an introduction to coordination compoundsfaculty.uscupstate.edu/cbender/courses/c511/c511 notes...
TRANSCRIPT
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”.
CHEM 511 Chapter 7 page 2 of 13
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
CHEM 511 Chapter 7 page 3 of 13
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+
CHEM 511 Chapter 7 page 4 of 13
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".”
CHEM 511 Chapter 7 page 5 of 13
..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
CHEM 511 Chapter 7 page 6 of 13
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)
CHEM 511 Chapter 7 page 8 of 13
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?
CHEM 511 Chapter 7 page 9 of 13
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?
CHEM 511 Chapter 7 page 10 of 13
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?
CHEM 511 Chapter 7 page 11 of 13
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
CHEM 511 Chapter 7 page 12 of 13
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