Chem Ch 22

Front 1

What are coordination compounds?

Back 1

A coordination compound is a compound that contains coordinate covalent bonds between a metal ion, often a (transition metal ion), and two or more polar molecules or ions.

Front 2

What is a coordinate covalent bond (also referred to as the dative bond)?

Back 2

A covalent bond in which one of the atoms donates both electrons. Although the properties of a coordinate covalent bond do not differ from those of a normal covalent bond (i.e. the electrons are shared in both cases), the distinction is useful for keeping track of valence electrons and assigning formal charges.

Front 3

How does the metal ion act in these reactions?

Back 3

As a Lewis acid, accepting electrons.

Front 4

How do the anions or polar molecules act in these reactions?

Back 4

Act as Lewis bases, donating pairs of electrons to form bonds to the metal ion.

Front 5

What is a complex ion?

Back 5

One in which a metal cation is covalently bound to one or more molecules or ions.

Front 6

What does a coordination compound usually consist of?

Back 6

a complex ion and one or more counter ions. In writing formulas for such coordination compounds, we use square brackets to separate the complex ion from the counter ion.

- some coordination compounds (like Fe(CO)5) don't contain complex ions. Most but not all of the metals in coordination compounds are transition metals.

Front 7

What is primary and secondary valence?

Back 7

primary valence corresponds to the oxidation number and secondary valence to the coordination number of the element. In CoCl3 • 6NH3, according to Werner, cobalt has a primary valence of 3 and a secondary valence of 6.

Front 8

What are transition metals?

Back 8

Transition metals are those that either have incompletely filled d subshells or form ions with incompletely filled d subshells. Incompletely filled d subshells give rise to several notable properties, including distinctive colors, the formation of paramagnetic compounds, catalytic activity, and the tendency to form complex ions. The most common transition metals are scandium through copper, which occupy the fourth row of the periodic table.

Front 9

Why aren't group 2B elements always considered transition metals?

Back 9

Note that although the Group 2B elements (Zn, Cd, Hg) are described as transition metals by some chemists, neither the metals nor their ions possess incompletely filled d-subshells. They are d-block metals but not actually transition metals.

Front 10

Explain transition metals and oxidation states?

Back 10

Transition metals exhibit variable oxidation states in their compounds. Figure 22.2 shows the oxidation states of the first row of transition metals. Note that all these metals can exhibit the oxidation state +3 and nearly all can exhibit the oxidation state +2. Of these two, the +2 oxidation state is somewhat more common for the heavier elements. The highest oxidation state for a transition metal is +7, exhibited by manganese (4s23d5). Transition metals exhibit their highest oxidation states in compounds that contain highly electronegative elements such as oxygen and fluorine—for example, V2O5, CrO3, and Mn2O7.

Front 11

What are ligands?

Back 11

The molecules or ions that surround the metal in a complex ion are called ligands (Table 22.3). The formation of covalent bonds between ligands and a metal can be thought of as a Lewis acid-base reaction. (Recall that a Lewis base is a species that donates a pair of electrons. To be a ligand, a molecule or ion must have at least one unshared pair of valence electrons, as these examples illustrate:

Ligands play the role of Lewis bases and the transition metal acts as a Lewis acid.

Front 12

What are common ligands?

Back 12

Front 13

What is the donor atom?

Back 13

The atom in a ligand that is bound directly to the metal atom is known as the donor atom. For example, nitrogen is the donor atom in the [Cu(NH3)4]2+ complex ion:

Front 14

What is a coordination number?

Back 14

The coordination number in a coordination compound refers to the number of donor atoms surrounding the central metal atom in a complex ion. The coordination number of Cu2+ in [Cu(NH3)4]2+ is 4. The most common coordination numbers are 4 and 6, although coordination numbers of 2 and 5 are also known.

Front 15

How are ligands classified?

Back 15

Depending on the number of donor atoms a ligand possesses, it is classified as monodentate (1 donor atom), bidentate (2 donor atoms), or polydentate (> 2 donor atoms).

Front 16

What are chelating agents?

Back 16

Bidentate and polydentate ligands are also called chelating agents because of their ability to hold the metal atom like a claw (from the Greek chele, meaning “claw”). One example is EDTA (Figure 22.4), a polydentate ligand used to treat metal poisoning. Six donor atoms enable EDTA to form a very stable complex ion with lead. This stable complex enables the body to remove lead from the blood.

Front 17

How do you find the oxidation state of a transition metal?

Back 17

The oxidation state of a transition metal in a complex ion is determined using the known charges of the ligands and the known overall charge of the complex ion. In the complex ion [PtCl6]2−, for example, each chloride ion ligand has an oxidation number of −1. For the overall charge of the ion to be −2, the Pt must have an oxidation number of +4.

Front 18

What are the rules for naming ionic coordination compounds?

Back 18

1. The cation is named before the anion, as in other ionic compounds. The rule holds regardless of whether the complex ion bears a net positive or a net negative charge. In the compounds K2[Fe(CN)6] and [Co(NH3)4]Cl, for example, we name the K+ and [Co(NH3)4]+ cations first, respectively. 2. Within a complex ion, the ligands are named first, in alphabetical order, and the metal ion is named last. 3. The names of anionic ligands end with the letter o, whereas neutral ligands are usually called by the names of the molecules. The exceptions are H2O (aqua), CO (carbonyl), and NH3 (ammine). 4.When two or more of the same ligand are present, use Greek prefixes di, tri, tetra, penta, and hexa, to specify their number. Thus, the ligands in the cation [Co(NH3)4Cl2]+ are “tetraamminedichloro.” 5.The oxidation number of the metal is indicated in Roman numerals immediately following the name of the metal. For example, the Roman numeral III is used to indicate the +3 oxidation state of chromium in [Cr(NH3)4Cl2]+, which is called tetraamminedichlorochromium(III) ion.

Front 19

What's the last rule?

Back 19

6.If the complex is an anion, its name ends in −ate. In K4[Fe(CN)6], for example, the anion [Fe(CN)6]4− is called hexacyanoferrate(II) ion. Note that the Roman numeral indicating the oxidation state of the metal follows the suffix −ate. Table 22.5 lists the names of anions containing metal atoms.

Front 20

What do you do when the name of a ligand already has a Greek prefix?

Back 20

Student Annotation: When the name of a ligand already contains a Greek prefix, a different set of prefixes is used to denote the number of the ligand: Two ethylenediamine ligands, for example, would be specified by bis(ethylenediamine).

- 2 = bis

- 3 = tris

- 4 = tetrakis

Front 21

What is the structure of ethylenediamine?

Back 21

H2N(2 dots above)-CH2-CH2-(2 dots above)NH2

Front 22

How to structure and coordination number relate to each other?

Back 22

Front 23

What do those look like?

Back 23

M is the metal and L is a monodentate ligand.

Front 24

What are stereoisomers?

Back 24

In studying the geometry of coordination compounds, we sometimes find that there is more than one way to arrange the ligands around the central atom. Such compounds in which ligands are arranged differently, known as stereoisomers, have distinctly different physical and chemical properties. Coordination compounds may exhibit two types of stereoisomerism: geometric and optical.

Front 25

What are geometrical isomers?

Back 25

Geometrical isomers are stereoisomers that cannot be interconverted without breaking chemical bonds. Geometric isomers come in pairs. We use the terms cis and trans to distinguish one geometric isomer of a compound from the other. Cis means that two particular atoms (or groups of atoms) are adjacent to each other, and trans means that the atoms (or groups of atoms) are on opposite sides in the structural formula. The cis and trans isomers of coordination compounds generally have quite different colors, melting points, dipole moments, and chemical reactivities.

Front 26

What does that look like?

Back 26

The (a) cis and (b) trans isomers of diamminedichloroplatinum( II). Note that the two Cl atoms are adjacent to each other in the cis isomer and diagonally across from each other in the trans isomer.

Front 27

What are optical isomers?

Back 27

Optical isomers are nonsuperimposable mirror images. (Superimposable means that if one structure is laid over the other, the positions of all the atoms will match.) Like geometric isomers, optical isomers come in pairs. However, the optical isomers of a compound have identical physical and chemical properties, such as melting point, boiling point, dipole moment, and chemical reactivity toward molecules that are not themselves optical isomers. Optical isomers differ from each other, though, in their interactions with plane-polarized light, as we will see.

- The structural relationship between two optical isomers is analogous to the relationship between your left and right hands. If you place your left hand in front of a mirror, the image you see will look like your right hand (Figure 22.8). Your left hand and right hand are mirror images of each other. They are nonsuperimposable, however, because when you place your left hand over your right hand (with both palms facing down), they do not match. This is why a right-handed glove will not fit comfortably on your left hand.

Front 28

What does chiral mean?

Back 28

Optical isomers are described as chiral (from the Greek word for “hand”) because, like your left and right hands, chiral molecules are nonsuperimposable. Isomers that are superimposable with their mirror images are said to be achiral. Chiral molecules play a vital role in enzyme reactions in biological systems. Many drug molecules are chiral, although only one of a pair of chiral isomers is biologically effective

Front 29

Why are chiral molecules said to be optically active?

Back 29

Chiral molecules are said to be optically active because of their ability to rotate the plane of polarization of polarized light as it passes through them. Unlike ordinary light, which vibrates in all directions, plane-polarized light vibrates only in a single plane.

Front 30

What is a polarimeter?

Back 30

A device used to measure the angle of rotation of plane-polarized light caused by a optically active compound. A beam of unpolarized light first passes through a Polaroid sheet, called the polarizer, and then through a sample tube containing a solution of an optically active, chiral compound. As the polarized light passes through the sample tube, its plane of polarization is rotated either to the right (clockwise) or to the left (counterclockwise). This rotation can be measured directly by turning the analyzer in the appropriate direction until minimal light transmission is achieved.

Front 31

Explain the rotation of plane polarization.

Back 31

If the plane of polarization is rotated to the right, the isomer is said to be dextrorotatory and the isomer is labeled d; if the rotation is to the left, the isomer is levorotatory and the isomer is labeled l. The d and l isomers of a chiral substance, called enantiomers, always rotate the plane of polarization by the same amount, but in opposite directions. Thus, in an equimolar mixture of two enantiomers, called a racemic mixture, the net rotation is zero.