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23-1 COORDINATION COMPOUNDS Unit 10A • Bonding • Terminology • Nomenclature • Chapter 20 (McM) • Chapter 23.4-23.5 Silberberg
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COORDINATION COMPOUNDSUnit 10A
• Bonding• Terminology• Nomenclature• Chapter 20 (McM)• Chapter 23.4-23.5 Silberberg
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Chapter 23
Transition Elements and Their Coordination Compounds
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The Transition Elements and Their Coordination Compounds
23.3 Coordination Compounds
23.4 Theoretical Basis for the Bonding and Properties of Complexes
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Figure 23.1 The transition elements (d block) and inner transition elements (f block) in the periodic table.
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Properties of the Transition Metals
All transition metals are metals, whereas main-group elements in each period change from metal to nonmetal.
Many transition metal compounds are colored and paramagnetic, whereas most main-group ionic compounds are colorless and diamagnetic.
The properties of transition metal compounds are related to the electron configuration of the metal ion.
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Figure 23.2 The Period 4 transition metals.
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Figure 23.5 Aqueous oxoanions of transition elements.
Mn2+ MnO42− MnO4
−+2 +6 +7
The highest oxidation state for Mn equals its group number.
VO43− Cr2O7
2− MnO4−
+5 +6 +7
Transition metal ions are often highly colored.
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Color and Magnetic Behavior
Most main-group ionic compounds are colorless and diamagnetic because the metal ion has no unpaired electrons.
Many transition metal ionic compounds are highly colored and paramagnetic because the metal ion has one or more unpaired electrons.
Transition metal ions with a d0 or d10 configuration are also colorless and diamagnetic.
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Figure 23.6 Colors of representative compounds of the Period 4 transition metals.
titanium(IV) oxide sodium chromatepotassium
ferricyanidenickel(II) nitrate
hexahydratezinc sulfate
heptahydrate
scandium oxide vanadyl sulfate dihydrate
manganese(II) chloride
tetrahydrate
cobalt(II) chloride
hexahydrate
copper(II) sulfate pentahydrate
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Lanthanides and Actinides
The lanthanides are also called the rare earth elements.
The atomic properties of the lanthanides vary little across the period, and their chemical properties are also very similar.
Most lanthanides have the ground-state electron configuation [Xe]6s24fx5d0.
All actinides are radioactive, and have very similar physical and chemical properties.
The +3 oxidation state is common for both lanthanides and actinides.
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Coordination Compounds
A coordination compound contains at least one complex ion, which consists of a central metal cation bonded to molecules and/or anions called ligands.
The complex ion is associated with counter ions of opposite charge.
The complex ion [Cr(NH3)6]3+ has a central Cr3+ ion bonded to six NH3 ligands. The complex ion behaves like a polyatomic ion in solution.
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Ammine Complexes
• The ammine complexes contain NH3 molecules bonded to metal ions by coordinate covalent bonds as in [Cu(NH3)4]2+.
• Common metal ions that form soluble ammine complexes with an excess of aqueous NH3 include Co2+, Co3+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+, and Hg2+.
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Terminology
• Ligand -- a Lewis base that coordinates to a central metal atom or ion.
• Center of Coordination -- the electron pair acceptor(Lewis acid) -- usually a metal atom or ion
• Donor atom -- the specific atom of a ligand that donates a lone pair of electrons to form a coordinate covalent bond
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Terminology
• Unidentate ligand -- a ligand that can bind through only one atom(has only one donor atom)
• Polydentate ligand -- a ligand that can bind through more than one donor atom at a time-2=bidentate, 3=tridentate, etc.
• Coordination number -- the number of donor atoms coordinated to the center of coordination
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Terminology
• Coordination sphere -- includes the metal atom or ion and the ligands coordinated to it--does not include uncoordinated counter ions--usually enclosed in brackets
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Coordination Number
The coordination number is the number of ligand atoms bonded directly to the central metal ion.
Coordination number is specific for a given metal ion in a particular oxidation state and compound.
The most common coordination number in complex ions is 6, but 2 and 4 are often seen.
- [Cr(NH3)6]3+ has a coordination number of 6.
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Figure 23.7 Components of a coordination compound.
[Co(NH3)6]Cl3 dissolves in water. The six ligands remain bound to the complex ion.
[Pt(NH3)4]Br2 has four NH3 ligands and two Br- counter ions.
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Table 23.5 Coordination Numbers and Shapes of Some Complex Ions
Coordination Number Shape Examples
2 Linear [CuCl2]-, [Ag(NH3)2]+, [AuCl2]-
4 Square planar [Ni(CN)4]2-, [PdCl4]2-,
[Pt(NH3)4]2+, [Cu(NH3)4]2+
4 Tetrahedral [Cu(CN)4]3-, [Zn(NH3)4]2+,
[CdCl4]2-. [MnCl4]2-
6 Octahedral [Ti(H2O)6]3+, [V(CN)6]4-,
[Cr(NH3)4Cl2]+, [Mn((H2O6]2+,
[FeCl6]3-, [Co(en)3]3+The geometry of a given complex ion depends both on the coordination number and the metal ion.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Ligands
The ligands of a complex ion are molecules or anions with one or more donor atoms.
Each donor atom donates a lone pair of electrons to the metal ion to form a covalent bond.
Ligands are classified in terms of their number of donor atoms, or “teeth”:- Monodentate ligands bond through a single donor atom.- Bidentate ligands have two donor atoms, each of which bonds to
the metal ion.- Polydentate ligands have more than two donor atoms.
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Table 23.6 Some Common Ligands in Coordination Compounds
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Chelates
Bidentate and polydentate ligands give rise to rings in the complex ion.
A complex ion containing this type of structure is called a chelate because the ligand seems to grab the metal ion like claws.
EDTA has six donor atoms and forms very stable complexes with metal ions.
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Terminology
• For the compound K3[Co(CN)6]– coordination sphere = – center of coordination = – ligands = – donor atoms = – coordination number = – uncoordinated counter ions =
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Formulas of Coordination Compounds
• A coordination compound may consist of– a complex cation with simple anionic counterions,
– a complex anion with simple cationic counterions, or
– a complex cation with complex anion as counterion.
• When writing the formula for a coordination compound– the cation is written before the anion,
– the charge of the cation(s) is/are balanced by the charge of the anion(s), and
– neutral ligands are written before anionic ligands, and the formula of the whole complex ion is placed in square brackets.
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Determining the Charge of the Metal Ion
The charge of the cation(s) is/are balanced by the charge of the anion(s).
K2[Co(NH3)2Cl4] contains a complex anion.
The charge of the anion is balanced by the two K+ counter ions, so the anion must be [Co(NH3)2Cl4]2-.
There are two neutral NH3 ligands and four Cl- ligands. To have an overall charge of 2-, the metal ion must have a charge of 2+.
Charge of complex ion = charge of metal ion + total charge of ligands 2- = charge of metal ion + [(2 x 0) + (4 x -1)]Charge of metal ion = (-2) – (-4) = +2 or 2+
The metal ion in this complex anion is Co2+.
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[Co(NH3)4Cl2]Cl contains a complex cation.
The charge of the cation is balanced by the Cl- counter ion, so the cation must be [Co(NH3)4Cl2]+.
There are four neutral NH3 ligands and two Cl- ligands. To have an overall charge of 1+, the metal ion must have a charge of 3+.
Charge of complex ion = charge of metal ion + total charge of ligands 1+ = charge of metal ion + [(4 x 0) + (2 x 1-)]Charge of metal ion = (+1) – (2-) = +3 or 3+
The metal ion in this complex cation is Co3+.
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Naming Coordination Compounds
• The cation is named before the anion.• Within the complex ion, the ligands are named in alphabetical order
before the metal ion.– Anionic ligands drop the –ide and add –o after the root name.
• A numerical prefix is used to indicate the number of ligands of a particular type.– Prefixes do not affect the alphabetical order of ligand names.– Ligands that include a numerical prefix in the name use the
prefixes bis (2), tris (3), or tetrakis (4) to indicate their number.• A Roman numeral is used to indicate the oxidation state for a metal
that can have more than one state.• If the complex ion is an anion, we drop the ending of the metal
name and add –ate.
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Typical Simple Ligands
• Ion/Molecule Name As Ligand • NH3
• CO• Cl-
• CN-
• F-
• OH-
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Typical Simple Ligands
• Ion/Molecule Name As Ligand• NO• NO2
-
• ONO-
• PH3
• OH2
• NH2CH2CH2NH2
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Typical Simple Ligands
Ion/Molecule Name As Ligand CO3
-2
C2O4-2
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Table 23.7 Names of Some Neutral and Anionic Ligands
Neutral Anionic
Name Formula Name Formula
Aqua H2O Fluoro F-
Ammine NH3 Chloro Cl-
Carbonyl CO Bromo Br-
Nitrosyl NO Iodo I-
Hydroxo OH-
Cyano CN-
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Table 23.8 Names of Some Metal Ions in Complex Anions
Metal Name in Anion
Iron Ferrate
Copper Cuprate
Lead Plumbate
Silver Argentate
Gold Aurate
Tin Stannate
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Sample Problem 23.3
PLAN: We use the rules for writing formulas and names of coordination compounds.
Writing Names and Formulas of Coordination Compounds
PROBLEM:
(a) What is the systematic name of Na3[AlF6]?(b) What is the sytematic name of [Co(en)2Cl2]NO3?(c) What is the formula of tetraamminebromochloroplatinum(IV) chloride?(d) What is the formula of hexaamminecobalt(III) tetrachloroferrate(III)?
SOLUTION:
(a) The complex ion is [AlF6]3-. There are six (hexa-) F- ions (fluoro) as ligands. The complex ion is an anion, so the ending of the metal name must be changed to –ate. Since Al has only one oxidation state, no Roman numerals are used.
sodium hexafluoroaluminate
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(b) There are two ligands, Cl- (chloro) and en (ethylenediamine). The ethylenediamine ligand already has a numerical prefix in its name, so we indicate the two en ligands by the prefix bis instead of di.
Sample Problem 23.3
The complex ion is a cation, so the metal name is unchanged, but we need to specify the oxidation state of Co. The counter ion is NO3
-, so the complex ion is [Co(en)2Cl2]+.
Charge of complex ion = charge of metal ion + total charge of ligands 1+ = charge of metal ion + [(2 x 0) + (2 x 1-)]Charge of metal ion = (+1) – (-2) = +3 or 3+
The ligands must be named in alphabetical order:
dichlorobis(ethylenediamine)cobalt(III) nitrate
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Sample Problem 23.3
(c) The central metal ion is written first, followed by the neutral ligands and then (in alphabetical order) by the negative ligands.
Charge of complex ion = charge of metal ion + total charge of ligands = (4+) + [(4 x 0) + (1 x 1-) + (1 X 1-)] = +4 + (-2) = +2 or 2+
We will therefore need two Cl- counter ions to balance the charge on the complex ion.
[Pt(NH3)4BrCl]Cl2
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Sample Problem 23.3
(d) This compound consists of two different complex ions. In the cation, there are six NH3 ligands and the metal ion is Co3+, so the cation is [Co(NH3)6]3+.
The anion has four Cl- ligands and the central metal ion is Fe3+, so the ion is [FeCl4]-.
The charge on the cation must be balanced by the charge on the anion, so we need three anions for every one cation:
[Co(NH3)6][FeCl4]3
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Nomenclature
• A. If the compound is a salt, name the cation first and then the anion.
• B. In naming a complex ion or a neutral complex, name the ligands first, in alphabetical order and then the center of coordination.– 1. Anionic ligands end in o– 2. The complex name is one word
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Nomenclature
• C. For more than one ligand of a single type, use the Greek prefixes to indicate number.– 2=di, 3=tri, 4=tetra, 5=penta, 6=hexa, etc.
• D. If the name of the ligand itself contains a Greek prefix, put the ligand name in parentheses and use an alternate prefix.– 2=bis, 3=tris, 4=tetrakis, 5=pentakis, etc.
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Nomenclature
• E. Use a Roman numeral in parentheses immediately following the name of the metal to indicate the metal’s oxidation state.
• F. In naming the metal, use the ending -ate if the metal is in an anionic complex.
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Examples
• Fill in the blanks for each of the following compounds:
• [Co(NH3)4Cl2]NO3
– coordination sphere=– ligands=– center of coordination=– coorrdination number=– name =
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Examples
• Fill in the blanks for each of the following compounds:
• K3[FeCl6]– coordination sphere=– ligands=– center of coordination=– coordination number=– name =
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Examples
• Fill in the blanks for each of the following compounds:
• [Co(en)2OCO2]Br – coordination sphere=– ligands=– center of coordination=– coordination number=– name =
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Examples
• Fill in the blanks for each of the following compounds:
• [Ni(NH3)4(OH2)2](NO3)2 – coordination sphere=– ligands=– center of coordination=– coordination number=– name =
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Examples
• Fill in the blanks for each of the following compounds:
• potassium tetracyanonickelate(II)
• Formula= – coordination sphere=– ligands=– center of coordination=– coordination number=
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Examples
– Fill in the blanks for each of the following compounds:
• tris(ethylenediamine)cobalt(II) nitrate• Formula=
– coordination sphere=– ligands=– center of coordination=– coordination number=
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Stereoisomers of Coordination Compounds
Stereoisomers are compounds that have the same atomic connections but different spatial arrangements of their atoms.
Geometric or cis-trans isomers occur when atoms or groups can either be arranged on the same side or on opposite sides of the compound relative to the central metal ion.
Optical isomers (enantiomers) are non-superimposable mirror images of each other.
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Figure 23.9A Geometric (cis-trans) isomerism.
In the cis isomer, identical ligands are adjacent to each other, while in the trans isomer they are across from each other.
The cis and trans isomers of [Pt(NH3)2Cl2].
The cis isomer (cisplatin) is an antitumor agent while the trans isomer has no antitumor effect.
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Figure 23.9B Geometric (cis-trans) isomerism.
The cis and trans isomers of [Co(NH3)4Cl2]+. Note the placement of the Cl- ligands (green spheres).
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Figure 23.10A Optical isomerism in an octahedral complex ion.
Structure I and its mirror image, structure II, are optical isomers of cis-[Co(en)2Cl2]+.
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Figure 23.10B Optical isomerism in an octahedral complex ion.
The trans isomer of [Co(en)2Cl2]+ does not have optical isomers. Structure I can be superimposed on its mirror image, structure II.
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Bonding in Complex Ions
In terms of valence bond theory, the filled orbital of the ligand overlaps with an empty orbital of the metal ion.
When a complex ion is formed, each ligand donates an electron pair to the metal ion.The ligand acts as a Lewis base, while the metal ion acts as a Lewis acid.
This type of bond is called a coordinate covalent bond since both shared e- originate from one atom in the pair.
The VB model proposes that the geometry of the complex ion depends on the hybridization of the metal ion.
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Figure 23.12 Hybrid orbitals and bonding in the octahedral [Cr(NH3)6]3+ ion.
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Figure 23.13 Hybrid orbitals and bonding in the square planar [Ni(CN)4]2- ion.
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Figure 23.14 Hybrid orbitals and bonding in the tetrahedral [Zn(OH)4]2- ion.
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Figure 23.15 An artist’s wheel.
Each color has a complementary color; the one opposite it on the artist’s wheel.
The color an object exhibits depends on the wavelengths of light that it absorbs.
An object will have a particular color because• it reflects light of that color, or• it absorbs light of the complementary color.
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Table 23.9 Relation Between Absorbed and Observed Colors
Absorbed Color λ (nm) Observed Color λ (nm)
Violet 400 Green-yellow 560
Blue 450 Yellow 600
Blue-green 490 Red 620
Yellow-green 570 Violet 410
Yellow 580 Dark blue 430
Orange 600 Blue 450
Red 650 Green 520
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Crystal Field Theory
Crystal field theory explains color and magnetism in terms of the effect of the ligands on the energies of the d-orbitals of the metal ion.
The bonding of the ligands to the metal ion cause the energies of the metal ion d-orbitals to split.Although the d-orbitals of the unbonded metal ion are equal in energy, they have different shapes, and therefore different interactions with the ligands.
The splitting of the d-orbitals depends on the relative orientation of the ligands.
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Figure 23.16 The five d-orbitals in an octahedral field of ligands.
The ligands approach along the x, y and z axes. Two of the orbitals point directly at the ligands, while the other three point between them.
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Figure 23.17 Splitting of d-orbital energies in an octahedral field of ligands.
The d orbitals split into two groups. The difference in energy between these groups is called the crystal field splitting energy, symbol Δ.
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Figure 23.18 The effect of ligands and splitting energy on orbital occupancy.
Weak field ligands lead to a smaller splitting energy.
Strong field ligands lead to a larger splitting energy.
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Figure 23.19 The color of [Ti(H2O)6]3+.
The hydrated Ti3+ ion is purple.
Green and yellow light are absorbed while other wavelengths are transmitted. This gives a purple color.
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Figure 23.19 The color of [Ti(H2O)6]3+.
When the ion absorbs light, electrons can move from the lower t2g energy level to the higher eg level. The difference in energy between the levels (Δ) determines the wavelengths of light absorbed. The visible color is given by the combination of the wavelengths transmitted.
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The Colors of Transition Metal Complexes
The color of a coordination compound is determined by the Δ of its complex ion.
For a given ligand, the color depends on the oxidation state of the metal ion.
For a given metal ion, the color depends on the ligand.
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Figure 23.20 Effects of oxidation state and ligand on color.
[V(H2O)6]2+ [V(H2O)6]3+[Cr(NH3)6]3+ [Cr(NH3)5Cl ]2+
A change in oxidation state causes a change in color.
Substitution of an NH3 ligand with a Cl- ligand affects the color of the complex ion.
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Figure 23.21 The spectrochemical series.
I- < Cl- < F- < OH- < H2O < SCN- < NH3 < en < NO2- < CN- < CO
WEAKER FIELD STRONGER FIELD
LARGER DSMALLER D
LONGER SHORTER
As Δ increases, shorter wavelengths (higher energies) of light must be absorbed to excite electrons. For reference H2O is considered a weak-field ligand.
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The Magnetic Properties of Transition Metal Complexes
Magnetic properties are determined by the number of unpaired electrons in the d orbitals of the metal ion.
Hund’s rule states that e- occupy orbitals of equal energy one at a time. When all lower energy orbitals are half-filled:
The number of unpaired e- will depend on the relative sizes of Epairing and Δ.
- The next e- can enter a half-filled orbital and pair up by overcoming a repulsive pairing energy, (Epairing).
- The next e- can enter an empty, higher, energy orbital by overcoming Δ.
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Figure 23.22 High-spin and low-spin octahedral complex ions of Mn2+.
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Figure 23.23 Orbital occupancy for high-spin and low-spin octahedral complexes of d4 through d7 metal ions.
high spin: weak-field
ligand
low spin: strong-field
ligand
high spin: weak-field
ligand
low spin: strong-field
ligand
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Table B23.1 Some Transition Metal Trace Elements in Humans
