chelating ligands, and they form chelate rings, closed by the central atom. The phenomenon is termed chelation. 

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