CHEM 511 Chapter 7 page 1 of 13

An Introduction to Coordination Compounds What is a complex?

Complex: a metal atom or ion surrounded by ligands

Ligand: an ion or molecule that can have an independent existence

Coordination compound: a neutral complex or an ionic compound in which at least one of the

ions is a complex

Complex Ligand CC? CC possibility?

Ag(NH3)2+

AgCl2-

Fe(H2O)63+

Ni(CO)4

With complexes, we can discuss the inner coordination sphere versus the outer coordination sphere.

EX. FeCl3∙6H2O EX. CoCl2∙6H2O

Chelating ligands

Chelate: a complex in which a polydentate ligand forms a ring that includes the metal atom

EX. [Co(OH2)4(ox)]+

This can sometimes lead to strain and can distort an octahedral complex. Measure the ligand's

“bite angle”.

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Nomenclature rules 1. Writing formulas: write cation, then anion (just like simple salts)

2. The inner coordination sphere is enclosed in [ ]. Within the coordination sphere the ligands are

named before the metal, but the metal ion is written first in a formula. Old rules wrote anionic

ligands, followed by neutral or cationic ligands; new rules list all ligands alphabetically by letter

of the ligand (not name). To throw a wrench into this: ligands are sometimes written to point out

their attachment sites.

3. The number of ligands is indicated with Greek prefixes. If the ligand name includes a Greek

prefix or is complicated, it is set off with parentheses and the second set of prefixes is used.

1. mono 6. hexa......hexakis

2. di.......bis 7. hepta.....heptakis

3. tri.......tris 8. octa......octakis

4. tetra.....tetrakis 9. nona.....nonakis

5. penta......pentakis 10. deca......decakis

EX. [X(NH3)2]n+ versus [X(CH3NH2)2]

n+

4. Ligand endings: (see Table 7.1, pages 211-212 for some of the more important ligands)

Anionic ligands are given an "o" suffix

Neutral or cationic ligands retain their name

Coordinated water is "aqua" (not aquo!)

1. acac is acetylacetonate

2. NH3 is called ammine

3. CN-, attaches through C (usually), cyano ligand

4. SCN-, S attachment is the thiocyanato ligand

5. SCN-, N attachment is the isothiocyanato ligand

6. NO2-, nitro if N attachment

7. NO2-, nitrito if O attachment

8. NCO-, N attachment is the isocyanato ligand

9. CNO-, C attachment is the fulminato ligand

IUPAC suggestion

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EX. Name the following ligands

Cl- CH3NH2

Br- NH3

SO42- CO

C2O42-

H2O

5. Write ligand names alphabetically (ignoring Greek prefixes), with no spaces between parts of

names. Avoid abbreviations in the name.

EX. [Cu(NH3)4]2+

[Cu(NH3)4]SO4

[Co(NH3)6]Cl3

[CoCl2(NH3)4]+

a complex with ethylenediamine (en), [CoCl2(NH2CH2CH2NH2)2]+

a complex with 2,2’-bipyridine (bpy), [Fe(C5H4N-C5H4N)3]2+

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6. Two ways to designate charge or oxidation number

(a) Stock system: use a Roman numeral in parentheses to indicate the oxidation # of metal

(b) Ewens-Bassett system:1 use an Arabic numeral in parentheses (with a + or -) indicating the

overall charge of a complex. Numbers are usually written as (2+), (3-), (1+), etc.

In either case, if the complex is negative, add "-ate" to the name of the coordination sphere.

EX. [Pt(NH3)4]2+ [PtCl6]

2-

[PtCl4]2- [Re2Cl8]

2-

7. cis- and trans- prefixes designate adjacent or opposite geometric locations

EX. [PtCl2(NH3)2]

[CoCl2(NH3)4]+

1 This footnote comes from a journal article (Kauffman, G. B.; Jørgensen, C. K.; J. Chem. Ed. 1985, 62, 474-476.) probing the history of the E-

B system. “Ronald Victor George Ewens was born in 1913, was educated at Queen Elizabeth's Hospital, Bristol, and received his doctorate in

1938 from Lincoln College, Oxford University as a pupil of Nevil Vincent Sidgwick. After a year's investigation of the inhibition of fermentation

and glycolysis by snake venom at the Sir William Dunn School of Pathology, Oxford, he became Demonstrator and later (1948) Reader in

chemistry at Guy's Hospital Medical School. His primary research interest was metal carbonyls and nitrosyls. As Bassett told Fernelius, the 35-

year-old Ewens committed suicide by throwing himself in front of a subway train on September 21, 1948, in the year preceding the publication

of the paper. He left behind a widow and a young daughter.

Henry Bassett (1881-1965). PhD. Munich; DSc, London; was Assistant Lecturer, Liverpool University; Professor and Head of the Department

of Chemistry, University College, Reading and Reading University; and Emeritus Fellow, University College, London. He was primarily

interested in double salts and solid ammoniates and was the author of "Theory of Qualitative Analysis" and "Chemical Laboratory for

Beginners".”

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..but can be more complicated with [MA2BC], [MABCD], or [M(AB)2] structures (geometric

isomers)

cis-[MA2BC] trans-[MA2BC] [MABCD], A trans to B [MABCD], A trans to C

cis-[M(AB)2] trans-[M(AB)2]

8. fac- and mer- prefixes used for MX3Y3 octahedral complexes

EX. [Co(NO2)3(NH3)3]

Can it be more complicated? Most certainly...

fac-isomer mer-trans-isomer

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9. Bridging ligands between two metal centers have the prefix (mu)

EX. [(NH3)5Cr-O-Cr(NH3)5]4+

EX. [(NH3)4Co(-OH-)2Co(NH3)4]4+

EX. [(NH3)4Co(-NH2-)(-OH-)Co(NH3)4]4+

EX. What is the formula of tri-μ-iodo-bis(tricarbonylrhenium)?

10. When the complex is negatively charged, a Latin name for the element is often used (see

highlighted section). Otherwise, use this list.

iron ferrate

silver argentate

antimony stibate

lead plumbate

tin stannate

gold aurate

copper cuprate

tungsten tungstate

mercury mercurate

scandium scandate

titanium titanate

chromium chromate

manganese manganate

cobalt cobaltate

nickel nickelate

zirconium zirconate

niobium niobate

ruthenium ruthenate

rhodium rhodate

palladium palladate

rhenium rhenate

molybdenum molybdate

tantalum tantalate

platinum platinate

CHEM 511 Chapter 7 page 7 of 13

EX. K3[Fe(CN)6] EX. Na[AgCl2]

Chirality

A chiral complex is not superimposable on its own mirror image.

Two mirror images = enantiomeric pair

What symmetry operations prevent a chemical from being chiral?

EX. trans-[FeCl2(OH2)4] trans-[CoCl(en)2(OH2)]2+ cis-[CoCl2(en)2]

+

[Cr(edta)]- [Ru(bpy)3]2+ [PtCl(dien)]+

(dien = diethylenetriamine)

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Designations of chiral Oh complexes

Instead of R and S designations, use Δ and Λ (delta and lambda)

Original designation viewed rotation through a 3-fold axis, but see article on

website for easier determination method (Journal of Chemical Education

72(12) 1995, 1065-1066).

Read sections 7.3-7.11 on your own, though take special note of section 7.9...

Trigonal bipyramidal and square pyramidal complexes.

Despite being different structures, there is often very little energy difference between these two

shapes—prediction of which structure is adopted can be difficult.

To make matters more complicated, trigonal bipyramidal structures can undergo “fluxional

isomerism” (aka Berry pseudorotation).

Are there ways to prevent this from happening?

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Thermodynamics of Complex Formation

Formation constants

Complexes form by reaction of Lewis acid/base pairs.

Lewis acid: a chemical that is an _____________ acceptor

Lewis base: a chemical that is an _____________ donor

When a metal acquires multiple ligands, it does so in a stepwise fashion, i.e., stepwise stability

constants (i.e. stepwise formation constants).

Ni(OH2)62+ + NH3 Ni(OH2)5(NH3)

2+ + H2O log Kf1 = 2.79

Ni(OH2)5(NH3)2+ + NH3 Ni(OH2)4(NH3)2

2+ + H2O log Kf2 = 2.26

Ni(OH2)4(NH3)22+ + NH3 Ni(OH2)3(NH3)3

2+ + H2O log Kf3 = 1.69

Ni(OH2)3(NH3)32+ + NH3 Ni(OH2)2(NH3)4

2+ + H2O log Kf4 = 1.25

Ni(OH2)2(NH3)42+ + NH3 Ni(OH2)(NH3)5

2+ + H2O log Kf5 = 0.74

Ni(OH2)(NH3)52+ + NH3 Ni(NH3)6

2+ + H2O log Kf6 = 0.03

A β-value will comprise multiple values of Kf: nz

n2

zn

n][L])[M(OH

][MLβ

Trend in Kf values? Why?

Sometimes “odd” values can be seen in successive Kf values:

[Fe(OH2)6]2+ + bpy [Fe(bpy)(OH2)4]

2+ + 2H2O log Kf1 = 4.2

[Fe(bpy)(OH2)4]2+ + bpy [Fe(bpy)2(OH2)2]

2+ + 2H2O log Kf2 = 3.7

[Fe(bpy)2(OH2)2]2+ + bpy [Fe(bpy)3]

2+ + 2H2O log Kf3 = 9.3

Change in Kf2 to Kf3 not due to statistics. Change is due to a change in electron configuration of

Fe (next chapter). Which complex is more stable?

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A change in coordination number or major distortion can cause sudden changes in Kf values, too.

[Hg(OH2)6]2+ + Cl- [HgCl(OH2)5]

+ + H2O log Kf1 = 6.74

[HgCl(OH2)5]+ + Cl- [HgCl2(OH2)4] + H2O log Kf2 = 6.48

[HgCl2(OH2)4] + Cl- [HgCl3(OH2)]

— + 3H2O log Kf3 = 0.95

Note that as anionic ligands attach to cations, the charges begin to cancel, resulting in a negative

enthalpy change...but the entropy change is positive. This latter value more than makes up for the

former, if the complex is stable.

Another noticeable effect is the chelate effect: metal ions form stronger complexes with

polydentate ligands than with monodentate ligands.

Consider the following reactions:

EX. Cd2+(aq) + 2en Cd(en)2

2+ + 4H2O K = 4 1010

Cd2+(aq) + 4CH3NH2 Cd(CH3NH2)4

2+ + 4H2O K = 4 106 Data from: Spike, C. G.; Perry. R.W. JACS 1953, 75, 2726-2729

Consider enthalpy issues: What bond is breaking? What bond is forming? Is it the same in both

reactions? Inductive effects?

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Consider the entropy issues: number of species in reactants and products? other effects?

Note that macrocycles can give rise to the macrocyclic effect—even stronger

complex formation that just chelate effect.

Complexes made of monodentate ligands are not generally stable with s-block elements

s-block elements will form complexes with polydentate ligands like crown ethers and cryptands

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Sulfur and nitrogen variants of crown ethers...

Crown ethers and cryptands can be used to isolate alkalide ions (M-)

EX. Dissolve Na in ethylenediamine, add 2.2.2 crypt (or 2.2.1 crypt) and you get a salt. Will this

species be diamagnetic or paramagnetic?

CHEM 511 Chapter 7 page 13 of 13

Also interesting is the dissolution of alkali metals in liquid ammonia

video of Na in NH3 http://www.youtube.com/watch?v=JefumJFatsw

video of concentrated Li in NH3 http://www.youtube.com/watch?feature=fvwp&NR=1&v=Qx-

gVTaRAq4