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19.2 Coordination chemistry of transition metals  (Page 6/25)

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Two structures are shown with a vertical dashed line segment between them. The structure left of this line segment has a central M representing a metal atom. To this atom, six N H subscript 2 groups are attached with single bonds. These bonds are indicated with line segments extending above and below, dashed wedges extending up and to the left and right, and solid wedges extending below and to the left and right. The bonds to these groups are all directed toward the N atoms. The N H subscript 2 groups are each connected to C atoms of C H subscript 2 groups extending outward from the central M atom. These C H subscript 2 groups are connected in pairs with bonds indicated by short line segments. This structure has the overall appearance of a flower with three petals, two of which are equidistant from the dashed line. A mirror image of this structure appears on the right side of the dashed line, again with two of the “petals” equidistant from the dashed line to its left.
The complex [M(en) 3 ] n+ (M n+ = a metal ion, en = ethylenediamine) has a nonsuperimposable mirror image.

The [Co(en) 2 Cl 2 ] + ion exhibits geometric isomerism ( cis / trans ), and its cis isomer exists as a pair of optical isomers ( [link] ).

This figure includes three structures. The first two are labeled “cis form (optical isomers).” These structures are followed by a vertical dashed line segment to the right of which appears a third structure that is labeled “trans form.” The first structure includes a central C o atom that has four N H subscript 2 groups and two C l atoms attached with single bonds. These bonds are indicated with line segments extending above and below, dashed wedges extending up and to the left and right, and solid wedges extending below and to the left and right. C l atoms are bonded at the top and at the upper left of the structure. The remaining four bonds extend from the central C o atom to the N atoms of N H subscript 2 groups. The N H subscript 2 groups are each connected to C atoms of C H subscript 2 groups extending outward from the central C o atom. These C H subscript 2 groups are connected in pairs with bonds indicated by short line segments, forming two rings in the structure. This entire structure is enclosed in brackets. Outside the brackets to the right is the superscript plus. The second structure, which appears to the be mirror image of the first structure, includes a central C o atom that has four N H subscript 2 groups and two C l atoms attached with single bonds. These bonds are indicated with line segments extending above and below, dashed wedges extending up and to the left and right, and solid wedges extending below and to the left and right. C l atoms are bonded at the top and at the upper right of the structure. The remaining four bonds extend from the central C o atom to the N atoms of N H subscript 2 groups. The N H subscript 2 groups are each connected to C atoms of C H subscript 2 groups extending outward from the central C o atom. These C H subscript 2 groups are connected in pairs with bonds indicated by short line segments, forming two rings in the structure. This entire structure is enclosed in brackets. Outside the brackets to the right is a superscript plus sign. The third, trans structure includes a central C o atom that has four N H subscript 2 groups and two C l atoms attached with single bonds. These bonds are indicated with line segments extending above and below, dashed wedges extending up and to the left and right, and solid wedges extending below and to the left and right. C l atoms are bonded at the top and bottom of the structure. The remaining four bonds extend from the central C o atom to the N atoms of N H subscript 2 groups. The N H subscript 2 groups are each connected to C atoms of C H subscript 2 groups extending outward from the central C o atom. These C H subscript 2 groups are connected in pairs with bonds indicated by short line segments, forming two rings in the structure. This entire structure is enclosed in brackets. Outside the brackets to the right is a superscript plus sign. This final structure has rings of atoms on opposite sides of the structure.
Three isomeric forms of [Co(en) 2 Cl 2 ] + exist. The trans isomer, formed when the chlorines are positioned at a 180° angle, has very different properties from the cis isomers. The mirror images of the cis isomer form a pair of optical isomers, which have identical behavior except when reacting with other enantiomers.

Linkage isomers occur when the coordination compound contains a ligand that can bind to the transition metal center through two different atoms. For example, the CN ligand can bind through the carbon atom (cyano) or through the nitrogen atom (isocyano). Similarly, SCN− can be bound through the sulfur or nitrogen atom, affording two distinct compounds ([Co(NH 3 ) 5 SCN] 2+ or [Co(NH 3 ) 5 NCS] 2+ ).

Ionization isomers (or coordination isomers ) occur when one anionic ligand in the inner coordination sphere is replaced with the counter ion from the outer coordination sphere. A simple example of two ionization isomers are [CoCl 6 ][Br]and [CoCl 5 Br][Cl].

Coordination complexes in nature and technology

Chlorophyll, the green pigment in plants, is a complex that contains magnesium ( [link] ). This is an example of a main group element in a coordination complex. Plants appear green because chlorophyll absorbs red and purple light; the reflected light consequently appears green. The energy resulting from the absorption of light is used in photosynthesis.

Structural formulas are shown for two complex molecules. The first has a central M g atom, to which N atoms are bonded above, below, left, and right. Each N atom is a component of a 5 member ring with four C atoms. Each of these rings has a double bond between the C atoms that are not bonded to the N atom. The C atoms that are bonded to N atoms are connected to C atoms that serve as links between the 5-member rings. The bond to the C atom clockwise from the 5-member ring in each case is a double bond. The bond to the C atom counterclockwise from the 5-member ring in each case is a single bond. To the left of the structure, two of the C atoms in the 5-member rings that are not bonded to N atoms are bonded to C H subscript 3 groups. The other carbons in these rings that are not bonded to N atoms are bonded to groups above and below. A variety of groups are attached outside this interconnected system of rings, including four C H subscript 3 groups, a C H subscript 2 C H subscript 2, C O O C subscript 20, H subscript 39 group, a C H C H subscript 2 group with a double bond between the C atoms, additional branching to form a five-member carbon ring to which an O atom is double bonded and a C O O C H subscript 3 group is attached. The second structure has a central C u atom to which four N atoms that participate in 5-member rings with C atoms are bonded. Unlike the first molecule, these 5-member rings are joined by N atoms between them, with a double bond on the counter clockwise side and a single bond on the clockwise side of each of the four N atoms that link the rings. On the side of each 5-member ring opposite its N atom, four additional carbon atoms are bonded, forming 6-member carbon rings with alternating double bonds. The double bonds are not present on the bonds that are shared with the 5-member rings.
(a) Chlorophyll comes in several different forms, which all have the same basic structure around the magnesium center. (b) Copper phthalocyanine blue, a square planar copper complex, is present in some blue dyes.

Transition metal catalysts

One of the most important applications of transition metals is as industrial catalysts. As you recall from the chapter on kinetics, a catalyst increases the rate of reaction by lowering the activation energy and is regenerated in the catalytic cycle. Over 90% of all manufactured products are made with the aid of one or more catalysts. The ability to bind ligands and change oxidation states makes transition metal catalysts well suited for catalytic applications. Vanadium oxide is used to produce 230,000,000 tons of sulfuric acid worldwide each year, which in turn is used to make everything from fertilizers to cans for food. Plastics are made with the aid of transition metal catalysts, along with detergents, fertilizers, paints, and more (see [link] ). Very complicated pharmaceuticals are manufactured with catalysts that are selective, reacting with one specific bond out of a large number of possibilities. Catalysts allow processes to be more economical and more environmentally friendly. Developing new catalysts and better understanding of existing systems are important areas of current research.

This figure includes three photographs. In a, a photo shows store shelving filled with a variety of brands of laundry detergent. In b, a photo shows a can of yellow paint being stirred. In c, a bag of fertilizer is shown.
(a) Detergents, (b) paints, and (c) fertilizers are all made using transition metal catalysts. (credit a: modification of work by “Mr. Brian”/Flickr; credit b: modification of work by Ewen Roberts; credit c: modification of work by “osseous”/Flickr)

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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