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AP TOPIC 10: Transition Metal Basics

• Introduction

TOPIC 10 of the AP course is really what I like to describe as a “bonus topic”. Many AP courses will not touch upon transition metal chemistry at all, and it is certainly possible to do extremely well on the AP exam without devoting any time to it. So why bother? Well, there are two reasons.

1. Some information on this topic may well help in the other, more important areas (for example, TOPIC 10 is related to REDOX chemistry). A little extra knowledge cannot hurt, and this constitutes some solid chemistry that can be viewed as reading around the subject, increasing chemical general knowledge and broadening chemical horizons.

2. Study of this topic may help in the equation writing section of the exam. Several equations involving this topic have been examined in the past and students often ignored them (before 2007 you could choose to ignore them since you only had to choose five equations from a total of eight but since 2007 there is no longer any choice in which equations to write). Below is a survey of the appearance of transition metal chemistry in the equation writing section of the exam since 1981.

2+ 2010B b (XS conc. NH3 + Ni ) 2008 a (XS conc. OH- + Al3+) 2006B d (conc. HCl + Ni2+) 2006 b (conc. HCl + AgCl) 2+ 2005B c (XS NH3 + Zn ) 2005 b (XS conc. OH- + Al3+) 2004B g (XS F- + Al3+) 2004 f (XS conc. OH- + Ni2+) 2003B g (XS OH- + Ni2+) + 2003 e (XS NH3 + Ag ) 2+ 2002B d (conc. NH3 + Cu ) 2002 d (Cl- + Co2+) 2001 b (SCN- + Fe3+) 2+ 2000 c (XS NH3 + Ni ) 1998 f (NH3 complex decomposition) 1997 a (XS conc. OH- + Al3+) 1994 a (CN- + Ag+) + 1993 g (XS NH3 + Ag ) 1992 g (NH3 complex decomposition) 2+ 1991 g (conc. NH3 + Zn ) 1989 f (SCN- + Fe3+) 2+ 1988 h (conc. NH3 + Cu ) 1987 d (XS conc. OH- + Al3+) 1985 d (XS conc. OH- + Zn2+) 2+ 1982 f (conc. NH3 + Zn ) 1981 g (SCN- + Fe3+)

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• What are transition metals?

A transition metal may be defined as one that forms stable ions that have incomplete d orbitals. (N.B. although zinc is in the d block it is often not regarded as a transition metal since in its only common oxidation state (+2), it has a complete d orbital (d10)). Transition metals tend to have several different oxidation states (charges) since the s and d electrons have similar energies meaning that there are no “big jumps” in successive ionization energies and therefore greater numbers of electrons can be removed.

It is vitally important to remember that the outer s electrons are the first to be removed before the d electrons when these elements form ions.

• Two characteristic properties of transition metals

1. Transition metals form a variety of colored ions.

Some common colored ions of the third period d block elements are listed below.

+1 +2 +3 +4 +5 +6 +7

Sc colorless

Ti violet colorless

V violet Green blue yellow

yellow orange blue green 2- 2- Cr (CrO4 ) (Cr2O7 )

Mn pale pink brown dark green purple

Fe pale green yellow/brown

Co pink orange/yellow

Ni green

Cu colorless blue

Zn colorless

Color is caused by the movement of electrons within the d orbitals. If an ion has either a completely filled or a completely empty d orbital then there is no scope for electron movement and the ion will be colorless.

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2. Transition metals form complex ions.

Since transition metal ions tend to attract electron pair donors (Lewis bases) such as water molecules. For example, Co2+ in aqueous solution exists attached to six water molecules.

2+ [Co(H2O)6] is an example of a complex ion and the electron pair donors (water molecules in this case) are called ligands. The number of ligands surrounding the central metal ion is known as the co-ordination number. The bonding between the ligands and the central metal ion is dative. The central metal ions will attract electrons in the water molecules which leads to the O-H bonds being weakened and to the release of H+ ions into solution. When this happens the complexes are acidic. Higher charge density cations lead to greater attraction, greater weakening of the O-H bonds and therefore greater acidity.

Naming complexes

(i) Neutral complexes have single word names (ii) As in ‘normal’ nomenclature, in ionic compounds the cation is named before the anion (iii) Ligands are named before the central metal. Some common ligands are listed below

Negative ligands end in ‘o’, for example.

Ligand Name Cl- Chloro OH- Hydroxo - NO2 Nitrito CN- Cyano 2- SO4 Sulfato

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Neutral ligands usually keep their normal names, except the following;

Ligand Name

H2O aquo or aqua NH3 Ammine CO Carbonyl NO Nitrosyl

The number of ligands present is shown using prefixes; mono =1, di =2, tri =3, tetra = 4, penta = 5, hexa = 6

(N.B. if the ligand name begins with di/tri and there is more than one present use bis/tris/tetrakis)

(iv) If the complex has a negative charge the ‘ate’ is added to the name of the metal, some metals in negative complexes may use their Latin names for example.

Metal Name Iron Ferrate Copper Cuprate Silver Argenate

(v) The oxidation state of the central metal ion is given in Roman numerals in brackets after its name. Examples;

CATION COMPLEX: [Fe(H2O)6] Br2 hexaquoiron (II) bromide

NEUTRAL COMPLEX: [Co(H2O)3(OH)3] triaquotrihydroxocobalt (III)

ANION COMPLEX: K2 [CoCl4] potassium tetrachlorocobaltate (II)

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Task 10a

1. Name the following compounds;

(i) [Cu(H2O)6]SO3

(ii) [NiCl3(NH3)3]

(iii) [Co(NH3)6]I3

(iv) Na3[Fe(CN)6]

(v) [Cu(NH3)2(H2O)4]F

2. Write the formulae of the following compounds;

(i) ammonium hexachlorotitanate (IV) (ii) triamminetriaquachromium (III) chloride (iii) pentaquahydroxoiron (II) chloride (iv) sodium hexanitritocobaltate (III) (v) chloropentaamminecobalt (III) bromide

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• Two reactions of complex ions

1. Ligand exchange reactions of transition metal ions (AND Al3+)

In these reactions it is acceptable to leave out the water ligands completely. For example, when a solution of nickel(II) ions reacts with excess or concentrated ammonia the reaction is really,

2+ 2+ [Ni(H2O)6] (aq) + 6NH3(aq) è [Ni(NH3)6] (aq) + 6H2O

But, simplier (and still acceptable) is,

2+ 2+ Ni + 6NH3 è [Ni(NH3)6]

The next question that may concern you is; “How do I know the number of ligands that are being introduced in the product?” Unfortunately there are no hard and fast rules, but it’s worth learning a few that have come up in the past.

Metal ion Ligand Number of ligands Ag+ Cl- 2 Ag+ CN- 2 + Ag NH3 1 or 2 Al3+ OH- 3 or 4 Al3+ F- 6 Co2+ Cl- 4 2+ Cu NH3 4 Fe3+ SCN- 1 2+ Ni NH3 Any between 1 and 6 Ni2+ Cl- 4 Ni2+ OH- 4 2+ Zn NH3 4 Zn2+ OH- 4

Two further useful pieces of information are;

(i) Often the number of ligands is twice the cation charge and,

(ii) It often doesn’t matter, as credit is often given for any number of ligands (up to a maximum of six) as long as the overall charge is consistent with the number of ligands.

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2. Decomposition of a complex by acid base neutralization

Complexes containing NH3 can be broken down via acid base decomposition by the addition of an acid. For example, hydrochloric acid is added to tetraamminecopper (II) sulfate

2+ + + 2+ [Cu(NH3)4] + 4H è 4NH4 + Cu

+ + The NH3 acts as a base by accepting the H ions to form NH4 and the complex is broken down.

Task 10b

1. Add name or formula to complete the table below.

Name Formula

Hexaquochromium (II) sulfate

[Cu(OH)2(H2O)4]

trichlorotriamminechromium (III)

2. Write the electronic configuration of these species.

(i) Fe2+ (ii) Co2+ (iii) Mn7+

3. What are the oxidation states of Manganese in these species?

2- (i) [MnCl5] 3+ (ii) [Mn(H2O)6] - (iii) [MnF6] 3- (iv) [Mn(CN)6]

2- 4. Yellow chromate (VI) ions (CrO4 ) can be converted to orange dichromate (VI) ions 2- + - (Cr2O7 ) via a reaction with acid (H ). On addition of base (OH ) the process can be reversed. Write a reversible equation to show the changes that are occurring in this reaction.

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