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19.2 Coordination chemistry of transition metals  (Page 5/25)

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Two structures are shown. In a, inside of brackets, a central Z n atom is bonded to 4 C atoms in a tetrahedral spatial arrangement. Short line segments are used to represent a bond extending above and down and to the left of the Z n atom. A dashed wedge with the vertex at the Z n atom and wide end at the C atom is used to represent a bond down and to the right of the Z n atom. The final bond is indicated by a similar solid wedge again directed down and only slightly right of the center beneath the Z n atom. Four groups of three parallel short line segments are shown indicating triple bonds extending from each C atom opposite the bond with Z n to an associated N atom. Outside the brackets a superscript of 2 negative is shown. In b, at the center of this structure is a P t atom. From this atom, a single bond represented by a dashed wedge extends from a vertex at the P t atom up and to the right to the N atom of an N H subscript 3 group. Similarly, a single bond represented by a solid wedge extends from a vertex at the P t atom down and to the right to the N atom of an N H subscript 3 group. Another single bond represented by a dashed wedge extends from a vertex at the P t atom up and to the left to a C l atom. Similarly, a single bond represented by a solid wedge extends from a vertex at the P t atom down and to the left to a C l atom.
Transition metals with a coordination number of four can adopt a tetrahedral geometry (a) as in K 2 [Zn(CN) 4 ] or a square planar geometry (b) as shown in [Pt(NH 3 ) 2 Cl 2 ].

Isomerism in complexes

Isomers are different chemical species that have the same chemical formula. Transition metals often form geometric isomers , in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH 3 ) 4 Cl 2 ] + ion has two isomers. In the cis configuration    , the two chloride ligands are adjacent to each other ( [link] ). The other isomer, the trans configuration    , has the two chloride ligands directly across from one another.

Two structures are shown. The first is labeled, “Violet, cis form.” Below this label inside brackets is a central C o atom. From the C o atom, line segments indicate bonds to a C l atom above and the O atom of an H subscript 2 O group below the structure. Above and to both the right and left, dashed wedges with their vertex at the C o atom widening as they move out from the atom indicate bonds with O atoms of H subscript 2 O groups. Similarly, solid wedges below to both the right and left indicate bonds to a C l atom on the right and the O atom of an H subscript 2 O group on the left. This structure is enclosed in brackets. Outside the brackets to the right is the superscript plus sign. The second is labeled, “Green, trans form.” Below this label inside brackets is a central C o atom. From the C o atom, line segments indicate bonds to C l atoms above and below the structure. Above and to both the right and left, dashed wedges indicate bonds with O atoms of H subscript 2 O groups. Similarly, solid wedges below to both the right and left indicate bonds to the O atoms of H subscript 2 O groups. This structure is also enclosed in brackets with a superscript plus sign outside the brackets to the right.
The cis and trans isomers of [Co(H 2 O) 4 Cl 2 ] + contain the same ligands attached to the same metal ion, but the spatial arrangement causes these two compounds to have very different properties.

Different geometric isomers of a substance are different chemical compounds. They exhibit different properties, even though they have the same formula. For example, the two isomers of [Co(NH 3 ) 4 Cl 2 ]NO 3 differ in color; the cis form is violet, and the trans form is green. Furthermore, these isomers have different dipole moments, solubilities, and reactivities. As an example of how the arrangement in space can influence the molecular properties, consider the polarity of the two [Co(NH 3 ) 4 Cl 2 ]NO 3 isomers. Remember that the polarity of a molecule or ion is determined by the bond dipoles (which are due to the difference in electronegativity of the bonding atoms) and their arrangement in space. In one isomer, cis chloride ligands cause more electron density on one side of the molecule than on the other, making it polar. For the trans isomer, each ligand is directly across from an identical ligand, so the bond dipoles cancel out, and the molecule is nonpolar.

Geometric isomers

Identify which geometric isomer of [Pt(NH 3 ) 2 Cl 2 ] is shown in [link] . Draw the other geometric isomer and give its full name.

Solution

In the [link] , the two chlorine ligands occupy cis positions. The other form is shown in [link] . When naming specific isomers, the descriptor is listed in front of the name. Therefore, this complex is trans -diaminedichloroplatinum(II).

A structure is shown with a central P t atom. From this atom, single bonds represented by short line segments extend from the P t atom up and to the right and below and to the left to the N atom of N H subscript 3 groups. Similarly, two additional single bonds extend up and to the left and down and to the right to C l atoms.
The trans isomer of [Pt(NH 3 ) 2 Cl 2 ] has each ligand directly across from an adjacent ligand.

Check your learning

Draw the ion trans -diaqua- trans -dibromo- trans -dichlorocobalt(II).

Answer:

The structure in this figure shows a structure inside brackets. A central C o atom is shown with line segments indicating bonds to C l atoms above and below the structure. Above and to the left, a dashed wedge with its vertex at the C o atom widening as it moves out from the atom indicates a bond with the O atom of a H subscript 2 O group. A second dashed wedge indicates a bond with the central C o atom with a B r atom up and to the right. Similarly, a solid wedge below and to the left indicates a bond with a B r atom. A second solid wedge below and to the right indicates a bond with the O atom of an H subscript 2 O group. This structure is enclosed in brackets. Outside the brackets to the right is the superscript 2 negative sign.
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Another important type of isomers are optical isomers , or enantiomers , in which two objects are exact mirror images of each other but cannot be lined up so that all parts match. This means that optical isomers are nonsuperimposable mirror images. A classic example of this is a pair of hands, in which the right and left hand are mirror images of one another but cannot be superimposed. Optical isomers are very important in organic and biochemistry because living systems often incorporate one specific optical isomer and not the other. Unlike geometric isomers, pairs of optical isomers have identical properties (boiling point, polarity, solubility, etc.). Optical isomers differ only in the way they affect polarized light and how they react with other optical isomers. For coordination complexes, many coordination compounds such as [M(en) 3 ] n+ [in which M n+ is a central metal ion such as iron(III) or cobalt(II)] form enantiomers, as shown in [link] . These two isomers will react differently with other optical isomers. For example, DNA helices are optical isomers, and the form that occurs in nature (right-handed DNA) will bind to only one isomer of [M(en) 3 ] n+ and not the other.

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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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