Chapter 23 Sections 2-4 Transition Metal Complexes © 2012 Pearson Education, Inc.

Chapter 23Sections 2-4

Transition Metal Complexes

© 2012 Pearson Education, Inc.

Complexes

• Commonly, transition metals can have molecules or ions that bond to them.

• These give rise to complex ions or coordination compounds.


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Ligands

The molecules or ions that bind to the central metal are called ligands (from the Latin ligare, meaning “to bind”).


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A. Yes. Ammonia is the Lewis acid.B. Yes. The metal cation is the Lewis acid.C. No. Ammonia is a neutral molecule and cannot

can not accept electron density from a Lewis base.

D. No. The metal cation cannot accept electron density from a Lewis base.

Coordination

One of the properties that has led to the fascination with complexes and transition metals is the wide range of stunning colors found in them.


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Werner Theory

The Swiss chemist Alfred Werner deduced that there was a difference between the oxidation number of a metal and the number of ligands it took on, which he called the coordination number.

Thus, although the first two complexes in the table above each have 3 chlorines, in the first all three serve as anions, while in the second one of the chlorines is tightly bound to the cobalt and the other two are counterions.

Sample Exercise 23.1 Identifying the Coordination Sphere of a Complex

Palladium(II) tends to form complexes with coordination number 4. A compound has the composition PdCl2 · 3 NH3. (a) Write the formula for this compound that best shows the coordination structure. (b) When an aqueous solution of the compound is treated with excess AgNO3(aq), how many moles of AgCl(s) are formed per mole of PdCl2 · 3 NH3?

(a) By analogy to the ammonia complexes of cobalt(III) shown in Figure 23.7, we predict that the three NH3 are ligands attached to the Pd(II) ion. The fourth ligand around Pd(II) is one chloride ion. The second chloride ion is not a ligand; it serves only as a counterion (a noncoordinating ion that balances charge) in the compound. We conclude that the formula showing the structure best is [Pd(NH3)3Cl]Cl.

(b) Because only the non-ligand can react, we expect to produce 1 mol of AgCl(s) per mole of complex. The balanced equation is

[Pd(NH3)3Cl]Cl(aq) + AgNO3(aq) → [Pd(NH3)3Cl]NO3(aq) + AgCl(s)

This is a metathesis reaction (Section 4.2) in which one of the cations is the [Pd(NH3)3Cl]+ complex ion.

Practice ExercisePredict the number of ions produced per formula unit in an aqueous solution of CoCl2 · 6 H2O.

Answer: three: [Co(H2O)6]2+ and two Cl–

The Metal–Ligand Bond

• The reaction between a metal and a ligand is a reaction between a Lewis acid (the metal) and Lewis base (the ligand).

• The new complex has distinct physical and chemical properties.


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A. [Fe(H2O)4]3+(aq) + SCN–(aq) [Fe(H2O)3NCS]2+(aq) + H2O(l)

B. [Fe(H2O)4]3+(aq) + S2+(aq) + CN–(aq) [Fe(H2O)5NCS]2+(aq) + H2O(l)

C. Fe3+(aq) + 3SCN–(aq) FeCN3 (aq) + 3S–(aq)

D. [Fe(H2O)6]3+ (aq) + SCN–(aq) [Fe(H2O)5NCS]2+(aq) + H2O(l)

Coordination Numbers• The number of atoms directly bonded to the

metal atom in a complex is the coordination number.


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Compound Coordination number

[Ag(NH3)2]+ 2

[Co(NH3)6]Cl3 6

[Co(NH3)5Cl]Cl2 6

trans -[Co(NH3)4Cl2]Cl 6

cis -[Co(NH3)5Cl2]Cl 6

Sample Exercise 23.2 Determining the Oxidation Number of a Metal in a Complex

What is the oxidation number of the metal in [Rh(NH3)5Cl](NO3)2?

Solve The NO3 group is the nitrate anion, which has a 1– charge. The NH3 ligands carry zero charge, and the Cl is a coordinated chloride ion, which has a 1– charge. The sum of all the charges must be zero:

x + 5(0) + (–1) + 2(–1) = 0

[Rh(NH3)5Cl](NO3)2

The oxidation number of rhodium, x, must therefore be +3.

Practice ExerciseWhat is the charge of the complex formed by a platinum(II) metal ion surrounded by two ammonia molecules and two bromide ions?

Answer: zero

Sample Exercise 23.3 Determining the Formula of a Complex Ion

A complex ion contains a chromium(III) bound to four water molecules and to two chloride ions.What is the formula and charge of this ion?

+3 + 4(0) + 2(–1) = +1

Cr(H2O)4Cl2The charge on the ion is 1+, [Cr(H2O)4Cl2]+.

Practice ExerciseWrite the formula for the complex described in the Practice Exercise accompanying Sample Exercise 23.2.Answer: [Pt(NH3)2Br2]

Coordination Numbers

• The coordination number of a metal depends upon the size of the metal and the size of the ligands.

• While iron(III) can bind to 6 fluorides, it can only accommodate 4 of the larger chlorides.


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NH3—Zn—NH3 NH3—Pt—NH3

A. 109.5° 90°

B. 120° 109.5°C. 180° 120°D. 120° 90°

Geometries: C.N. = 4 C.N. = 6A. Octahedral TetrahedralB. Tetrahedral OctahedralC. Octahedral Square planarD. Square planar Tetrahedral

A. Two for bothB. Three for bothC. Four for bothD. Six for both

A. Bidentate ligandB. Monodentate ligand

Common Ligands

The table above contains some ligands commonly found in complexes.


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Monodentate ligands coordinate to one site on the metal, bidentate to two, and so forth.

Common Ligands

Bi and polydentate ligands are also called chelating agents.


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Chelates in Biological Systems

• There are many transition metals that are vital to human life.

• Several of these are bound to chelating agents.


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• For instance, the iron in hemoglobin carries O2 and CO2 through the blood.


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A. On the protein groupB. On the heme groupC. On the NH groupD. Where O2 binds


• For instance, the iron in hemoglobin carries O2 and CO2 through the blood.

• Carbon monoxide and cyanide are poisonous because they will bind more tightly to the iron than will oxygen.


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A. 420 nmB. 500 nmC. 610 nmD. 650 nm

A. It has a planar structure that enables the C-H bonds to absorb light in the visible region.

B. The C-C bonds absorb light in the ultraviolet region.

C. It has conjugated double bonds that absorb light in the visible region.

D. It has C-H bonds that absorb light in the infrared region.

Nomenclature in Coordination Chemistry

1. In naming complexes that are salts, the name of the cation is given before the name of the anion.


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2. In naming complex ions or molecules, the ligands are named before the metal. Ligands are listed in alphabetical order, regardless of their charges.


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3. The names of anionic ligands end in the letter o, but electrically neutral ligands ordinarily bear the name of the molecules.


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4. Greek prefixes (di-, tri-, tetra-, etc.) are used to indicate the number of each kind of ligand when more than one is present. If the ligand contains a Greek prefix or is polydentate, the prefixes bis-, tris-, tetrakis-, etc. are used and the ligand name is placed in parentheses.


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5. If the complex is an anion, its name ends in -ate.

6. The oxidation number of the metal is given in parentheses in Roman numerals following the name of the metal.


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Sample Exercise 23.4 Naming Coordination Compounds

Name the compounds (a) [Cr(H2O)4Cl2]Cl, (b) K4[Ni(CN)4].

+3 + 4(0) + 2(–1) + (–1) = 0

[Cr(H2O)4Cl2]Cl

tetraaquadichlorochromium(III) chloride

4(+1) + 0 + 4(–1) = 0

K4[Ni(CN)4]

potassium tetracyanonickelate(0)

Solve(a) The ligands are four water molecules—tetraaqua—and two chloride ions—dichloro. By assigning all the oxidation numbers we know for this molecule, we see that the oxidation number of Cr is +3:

Thus, we have chromium(III). Finally, the anion is chloride. The name of the compound is

(b) The complex has four cyanide ion ligands, CN–, which means tetracyano, and the oxidation state of the nickel is zero:

Because the complex is an anion, the metal is indicated as nickelate(0). Putting these parts together and naming the cation first, we have

Practice ExerciseName the compounds (a) [Mo(NH3)3Br3]NO3, (b) (NH4)2[CuBr4]. (c) Write the formula for sodium diaquabis(oxalato)ruthenate(III).Answer: (a) triamminetribromomolybdenum(IV) nitrate, (b) ammonium tetrabromocuprate(II) (c) Na[Ru(H2O)2(C2O4)2]

Chapter 23 Sections 2-4 Transition Metal Complexes © 2012 Pearson Education, Inc.

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Transcript of Chapter 23 Sections 2-4 Transition Metal Complexes © 2012 Pearson Education, Inc.