Transition Metals _ Coordination Chemistry

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					Transition Metals & Coordination Chemistry

Uses of Transition Metals
    

Iron for steel Copper for wiring and pipes Titanium for paint Silver for photographic paper Platinum for catalysts

Importance of Transition Metals

U.S. imports 60 “strategic and critical” minerals
Cobalt  Manganese  Platinum  Palladium  Chromium


Important for economy and defense

Transition Metals and Living Organisms

Iron – transport & storage of O2 Molybdenum and Iron


Zinc – found in more than 150 biomolecules Copper and Iron – crucial role in respiratory cycle Cobalt – found in vitamin B12

Catalysts in nitrogen fixation

Transition Metals: A survey
Representative elements
Chemistry changes across a period  Similarities occur within a group


Transition Metals


Similarities occur within a period as well as within a group


Due to last electrons being “d” (or “f”) orbital electrons

Transition Metals: A Survey
“d” and “f” electrons cannot easily participate in bonding, so chemistry of transition elements are not affected by increased number of these electrons

Transition Metal Behavior
Typical metals
Metallic Luster  Relatively high electrical conductivity  Relatively high thermal conductivity


Silver is the best conductor of heat and electricity  Copper is second best


Properties of Transition Metals
Transition metals vary considerably in some properties


Melting point


W – 3400oC vs. Hg, a liquid at 25oC



Hardness
Iron and Titanium are very hard  Copper, gold, and silver are relatively soft


Properties of Transition Metals
Chemical Reactivity


Reaction with oxygen
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Some form oxides that adhere to the metal, protecting the metal from further corrosion
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Cr, Ni, Co



Some form oxides that scale off, resulting in exposure of the metal to further corrosion
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Fe Au, Ag, Pt, Pd



Some noble metals do not form oxides readily


Properties of Transition Metals
Forming Ionic Compounds


Transition Metals can form more than one oxidation state


Fe+2 and Fe+3

Complex Ions
Formed by the cations  The transition metal ion is surrounded by a certain number of ligands (Lewis bases)


Properties of Transition Metals
In forming ionic compounds


Most compounds are colored


Transition metal ion can absorb visible light The transition metal ion contains unpaired electrons



Most compounds are paramagnetic


Electron Configurations
Energies of the 4s and 3d electrons are very similar Chromium is an exception to the diagonal rule, can be explained in terms of the similar energies of the 4s and 3d electrons  4s __ 3d __ __ __ __ __  Less electron-electron repulsion

Electron Configurations
Transition metal ions
Energy of the 3d orbital in transition metal ions is lower than the energy of the 4s orbital  In other words, in forming a transition metal ion, the electrons are lost from the 4s orbital before the 3d orbitals.  Mn: [Ar]4s23d5 Mn+2: [Ar]3d5


Oxidation States & I.E.

First five transition metals


Maximum possible oxidation state is the result of losing the 4s and the 3d electrons




At the end of the period, +2 is the most common oxidation state.


Cr: [Ar]4s13d5; max. ox. state = +6

Too hard to remove the d electrons as they become lower in energy as the nuclear charge increases

Standard Reduction Potentials
Metals act as reducing agents
 

All the metals except Cu can reduce H+ to H2  Reducing ability decreases going across the period


M  M+n + neMetal with the most positive reducing potential is the best reducing agent  Sc  Sc+3 + 3 eEored = 2.08 V  Ti  Ti+2 + 2eEored = 1.63 V

4d and 5d Transition Series

Radius increases in going from 3d to the 4d metals Radius of the 4d metals is similar to the 5d metals due to the lanthanide

contraction

Lanthanide Contraction
Adding 4f electrons does not add to the size of the atom (as inner electrons) However, nuclear charge is still increasing. Increased nuclear charge offsets the normal increase in size in filling the next higher energy level Chemistry of 4d and 5d elements are very similar

4d and 5d transition metals
Zr and ZrO2 – great resistance to high temperature, used for space vehicle parts exposed to high temperatures of reentry Niobium and Molybdenum – important alloying materials for steel Tantalum – resists attacks by body fluids, used for replacement of bones Platinum group: Ru, Os, Rh, Ir, Pd, Pt


Used as catalysts

Read
Pg. 971 – 977 Look at pictures, note colors

Coordination Compounds Coordination compound


Formed by transition metal ions  Usually colored  Often paramagnetic  Consists of


A complex ion




Counterions (the anions or cations needed to produce a neutral compound)

Made up of the transition metal ion with its attached ligands

Coordination Compounds

[Co(NH3)5Cl]Cl2 Brackets hold the complex ion


The “Cl2” outside the brackets are the 2 Cl- counterions In solution:
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(Co(NH3)5Cl+2

[Co(NH3)5Cl]Cl2 Co(NH3)5Cl+2 + 2 Cl-

Coordination Compounds

Alfred Werner in the 1890’s


Transition metals have two types of valence (combining abilities)
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with oppositely charged ions  Secondary valence – ability to to bind to Lewis bases (ligands) to form complex ions

Primary valence – ability to form ionic bonds

Coordination Compounds

Primary Valence = Oxidation State Secondary Valence = Coordination Number


number of bonds formed between the metal ion and the ligands in the complex ion.

Coordination Number Coordination number
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Varies from two to eight


Depends on the size, charge, and electron configuration of the transition metal

Most common coordination number is 6  Next is 4, then 2  Many metals show more than one coordination number



No way to predict which coordination number

Coordination Compounds

6 ligands – octahedral geometry 4 ligands – square planar or tetrahedral geometry 2 ligands - linear

Ligands

Ligand
Neutral molecule or ion having a lone electron pair that can be used to form a bond with a metal ion  Metal-ligand bond


Interaction between a Lewis acid and a Lewis base  Also known as a coordinate covalent bond
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Ligands

Unidentate (one tooth) ligand
Can only form one bond with the metal ion  H2O, CN-, NH3, NO2-, SCN-, OH-, Cl-, etc
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Bidentate ligand
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Can form two bonds to a metal  Ethylenediamine, aka en, (H2N-CH2- CH2-NH2), oxalate

Ligands

Polydentate ligands (chelating ligands)
EDTA, ethylenediaminetetraacetate  Surrounds the metal  Forms very stable complex ions with most metal ions  Used as a scavenger to remove toxic heavy metals, e.g., lead, from the body  Found in numerous consumer products to tie up trace metal ions


Nomenclature
Cation is named before the anion Ligands are named before the metal ion


Naming ligands  Add an o to the root name of an anion (fluoro, chloro, hydroxo, cyano, etc.)  Neutral ligand, use the name of the molecule except for the following:
    

H2O = aqua NH3 = ammine CH3NH2 = methylamine CO = carbonyl NO = nitro

Nomenclature


Use prefixes to indicate number of simple ligands (mono, di, tri, tetra, penta, hexa) Use bis, tris, tetrakis for complicated ligands that already contain di, tri, etc)

Oxidation state of central metal ion is designated by a Roman numeral in parentheses When more than one type of ligand is present, they are named alphabetically, where prefixes do not affect the order. If the complex ion has a negative charge, add – ate to the name of the metal (eg. ferrate or cuprate)

Nomenclature

[Co(NH3)5Cl]Cl2


Pentaamminechlorocobalt(III) chloride

K3Fe(CN)6
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Potassium hexacyanoferrate(III)
Bis(ethylenediamine)dinitroiron(III)sulfate

[Fe(en)2(NO2)2]2SO4


Nomenclature

Triamminebromoplatinum(II) chloride


[Pt(NH3)3Br]Cl

Potassium hexafluorocobaltate(III)


K3[CoF6]

The Crystal Field Model and Bonding in Complex Ions Crystal field model focuses on the energies of the d orbitals


Color and magnetism of complex ions are due to changes in the energies of the d orbitals caused by the metal-ligand interaction

The Crystal Field Model

Crystal Field Model assumes In the free metal ion, all the d orbitals are degenerate, they have the same energies
Ligands are like negative point charges  Metal-ligand bonding is entirely ionic


The Crystal Field Model

In the complex ion, the d orbitals are split into two sets with two different energies.


Lower energy set
The negative point charge ligands are farthest from the dxz, dyz, and dxy orbitals (the orbitals that point between the ligands)  Electron pair repulsions are minimized


The Crystal Field Model

In the complex ion, the d orbitals are split into two sets with two different energies.


Higher energy set
dz2, dx2-y2 point at the ligands  More electron repulsions


The Crystal Field Model

Splitting of the 3d orbital energies


Results in the color and magnetism of the complex ions

The Crystal Field Model

Strong field case (or low spin case)
Splitting produced by the liqands is very large  Electrons will pair in the lower energy orbitals (the ones pointing between the ligands)  Result – a diamagnetic complex in which all electrons are paired


The Crystal Field Model

Weak Field Case (or high spin case)
Splitting produced by the ligands is very small  Electrons will fill each of the five d orbitals (Hund’s rule) before pairing  Will result in paramagnetism with unpaired electrons


The Crystal Field Model Ligands have different abilities to produce d-orbital splitting
Strong Field ligands -----> Weak Field ligands Large D -------> Small D
CN- > NO2- > en > NH3 > H2O > OH- > F-> Cl- > Br- > I-

D increases as the charge on the metal ion increases


Larger charge on ion pulls the ligands closer, results in greater splitting to minimize repulsions

The Crystal Field Model and Colors

Colors of complex ions
A complex ion will absorb certain wavelengths of light  The color we see is complementary to the color absorbed.



If yellow and green light is absorbed, then red and blue light passes through, so we would see violet.

The Crystal Field Model and Colors
A complex ion will absorb a specific wavelength depending on the D between the d orbitals. Different ligands on the same metal ion will result in different colors because of the different D’s. DE = hc/l…for octahedral complex ions, the l is usually in the visible region

Metallurgy Steps in the process of separating a metal from its ore (metallurgy)
Mining  Pretreatment of the ore  Reduction to the free metal  Purification of the metal (refining)  Alloying


Metallurgy Ores are mixtures containing
Minerals (relatively pure metal compounds)  Gangue (sand, clay, and rock)
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After mining, treat ores to remove the gangue and concentrate the mineral
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Pulverize and process ore

Metallurgy Flotation process


Allows minerals to float to the surface of a water-oil-detergent mixture

Alter the mineral to prepare it for the reduction step
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Carbonates and hydroxides are heated
CaCO3  CaO + CO2  Mg(OH)2  MgO + H2O


Metallurgy


Sulfides are converted to oxides by heating in air at temperatures below their melting points (roasting)
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2 ZnS + 3 O2  2 ZnO + 2 SO2

Metallurgy

Smelting – method used to reduce the metal ion to the free metal
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Depends on the affinity of the metal ion for electrons
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Good oxidizing agents produce the free metal in the roasting process
HgS + O2  Hg(l) + SO2

Metallurgy


More active metals  Use coke (impure carbon), carbon monoxide, or hydrogen, as a strong reducing agent  Fe2O3 + 3 CO  2 Fe + 3 CO2  WO3 + 3 H2  W(l) + 3 H2O  ZnO + C  Zn(l) + CO

Metallurgy

Most active metals (Al and alkali metals)
 must

be reduced electrolytically from the molten salts.

Iron ores


Metallurgy of Iron

Concentrate iron in iron ores


pyrite (FeS2), siderite (FeCO3), hematite(Fe2O3, magnetite (Fe3O4)

Separate Fe3O4 mineral from the gangue by magnets  Iron ores that are not magnetic are converted to Fe3O4, or are concentrated using the flotation process

Metallurgy of Iron

Reduction process
Occurs in the blast furnace  Uses coke which is converted to CO in the blast furnace  Reduction occurs in steps:


3Fe2O3 + CO  2 Fe3O4 + CO2  Fe3O4 + CO  3 FeO + CO2  FeO + CO  Fe + CO2


Metallurgy of Iron

The iron can reduce the CO2:


So the excess CO2 needs to be removed by adding excess coke:


Fe + CO2  FeO + CO

CO2 + C  2 CO


				
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