Chemistry part 35, Julia Burdge,2e (2009) docx

SECTION 22.3 Bonding in Coordination Compounds: Crystal Field Theory 86 7.. Bonding in Coordination Compounds: Crystal Field Theory A satisfactory theory of bonding in coordination co

864 CHAPTER 22 Coordination Chemistry

Think About It Although ligand s

are alphabeti zed in a compound' s

name, they do not nece ssa ril y

appear in alphabetical order in the

compound's formula

Figure 22 5 Common geometries

of comp l ex ions In each case M i s a

metal and L i s a monodentate ligand

Strategy If you can't remember them yet, refer to Tab l es 22.4 and 22.5 for the names of ligands

and anions containing metal atoms

Setup (a) There are s ix ligands: five NH 3 molecule s and one Cl- ion Tb.e oxidation state of cobalt

i s +3, making the overall charge on the complex ion +2 Therefore, there are two chloride ions as

counter ions

(b) There are four ligand s: two bidentate ethylenediamines and two Cl - ions The oxidation state of platinum i s +4, making the overall charge on the complex ion +2 Therefore, there are two nitrate

ions as counter ions

Solution (a) [Co(NH3)sC l]CI2

Select the correct name for the

compound [Cu ( NH3)4]CI2

c) He xaftuo roiron(III) pota ss ium

d ) Pota ssi um hexaftuoroferrate ( lII ) e) Pota ss ium i ronhexaftuorate

Structure of Coordination Compounds

The geometry of a coordination compound often plays a s i gnificant role in determining i t s proper

-ties Figure 22.5 s how s four different geometric arrangements for metal atoms with monodentate ligands In these diagrams we See that structure and the coordination number of the meta l re l ate to

each other as follow s :

SECTION 22.2 Structure of Coordination Compounds 865

In studying the geometry of coordination compounds, we sometimes find that there is more than

one way to an'ange the ligands around the central atom Such compounds in which ligands are

• • • • • • • • • • • • • < ' • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • arranged differently, known as stereoisomers, have distinctly different physical and chemical prop-

erties Coordination compounds may exhibit two types of stereoisomerism: geometric and optical

Geometric isomers are stereoisomers that cannot be interconverted without breaking cal bonds Geometric isomers come in pairs We use the terms cis and trans to distinguish one '

chemi-geometric isomer of a compound from the other Cis means that two palticular 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

Figure 22.6 shows the cis and trans isomers of diamrninedichloroplatinum(II) Note that although

the types of bonds are the same in both isomers (two Pt-N and two Pt-Cl bonds), the spatial

arrangements are different Another example is the tetraamminedichlorocobalt(III) ion, shown in

Figure 22.7

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

reac-tivity 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

Figure 22.9 shows the cis and trans isomers of dichlorobis(ethylenediamine)cobalt(III) ion and the mirror image of each Careful examination reveals that the trans isomer and its mirror

Minor image

of left hand L e ft hand

In genera l, stereoisomers are compounds that are made up of the same types and numbers of atoms, bonded together in the same sequence, but with d iff erent spatial arrangements

Figure 22 6 The ( a) cis and (b)

trans isomer s of platinum(II) Note that the two Cl atoms are adjacent to each other in the

diamminedichloro-cis isomer and diagonally across from each other in the trans isomer

Figure 22.7 The ( a) cis and (b)

trans i so mer s of cobalt(III) ion , [Co(NH 3)4 CI 2t The ion has only two geometric isomers

tetraamminedichloro-Figure 22.8 A left hand and its

II1lrror Image

,

866 CHAPTER 22 Coordination Chemistry

Figure 22.9 The (a) cis and (b)

trans isomers of dichlorobis(ethyl e

ne-diamine )cobalt(III) ion and their mirror

images If you could rotate the mirror

image in (b) 90° clockwise about the

vertical position and place the ion over

the trans isomer, you would find that

the two are superimposable No matter

how you rotate the c i s isomer and it s ,

mirror image in (a), however, you

cannot superimpose one on the other

Light

so urce

Fixed polarizer

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 We use a polarimeter to mea-sure the rotation of polarized light by optical isomers (Figure 22.10) A beam of unpolarized light

contain-ing 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 (coun-terclockwise) This rotation can be measured directly by turning the analyzer in the appropriate direction until minimal light transmission is achieved (Figure 22.11) 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 rota-tion 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

-90'

•

Figure 22.10 Operation of a polarimeter Initially, the tube i s filled with an achiral compound The analyzer is rotated so that its plane of

polarization is perpendicular to th a t of the polari ze r U nder thi s condition, no light reache s the observer Next, a chiral compound is placed in the tube

as shown The plane of polarization of the polarized light i s rotated a s it travels through the tube so that so me light reache s the observer Rotating the

analyzer (either to the left or to the right ) until no light reaches the observer again allows the angle of optical rotation to be measured

SECTION 22.3 Bonding in Coordination Compounds: Crystal Field Theory 86 7

1

"

Bonding in Coordination Compounds:

Crystal Field Theory

A satisfactory theory of bonding in coordination compounds must account for properties such as

color and magnetism, as well as stereochemistry and bond strength No single theory as yet does

all this for us Rather, several different approaches have been applied to transition metal

com-plexes We will consider only one of them here crystal field theory because it accounts for both

the color and magnetic properties of many coordination compounds

We will begin our discussion of crystal field theory with the most straightforward namely, complex ions with octahedral geometry Then we will see how it is applied to tetrahedral

case-and square-planar complexes

Crystal Field Splitting in Octahedral Complexes

Crystal field theory explains the bonding in complex ions purely in terms of electrostatic forces In

a complex ion, two types of electrostatic interaction come into play One is the attraction between

the positive metal ion and the negatively charged ligand or the negatively charged end of a polar

ligand This is the force that binds the ligands to the metal The second type of interaction is the

electrostatic repulsion between the lone pairs on the ligands and the electrons in the d orbitals of

the metals

The d orbitals have different orientations [ ~~ Section 6.7], but in the absence of an nal disturbance, they all have the same energy In an octahedral complex, a central metal atom

exter-is surrounded by six lone pairs of electrons (on the six ligands), so all fi ve d orbitals experience

electrostatic repulsion The magnitude of this repulsion depends on the orientation of the d orbital

that is involved Take the di _ / orbital as an example In Figure 22.12, we see that the lobes of this

orbital point toward the comers of the octahedron along the x and y axes, where the lone-pair

elec-trons are positioned Thus, an electron residing in this orbital would experience a greater repulsion

from the ligands than an electron would in the d X) , d yz ' or d x z orbitals For this reason, the energy

of the d x 2 _ / orbital is increased relative to the d X)' d yZ ' and d xz orbitals The d z 2 orbital's energy

is also greater, because its lobes are pointed at the ligands along the z axis As a result of these

metal-ligand interactions, the five d orbitals in an octahedral complex are split between two sets of

energy levels: a higher level with two orbitals (d } _ i and d l ) having the same energy, and a lower

level with three equal-energy orbitals (d X)' d yz , and dx z ) ' as shown in Figure 22.13 The crystal field

splitting (Ll) is the energy difference between two sets of d orbitals in a metal atom when ligands

are present The magnitude of ~ depends on the metal and the nature of the ligands; it has a direct

effect on the color and magnetic properties of complex ions

Figure 22.11 Polarized len s e s 0 light pa s se s through the lense s when they are rotated so that their pl a ne s of

polarization are perpendicular

•

868 CHAPTER 22 Coordination Chemistry

Figure 22.12 The fi v e d orbital s in

an octahedral e n v ironment The met a l

atom ( or ion ) i s at the c e nter o f t h e

octahedron , and the s i x lone pair s on

the donor atom s of the ligand s are a t

the c orners

•

Figure 22.13 Cr ys tal field s plittin g

between d orbital s in an oc tahed ra l

Figure 22.14 A color w heel

with appropriate wa v elen g th s

Complementary color s, s uch a s red

and green , are on oppo s ite s ide s of the

Cr ys tal field s plitting, ~

In Chapter 6 we learned that white light, such as sunlight, is a combination of all colors A

sub-s tance appear s black if it ab s orbs all the visible light that s trike s it If it absorbs no visible light,

it is white or colorle ss An object appears green if it absorb s all light but reflects the green ponent An object al s o look s green if it reflects all colors except red, the complementary color of

( to u s ) a s the incident light-white - and the ion appears colorless

The be s t way to mea s ure crystal field splitting is to use spectroscopy to determine the

wa v elength at which light i s absorbed The [Ti(H20)6] 3+ ion provides a straightforward ple , becau s e Ti 3+ ha s only one 3d electron ( Figure 22.15) The [Ti(H20)6] 3+ ion absorbs light in the v i s ible region of the spectrum (Figure 22.16) The wavelength corresponding to maximum

exam-SECTION 22.3 Bonding in Coordination Compounds: Crystal Field Theory 869

•

Photon of energy hv

~

DD [JDD

This is the energy required to excite one [Ti(H20)6]3+ ion To express this energy difference in the

more convenient units of kJ/mol , we write

~ = (3.99 X 10- 19 Jlion)(6.02 X 1023 ionslmol)

= 240,000 J/mol

Aided by spectroscopic data for a number of complexes, all having the same metal ion but different ligands, chemists calculated the crystal field splitting for each ligand and established the

following spectrochemical series, which is a list of ligands arranged in increasing order of their

abilities to split the d orbital energy levels:

1- < Br- < CI- < OH- < F- < H20 < NH3 < en < CN- < CO

These ligands are arranged in the order of increasing value of ~ CO and CN- are called

strong-field ligands, because they cause a large splitting of the d orbital energy levels The halide ions and

hydroxide ion are weak-field li ga nds, because they split the d orbitals to a lesser extent

Magnetic Properties

The magnitude of the crystal field splitting also determines the magnetic properties of a

com-plex ion The [Ti(H20)6f+ ion, having only one d electron, is always paramagnetic However,

for an ion with several d electrons, the situation is less immediately clear Consider, for example,

the octahedral complexes [FeF6]3- and [Fe(CN)6]3- (Figure 22.17) The electron configuration

of Fe3+ is [Ar]3d s, and there are two possible ways to distribute the five d electrons among the

d orbitals According to Hund's rule [I ~~ Section 6.8], maximum stability is reached when the

Figure 22.15 (a) The proce ss

of photon absorption , and (b) a

graph of the absorption spectrum

of [Ti (H20)6lH The energy of the

incoming photon is equal to the crystal field splitting The maximum absorpt ion peak in the visible region

870 CHAPTER 22 Coordination Chemistry

Figure 22 17 Energy -le vel

diagrams for the Fe 3+ ion and for the

[FeF6] 3 - and [Fe(CN)6] 3 - complex

•

IOns

•

Figure 22 18 Orbital diagrams for

the hi g h- spin and lo w - s pin octahedral

complexes correspo nding to the

electron configurations of (a) d 4 , ( b ) d 5 ,

electrons are placed in five separate orbitals with parallel spins This arrangement can be achieved

only at a cost, however, because two of the five electrons must be promoted to the higher-energy

and d xz orbitals According to Pauli's exclusion principle [ ~~ Section 6.8], there will be only one

Figure 22.1 8 shows the distribution of electrons among d orbitals that results in low- and high-spin complexes The actual arrangement of the electrons is determined by the amount of

SECTION 22.3 Bonding in Coordination Compounds: Crystal Field Theory 871

stability gained by having maximum parallel spins versus the investment in energy required to

promote electrons to higher d orbitals Because F- is a weak-field ligand, the five d electrons enter

five separate d orbitals with parallel spins to create a high-spin complex The cyanide ion is a

strong-field ligand, though, so it is energetically preferable for all five electrons to be in the lower

orbitals, thus forming a spin complex High-spin complexes are more paramagnetic than

low-spin complexes

The actual number of unpaired electrons (or spins) in a complex ion can be found by netic measurements, and in general, experimental findings support predictions based on crystal

mag-field splitting However, a distinction between low- and high-spin complexes can be made only

if the metal ion contains more than three and fewer than eight d electrons, as shown in Figure

22.18 Sample Problem 22.4 shows how to determine the number of spins in an octahedral

complex

Predict the number of unpaired spins in the [Cr(en) 3 f + ion

Strategy The magnetic properties of a complex ion depend on the strength of the ligands

Strong-field ligands, which cause a high degree of splitting among the d orbital energy levels, result in

low-spin complexes Weak-field ligands, which cause only a small degree of splitting among the d orbital

energy levels, result in high-spin complexes

Setup The electron configuration of Cr 2+ is [Ar]3d 4; and en is a strong-field ligand

Solution Because en is a strong-field ligand, we expect [Cr(en) 3 f + to be a low-spin complex

According to Figure 22.18, all four electrons will be placed in the lower-energy d orbitals (d X) , d yz ,

and d x z) and there will be a total of two unpaired spins

Practice Problem How many unpaired spins are in [Mn(H20)6f +? (Hint: H20 is a weak-field

ligand.)

Tetrahedral and Square-Planar Complexes

So far we have concentrated on octahedral complexes The splitting of the d orbital energy

levels in tetrahedral and square-planar complexes, though, can also be accounted for

satisfac-torily by the crystal field theory In fact, the splitting pattern for a tetrahedral ion is just the

reverse of that for octahedral complexes In this case, the d xy> d yz ' and d xz orbitals are more

closely directed at the ligands and therefore have more energy than the d x 2-l and d z 2 orbitals

(Figure 22.19) Most tetrahedral complexes are high-spin complexes Presumably, the

tet-rahedral arrangement reduces the magnitude of the metal-ligand interactions, resulting in a

smaller Ll value This is a reasonable assumption because the number of ligands is smaller in

a tetrahedral complex

As Figure 22.20 shows, the splitting pattern for square-planar complexes is the most cated The d x 2- i orbital possesses the highest energy (as in the octahedral case), and the d X) , orbital

compli-is the next highest However, the relative placement of the d i and the d x z and d yZ orbitals cannot be

determined simply by inspection and must be calculated

/ d ry d yZ d xz

/ /

[I 1(, Cry s tal field s plitting

L -'-= ==,'== -'=~~ J ,

"0

the wrong conclusion regarding high- and low-spin complexes

Remember that the term high spin refers to the number of spins

(unpaired electrons), not to the energy levels of the d orbitals The greater the energy gap between the lower-energy and higher-energy d

orbitals, the greater the chance that

the complex will be low spin

•

Figure 22.19 Crystal field splitting between d orbitals in a tetrahedral

complex

872 CHAPTER 22 Coordination Chemistry

Figure 22.20 Energy-level

dia g r a m for a sq uar e -planar complex

Be ca u se there are more than two e n ergy

lev e l s, we cannot defi ne c r ys tal field

s plitt i ng as we can for octahedral and

tetrahedral complexe s

•

22 3 1

Bonding in Coordination Compounds:

Crystal Field Theory

expect the [Mn ( CO )6 l 2+ ion to ha ve?

22 3 2 Which of t h e following metal ions can

p ote nti a ll y form both l ow - s pin and

hi g h -sp in co mple xes? (Se lect all that appl y )

Reactions of Coordination Compounds

Complex ions undergo l i ga nd exchange (or substitution) reactions in solution The rates of these

reactions vary widely, depending on the nature of the metal ion and the ligands

In studying ligand exchange reactions, it is often useful to distinguish between the stability

of a complex ion and its tendency to react, which we call k inetic lability Stability in this context

is a thermodynamic property, which is measured in terms of the species' formation constant K f

[ ~~ Section 17.5] For example, we say that the complex ion tetracyanonickelate(II) is stabl e

because it has a large formation constant ( K f = 1 X 1030):

By using cyanide ions labeled with the radioactive isotope carbon-14, chemists have shown that [Ni(CN)4]2- undergoes ligand exchange very rapidly in solution The following equilibrium is

established almost as soon as the species are mixed:

where the asterisk denotes a 14C atom Complexes like the tetracyanonickelate(II) ion are termed

labil e co mplexes because they undergo rapid ligand exchange reactions Thus, a

thermodynami-cally stable species (i.e , one that has a large formation constant) is not necessarily unreactive

A complex that is thermodynamically unstable in acidic solution is [Co(NH3)6]3+ The librium constant for the following reaction is about 1 X 1020:

equi-When equilibrium is reached, the concentration of the [Co(NH3)6]3+ ion is very low This reaction

requires several days to complete, however, because the [Co(NH3)6]3+ ion is so inert This is an example of an inert complex -a complex ion that undergoes very slow exchange reactions (on the

order of hours or even days) It shows that a thermodynamically unstable species is not necessarily chemically reactive The rate of reacti on is determined by the energy of activation, which is high

in this case

Most complex ions containing Co3+, Cr3+, and Pt2+ are kinetically inert Because they

exchange ligands very slowly, they are easy to study in solution As a result, our knowledge of the

SECTION 22.S Applications of Coordination Compounds 873

bonding, structure, and isomerism of coordination compounds has come largely from studies of

these compounds

and in medicine We briefly describe a few examples in this section

Metallurgy

nickel by converting the metal to the gaseous compound Ni(CO)4 are typical examples of the use

Chelation Therapy

Earlier we mentioned that chelation therapy is used in the treatment of lead poisoning Other

Chemotherapy

mechanism for the action of cisplatin is the che lation of DNA, the molecule that contains the

which must be accurately copied in order for the new cells to be identical to their parent cell

the DNA (Guanine is one of the four bases in DNA [ ~~ Section 10.6, Figure 10.1SJ ) This

uc-tural distortion is a key factor in inhibiting replication The damaged cell is then destroyed by

the body's immune system Because the binding of cisplatin to DNA requires both Cl atoms to

Cisplatin

Chemical Analysis

brick-red solid with Ni2+ and an insoluble bright-yellow solid with Pd2+ These characteristic

quanti-ties of ions present can be determined by gravimetric analysis [ ~~ Section 4.6J as follows: To a

precipitate forms The precipitate is then filtered , dried, and weighed Knowing the formula of

874 CHAPTER 22 Coordination Chemistry

The cleansing action of soap in hard water is hampered by the reaction of the Ca2+ ions in the

water with the soap molecules to form insoluble salts or curds In the late 1940s the detergent

Ca2+ ions Sodium tripolyphosphate revolutionized the detergent industry Because phosphates

are plant nutrients, however, wastewater containing phosphates discharged into rivers and lakes

causes algae to grow, resulting in oxygen depletion Under these conditions, most or all aquatic

life eventually succumbs This process is called eutrop hication Consequently, many states have

to eliminate phosphates

Sequestrants

In addition to its use in medicine and chemical analysis, EDTA is used as a food additive to

the oxidation reactions that cause food to spoil EDTA is a common preservative in a wide variety

of consumer products

Bringing Chemistry to life

The Coordination Chemistry of Oxygen Transport

prob-ably the most studied of all the proteins The molecule contains four folded long chains called

oxygen molecules to myoglobin Myoglobin, which is made up of only one subunit, stores oxygen

for metabolic processes in the muscle

coordinated to the four nitrogen atoms in the porphine group and also to a nitrogen donor atom in a

ion on the other side of the ring to complete the octahedral complex This hemoglobin molecule

is called deoxyhemoglobin and imparts a bluish tinge to venous blood The water ligand can be

I

;

!

I

APPLYING WHAT YOU'VE LEARNED

Applying What You've Learned

Elevated BLL and other heavy metal poisoning can be treated with one of several ing agents, including DMSA and EDTA EDTA is administered intravenously as either the sodium salt (Endrate) or as the calcium disodium salt (Versenate) Endrate is not approved for the treatment of lead poisoning because of its high affinity for calcium It

chelat-is approved, however, for treating hypercalcemia, a condition in which there chelat-is excess

calcium in the blood usually as a result of bone cancer The accidental use of Endrate during treatment for lead poisoning resulted in the death of a 2-year-old girl in February

of 2005 The girl's death was attributed to sudden cardiac arrest caused by the removal of too much calcium from her blood

Chelation therapy works by the administration of a ligand, which binds to metal ions already in the body Many drugs, including cisplatin [ ~ Chapter 3, Applying

What You've Learned], are themselves coordination compounds in which the trill metal ion binds to electron-rich sites (such as oxygen or sulfur atoms) in biological molecules

cen-Problems:

a) Determine the oxidation state of platinum in cisplatin, [Pt(NH3hCI2J

[I •• Sample Problem 22.1]

b) Give the systematic name for cisplatin [ ~ Sample Problem 22.2]

c) Write the formula for the compound potassium hexachloroplatinate(IV)

[ ~ Sample Problem 22.3]

875

876 CHAPTER 22 Coordination Chemistry

CHAPTER SUMMARY

Section 22.1

• Coordination compounds contain coor dinat e covalent bonds between

a metal ion (ofte n a tran s ition metal ion ) and t wo or more polar

m o lecule s or ions

• The molecule s or anions that s UlTound a m e tal in a coordination

• Many coordination compounds consist of a complex ion and a counter

Ion

• Tran s iti on metal s are tho se that ha ve in comp letel y filled d s ub s hell s

-or that give ri se to ions with incompletel y filled d subs hell s

• Tran s ition metals exhibit v ariable oxidation s tate s ranging f rom + 1 to

+7

• To act as a ligand , a inolecule or ion mu s t h ave at least one un s hared

pair of electrons The atom that bear s the un s har e d pair of electrons i s

the donor atom

• Ligands are cla ss ified as monodentate , bidentate , or pol y dentate,

based on the number of donor atom s they co ntain Bidentate and

polydentate ion s are also kno w n a s chelating agents

• The coordination number i s the number of donor atoms s UlToundin g a

metal in a complex

• Ionic coordination compounds are named b y first naming the cation

and then the anion Complex ion s are named by li s ting the ligand s in

alphabet i ca l order, followed b y the metal and it s oxidation state (as

a Roman numeral ) When the complex i o n i s the anion , the anion 's

name ends in -ate

Section 22.2

• The coordination number larg e ly determine s the geo metr y of a

coordination complex

same li gands are stereoisomers The two types of s tereoisomerism are

geometric and optical

• Geometric isomers contain the same atoms and bonds alTanged

differently in sp ace

!(EY WORDS

Chelating agent, 861

Coordination compound, 858

Coord in ation number, 861

Crystal field s plitting ( /1), 867

Dextrorotatory, 866

Donor atom, 860 Enantiomers, 866 Geometric i some r s, 865

QUESTIONS AND PROBLEMS

• Optical isomers are non s uperimpo sable min·or image s We call a pair

of optical i s omer s enantiomers The rotation of polari ze d li ght i s

• Enantiomers rotate the plan e of plane-polarized light in opposite

directions The enantiomer that rotates it to the right is called

dextrorotatory and i s labeled d The enantiomer that rotate s it to the left is ca lled levorotatory and is l a beled l An equal mixture of a pair

of ena ntiomer s, called a racemic mixture, does not cause any net

rotation of plane-polarized light

Ligands in a coordination comp l ex cause the energy level s of the d

orbitals on a metal to s plit T h e difference in energy between the lower and higher d orbital energy l eve l s i s called the crystal field splitting (Ll)

The magnitude of /1 depend s on the nature of the ligand s in the

co mplex The spectrochemical series orders some common li gands in

o rder of increa si ng field strength

Strong-field ligand s give ri se to a l arger /1 val u e; weak-fie l d li gands

yield a sm aller /1 va lue

Crystal field s plittin g so metime s changes the number of unpaired

e l ectro n s, and therefore the magnetic properties, of a metal

Complexes containing tran s iti on metal s with d 4 , d S , d 6 , or d 7

configuratio n s may be high spin or low spin In high- spin comp le xes, the number of unpaired electrons i s maximized because /1 i s small; in

low-spin complexes, the number of unpaired electrons i s minimized

because /1 i s large

Section 22.4

• Complex ion s undergo ligand exchange in so lution The rate at

w hi c h ligand exchange occurs i s a mea s ure of a complex 's kinetic

lability and doe s not nece ssa rily correspond directly to the complex's

thermodynamic stability

Section 22.5

• Coordination chemistry i s important in many biological , medical , a nd

indu s trial processes

Le voro tatory, 866

Li ga nd , 860

Racemic mixture , 866

Spectrochemical series, 869 Stereo isomers, 865

22 1 What di s tingui s hes a tran s ition metal from a main group met a l ?

22 2 Why i s z inc not considered a transition metal ?