Bonding with Metals Ligands

They do this based on Lewis acid-base chemistry, which is another way of saying it s about accepting and donating electrons. (See Chapter 5 for a refresher on acid-based chemistry.) The ligands act as a Lewis base because they re an electron pair donor. The central metal atom acts as a Lewis acid because it s an electron pair accepter. 

The number of ligands bound to the metal ion is designated as the coordination number of the metal. Some main group elements form complexes that are similar to transition metal complexes. These include aluminum, tin, and lead, for example, but for the most part organometallic chemistry employs transition metals. This is due to the partially-filled d-orbitals. 

When one bond is formed between the metal and the ligand, it s known as monodentate with two bonds it s known as multidentate. Multidentate compounds form a class of materials known as chelating complexes. (For more details about this see. Chapter 9 about coordination complexes.) Think of a chelating molecule like a finger and thumb that can hold onto a tennis ball. 

If the number of ligands present is not sufficient to fulfill the electron counting rules, then cluster compounds can form. This also occurs if metals are not able to satisfy the 16-electron or 18-electron rule. This can force metals to bond with each other, so you have metals bonded to one another (often with a strong double bond) that are stabilized by a series of ligands airound them. 

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Until about 20 years ago, the valence bond model discussed in Chapter 7 was widely used to explain electronic structure and bonding in complex ions. It assumed that lone pairs of electrons were contributed by ligands to form covalent bonds with metal atoms. This model had two major deficiencies. It could not easily explain the magnetic properties of complex ions. 

Fig.1. Structures of porphyrin 1, chlorophyll 2, and phthalocyanine 3. In the presence of metal salts M"+X (M=metal, X=counter anion, n=oxidation state or number of counter anions), porphyrins produce chelate complexes. Some metal chelates of the porphyrins, such as ZnPor, form further coordination bonds with other ligands such as pyridines...

M[pz(A4)] A = S2ML2. The octakis(.V-R)porphyra/,ines reported by Schramm and Hoffman (2), M[pz(S-R)8 (M = Ni, Cu), (60), can be converted to the octathiolate M[pz(S )g] (Scheme 11) via reductive cleavage of the sulfur-carbon bond when R = benzyl (Bn), and this tetra-bis(dithiolate) can then be peripherally capped with metal-ligand systems to yield peripherally tetrametalated star porphyrazines. The benzyl dinitrile 57 can be macrocyclized around magnesium butoxide to form [Mg[pz(S-Bn)8] (58) (35-40%), which can then be demetalated with trifluoroacetic acid to form 59 (90%), which is subsequently remetalated with nickel or copper acetate to form 60a (95%) and 60b (70%) (Scheme 11) (3, 23, 24). Deprotection of 60a or 60b with sodium in ammonia yields the Ni or Cu tetra-enedithiolates, 61a or 61b to which addition of di-ferf-butyl or n-butyl tin dinitrate produces the peripherally metalated star porphyrazines 62a (37%), 62b (80%), and 62c (41%). 

In a metalloid cluster [2] more metal-metal bonds than metal-ligand bonds are involved, which means n > r. The largest structurally characterized compounds of this type contain 77 A1 or 84 Ga atoms, respectively [3, 4], Metal-metal bonds dominate these clusters and the framework of the resulting metal-metal bonds exhibits a geometry similar to the bulk metal itself. With respect to the Greek word ei8o< (idea, prototype) the suffix -oid indicates that the bulk metal element is actually visible in the metal atom core of the metalloid or more generally, elementoid clusters. 

The orbitals d, dt and d can, however, be used in case that the central atom of the complex forms multiple bonds with the ligands. Some of the octahedral complexes of the transition metals contain bonds with a large amount of double-bond character. These complexes will be discussed in Chapter 9. 

The weakness of most metal-metal bonds compared with metal-ligand bonds makes cleavage of metal-metal bonds by nucleophiles a common process (15). In the case of metal-metal double bonds this corresponds to nucleophilic addition to the metal-metal bonded systems. Since unsaturated clusters exist which can be considered to contain metal - metal double bonds, this should be an important aspect of substrate activation by clusters. 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

Ligands generally form cr-donor bonds with metal ion sites. Several common ligands in bioinorganic chemistry also have strong re-donor interactions with the metal (thiolates and phenolates, in particular). 

The third case includes those ligands which possess both filled and empty n orbitals. Examples are the Br, Cl" and CN" anions. However, in order to understand the chalcophilic properties of transition elements, only the second case involving n-bonding with metal t2g orbitals needs to be considered. 

The interaction of saturated C—H bonds with metal ions is facilitated when the alkyl group foims part of a coordinated ligand (proximity effect). Several intramolecular reactions of metal ions with saturated C—H bonds have been reported. For example,60831 b... 

Manganese appears to be unique in catalyzing 02 evolution in all aerobic pho-totrophs. No other metal ion has been found to date that can replace it. Manganese was probably selected via evolution for this function because of its ability to exist in multiple oxidation states (Mn2+, Mn3+, Mn4+, Mn5+) and to form strong bonds with oxygen ligands [29],... 

D. Dithiolene Complexes with Metal-Ligand Multiple Bonds / 303... 

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