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TRANSITION METAL CHEMISTRY

By Betty Miller,2014-11-17 00:16
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TRANSITION METAL CHEMISTRYMetal,metal,METAL

    TRANSITION METAL CHEMISTRY

    The term ‘transition elements’ originally was coined to denote elements in the middle of the

    ’ on the left (Groups 1A and periodic table that provided a ‘transition’ between the ‘base formers

    2A) and the ‘acid formers’ on the right (Groups 5A through 7A).

    Recall that metal oxides typically form basic aqueous solutions whereas non metal oxides

    typically form acidic aqueous solutions.

    The term ‘transition elements’ actually applies to both ‘d’ and ‘f’ transition elements, all of which

    are metals, but commonly is used to denote only ‘d’-transition metals. The f-transition metals are usually called ‘rare earths’ or ‘inner transition metals’.

    Transition metals are located between Groups 2A and 3A. Strictly speaking, d-transition metals

    must have partially filled d-orbitals. Zn, Cd, and Hg (Group 2B) have completely filled d-orbitals

    and are actually ‘post transition metals’ but they are often referred to as transition metals because of similar properties.

    Cu, Ag, and Au (Group 1B) and Pb (Group 8B) also have filled d-orbitals however their cations +(except Ag) have partially filled d-orbitals.

General Properties of Transition Metals:

    ; All are metals

    ; Most are harder, more brittle, have higher mp and bp and Hvap than non transition

    metals.

    ; Their ions and compounds are often colored.

    ; They form many coordination complexes

    ; Most have multiple oxidation states

    ; Many of the metals and their compounds are good catalysts.

Electron Configuration:

    Period 4 Period 5 Period 6

    12 12 12 Sc [Ar] 3d 4sY [Kr] 4d 5sLa [Xe] 5d 6s213957

    22 22 1422 Ti [Ar] 3d 4sZr [Kr] 4d 5sHf [Xe] 4f 5d 6s407222

    32 41 1432 V [Ar] 3d 4sNb [Kr] 4d 5sTa [Xe] 4f 5d 6s234173

    51 51 1442 Cr [Ar] 3d 4sMo [Kr] 4d 5sW [Xe] 4f 5d 6s244274

    52 52 1452 Mn [Ar] 3d 4sTc [Kr] 4d 5sRe [Xe] 4f 5d 6s254375

    62 71 1462 Fe [Ar] 3d 4sRu [Kr] 4d 5sOs [Xe] 4f 5d 6s264476

    72 81 1472 Co [Ar] 3d 4sRh [Kr] 4d 5sIr [Xe] 4f 5d 6s274577

    82 10 0 1491 Ni [Ar] 3d 4sPd [Kr] 4d5sPt [Xe] 4f 5d 6s284678

    101 10 1 14101 Cu [Ar] 3d 4sAg [Kr] 4d5sAu [Xe] 4f 5d 6s294779

    102 10 2 14102 Zn [Ar] 3d 4sCd [Kr] 4d5sHg [Xe] 4f 5d 6s304880

bb/CH306/Transitionmetals.doc 1

    Energies of 3d and 4s orbitals are nearly equal. Generally the 3d orbitals fill after the 4s-orbital is filled. Cr and Cu are exceptions to the filling order in the first transition series. They have only one electron in their 4s orbital and one ‘extra’ electron in their 3d orbital. This gives a more favorable configuration in accordance with Hund’s rule.

    The 4d and 5s, and 5d and 6s orbitals are even closer in energy than the 3d and 4s orbtitals ndrdmaking electron configurations difficult to predict for the 2 and 3 transition series.

    The properties of the transition metals can be correlated roughly with either the total number of d-electrons or the number of unpaired electrons.

Melting Points:

    Alkali metals melt below 200 ?C. Several post transition metals are low melting (Ga = 30 ?C). Transition metals typically melt above 1000 ?C. Tungsten is highest melting, above 3400 ?C. Of all the elements, carbon has the highest melting point, i.e., ca. 3800 ?C. Hg is the exception to the rule. Hg, a liquid at room temperature, has the lowest mp of all metals (-39?C).

    METALS mp vs. Group Number

    4000

    Period 6W3500RePeriod 5TaOs3000

    MoNbIrRu2500TcHfRh2000PtZrCrVFeScPdTi1500Ymp ()CoAuNiMnBaCu1000GeAgSrLaLaCaZn500PbPeriod 4InCsSnCdGaRb0HgK

    -5001A 2A 3B 4B 5B 6B 7B 8B 8B 8B 1B 2B 3A 4A

    Group Number

    bb/CH306/Transitionmetals.doc 2

Atomic Size:

    Atomic size decreases left to right across each transition metal series as net core charge increases but size then increases at the far right (Group 1B and 2B) as valence electrons repulsion increases as d-orbitals fill.

    Atomic size, as expected, increases down all Groups in the periodic table as additional shells of stndelectrons are added with increasing atomic mass. This holds for the 1 and 2 row transition rdndmetals but the 3 row transition metals are the same size as the 2 row. This unexpected

    ‘shrinkage’ is called the ‘lanthanide contraction’.

    rdThe lanthanide contraction of period 6 (the 3 transition series) occurs because these elements contain an ‘additional’ 14 electrons in the 4f orbitals. The effective nuclear charge increases by stndrd15 between La and Hf (the 1 and 2 elements in the 3 transition series).

    Results of the Lanthanide Contraction:

    rd1. 3 transition series metals have the highest densities of all elements, for example:

     (g/mL) (g/mL)

    Os 22.6 W 19.3

    Ir 22.4 Au 19.3

    Pt 21.5 Hg 13.5

    Re 20.8 Pb 11.3

    Fe = 7.9 g/mL

    rd2. 3 transition series metals have unusually high ionization energies, i.e., are rather

    unreactive. rd3. 3 transition series metals have much higher oxidation resistance. The platinum metals,

    i.e., Ru, Os, Rh, Ir, Pd and Pt (plus Au) do not form simple cations or even oxyanions.

    Au and Pt are especially useful in low voltage circuits where trace oxidation is

    problematic.

    METALS Density vs. Group Number

    25OsIrPtRe

    Au20WPeriod 6Period 5Ta

    Hg15RhHfRuPdLaTcAgPbMoCdNb10InCoZrNiCuSnLaFeDensity (g/mL)MnZnCrGeVBa5GaYCsTiPeriod 4SrScRbCaK0

    1A 2A 3B 4B 5B 6B 7B 8B 8B 8B 1B 2B 3A 4A

    Group Number

    bb/CH306/Transitionmetals.doc 3

Acidity and Basicity of Transition Metals:

    Recall that nonmetal hydroxides and oxyacids increase in acidity as the oxidation number of the central nonmetal increases. For example HClO (Cl = +7) is more acidic than HClO (Cl = +5). 43

    Similarly, the acidity of the hydroxides and oxyacids of transition metals increases as the oxidation number of the central transition metal increases.

    Oxid. # +1 +2 +3 +4 +5 +6 +7

     MnO MnOMnO MnOMnO23 2 3 27

     ( HO ( HO ( HO ( HO ( HO Mn 22222

     Mn(OH) Mn(OH) MnO(OH) MnO(OH) (OH) MnO232 223(HMnO) (HMnO) (HMnO) 23244manganese(II) manganese(III) manganous manganic permanganic hydroxide hydroxide acidacid acid

    Basic Amphoteric Acidic

    Note: The oxyacids of Cl are all acidic. They are shown here only to see an analogous oxidation pattern. Oxid. # +1 +2 +3 +4 +5 +6 +7

    Cl HClO HClO HClO HClO234[ClOH] [ClO(OH)] [ClO(OH)] [ClO(OH)] 23

    chlorous acid hypochlorous chloric perchloric

    acid acid acid

     CrO CrO CrO23 3

     Cr ( HO ( HO ( HO 222

     Cr(OH) Cr(OH) CrO(OH) 2322 chromium(III) (HCrO) 24chromium(II) hydroxide(HCrO) 227hydroxide chromic

    acid

General Trends in Acidity:

    ; The more electronegative (EN) the central atom in an oxyacid, the greater the acidity.

    The following sequence of decreasing acidity (from left to right) with decreasing EN

    illustrates. Get the pKa values from the unit on ‘The Atom’ and compare EN values.

     HSO > HSeO > HPO ; HAsO > HGeO 2424343444

    ; For a given central atom, the acid strength increases with the number of oxygens it holds.

     HSO > HSO and HNO > HNO 242332

    Problem 1: Write the formulas of all four oxyacids of bromine. Name them and number them in order of acidity where 1 is most acidic and 4 is least acidic. Explain why this trend in acidity

    occurs. Hint: Consider the oxidation state of bromine and consider the stability of the conjugate base that forms when the acid gives up an acidic hydrogen atom.

    bb/CH306/Transitionmetals.doc 4

Transition Metals as Catalysts:

    Transition metals and their compounds function as effective catalysts in both homogeneous (single phase) and heterogeneous (multiple phase) reactions.

    Unreactive metals such as Pt, Pd, Ni and Au are sometimes used in a finely divided state to provide surfaces upon which heterogeneous reactions occur, e.g., hydrogenation of unsaturated organics.

    Other transition metals act as homogeneous catalysts having d-orbital vacancies that can accept electrons from reactants to form intermediates that subsequently decompose. Some typical reactions catalyzed by transition metals follow.

    1. Haber Process: [FeO, 500 ?C, 400 atm] 23

     N + 3 H 2 NH 22 3

    2. Contact Process: [VO, 400 ?C] 25

     2 SO + O 2 SO 22 3

    3. Hydrogenation: [H /Ni @ 25 ?C, 1 atm] 2

     CH=CH-CHCH CHCHCHCH 2233223

    4. Ostwalt Process: [Pt, 850 ?C]

     4 NH + 5 O 4 NO + 6 HO 322

     2 NO + O 2 NO 22

     NO + HO HNO 223

    5. Catalytic Converters: [Pd or Rh]

     CO + HC’s + O CO + HO 2 22

     NO and NO N + O 222

    6. Linear Polyethylene: [TiCl or MoO] 43

     CH=CH (CHCH) 2222n

    7. Redox Reactions: [Mo(PO)] n4m

     AsF + HO AsF 3225

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Classification into Subgroups:

    The transition metals including Zn, Cd and Hg (Group 2B) are divided into eight Groups designated as B-Group elements. The number designates the maximum oxidation number of the Group members. No simple ion of these elements possesses a charge greater than +3. Elements in corresponding A and B-Groups form many compounds of identical stoichiometry as illustrated by the following examples.

    1A 2A 3A 4A 5A 6A 7A

    -2 NaCl MgBrAl(NO)CClPOClSOClO2 33 4 3 427

    -3 KNOCaCl Ga(OH)PbOPOHSOHClO3 23 2 4227 4

    1B 2B 3B 4B 5B 6B 7B

    -2 CuCl ZnBrSc(NO)TiClVOClCrOMnO2 33 4 3 427

    -3 AgNOCdClY(OH)ZrOVOHCrOHMnO3 2 3 2 4227 4

    Despite similar stoichiometry in compounds, the chemical properties of A-Group and B-Group elements are dissimilar.

    Group 1B and 2B metals have filled d-orbitals, and d- and s-orbitals, respectively (pseudo-noble gas configurations) are unusually stable. In contrast, Groups 1A and 2A metals are very reactive and are never found unreacted in the native state.

    Group 8B consists of three columns of three metals each. Each Group 8B horizontal row is called ‘triad’ and is named after the best known

    metal of the row, i.e., the iron triad, the palladium triad, and the Fe Co Ni platinum triad. Greater horizontal similarities than vertical Ru Rh Pd similarities are found in Group 8B.

    Os Ir Pt The iron triad metals (Fe, Co, and Ni) are the only elements in the

    periodic table that exhibit ‘ferromagnetism’ in the uncombined

    state.

Problem 2: Without looking this up, write the formula of

    a) a common oxide of tungsten

    b) a naturally occuring oxide of osmium

    bb/CH306/Transitionmetals.doc 6

Oxidation States:

    Characteristically, transition metals can exhibit more than one oxidation state. In most cases,

    the maximum oxidation state found in a group is the same as the group number, but this is

    usually not the most common oxidation state. The tables below list common oxidation states.

    st1 Transition Series 3B 4B 5B 6B 7B 8B 1B 2B Sc Ti V Cr Mn Fe Co Ni Cu Zn

     +7

     +6 +6

     +5

     +4 +4 +4 +3 +3 +3 +3 +3 +3 +3 +3

     +2 +2 +2 +2 +2 +2 +2 +2 +2

     +1 The most common oxidation states are in bold. Not all oxidation states are shown.

    nd2 Transition Series 3B 4B 5B 6B 7B 8B 1B 2B

    Y Zr Nb Mo Tc Ru Rh Pd Ag Cd

     +8

     +7

     +6 +6

     +5 +5

     +4 +4 +4 +4 +4 +3 +3 +3 +3 +3

     +2 +2 +2 +2 +2

     +1

    ; s-electrons are outside the d-electrons and are removed first.

    ; Grp. 2B & 3B elements have fixed oxidation states, +2 and +3, respectively (except Hg) st; 1 transition series metals form ionic compounds which dissolve in water giving solvated +3+3+2ndrdcations (Cr, Fe, Co) whereas 2 and 3 transition series metals form only water -2-2soluble oxyanions, e.g., MoO, WO, etc. 44ndrd; Higher oxidation states are more common in the 2 and 3 transition series

    Element Half-Reaction Reduction Potential, E?, (V) +3-Sc -2.08 Sc + 3e Sc +2-Ti -2.00 Ti + 2e Ti +2-V -0.26 V + 2e V +2-. 2Cr -0.56 Cr + 2e Cr +2-Mn -1.02 Mn + 2e Mn +2-Fe -0.41 Fe + 2e Fe +2-Co -0.28 Co + 2e Co +2-Ni -0.23 Ni + 2e Ni +2-These metals (except Cu) are Cu +0.52 Cu + 2e Cu stronger reducing agents +2-Zn -0.76 Zn + 2e Zn than H

    bb/CH306/Transitionmetals.doc 7

    ; Elements with oxidation states below the most common for a particular metal usually act as

    reducing agents. Elements with oxidation states above the most common for a particular

    metal usually act as oxidizing agents.

    ; Most transition metals are moderately strong reducing agents that react with dilute mineral

    acids (HCl, HSO, HNO) liberating H gas. The previous table shows that the reduction 2432

    potential of most transition metal ions is unfavorable (negative values, i.e., not

    spontaneous). Rather the metals are readily oxidized to their ions. For this reason, few

    are found as pure metals in the native state. Most are readily oxidized and thus occur

    naturally as oxides. Examples include TiO (rutile), MoS (molybdenite), WO (wolframite), 223

    FeO (hematite and rust), FeOCrO (chromite), MnOHO (manganite), etc. 2323232

    ; Cu and the ‘noble metals’ ( Hg, Ag, Pt, Au, Pd, Ir, Ru, Re and Os) are the exceptions. They

    are not readily oxidized and can only be dissolved in strong mineral acids or mixtures of

    these acids. A number of these metals occur naturally in the elemental state, e.g., Ag, Au,

    Cu, etc.

    ; Study the Electrochemical Series table on the next page and memorize the mnemonic

    ‘LiKe CaBaNa MAZICNTL H. CHAPA’. This is not a complete list but it is useful to the

    chemist and is easily remembered.

Problem 3: On a periodic table, mark all the ‘noble metals’ to learn where they are located.

    Problem 4: Write the formula of

    a) the most common chloride salt of lanthanum

    b) the only sulfide of zinc

    c) the most common phosphate of mercury.

    Problem 5: Write out the electrochemical series in order of decreasing reactivity as metals. Problem 6: State whether the following reactions will occur spontaneously at room temperature. If the reaction does occur, write a balanced equation for the reaction. If the reaction is not spontaneous, state ‘no reaction’.

    a) Zn reacts with water to evolve H gas. 2

    b) Hg reacts with cold dilute HNO 3

    c) Ba reacts with water

    d) Ca reacts with concentrated HCl

    e) Cd reacts with dilute HSO 24

    +2f) Sc reacts with Cr in solution

    +2g) Ti reacts with V in solution

    +2h) Cu reacts with Ni in solution

    bb/CH306/Transitionmetals.doc 8

    ACITIVITY SERIES OF METALS

    half cell are listed. The order of this series can The reduction potentials of various metals ions vs. the H2

    be easily remembered using the mnemonic ‘LiKe CaBaNa MAZICNTL H. CHAPA’

     Ion Metal Reduction Potential (V) +- + Li (s) - 3.0 Li (aq) 1 e These metals are powerful +-+ K (s) -2.92 K (aq) 1 e reducing agents which

    react even with HOH to +2-+ Ca (s) -2.90 Ca (aq) 2 e liberate H gas 2+2-+ Ba (s) -2.87 Ba (aq) 2 e

     +-+ Na (s) -2.71 Na (aq) 1 e

     +2-+ Mg (s) -2.37 Mg (aq) 2 e +3-These metals are + Al (s) -1.66 Al (aq) 3 e moderate reducing +2-+ Zn (s) -0.763 Zn (aq) 2 e reagents which react with

    +2-dilute mineral acids (HCl, + Fe (s) -0.440 Fe (aq) 2 e HSO, HNO, etc.) and 243+2-+ Cd (s) -0.403 Cd (aq) 2 e liberate H gas. 2+2- + Ni (s) -0.28 Ni (aq) 2 e +2-+ Sn (s) -0.136 Sn (aq) 2 e

    +2- + Pb (s) -0.126 Pb (aq) 2 e

     +-+ 0.00 2 H (aq) 2 e H (g) 2

    These ‘noble’ metals are +2-+ Cu (s) +0.337 Cu (aq) 2 e poor reducing agents and +2-do not react with dilute + Hg (s) +0.778 Hg (aq) 2 e mineral acids. They are +1-+ Ag (s) +0.799 Ag (aq) 1 e weaker reducing agents

    +2-than H (g) + Pt (s) +1.2 2Pt (aq) 2 e

    +1-+ Au (s) +1.68 Au (aq) 1 e

A ‘+’ voltage indicates that a reaction is favorable (compared to hydrogen) in the direction shown.

    A ‘-’ voltage indicates that a reaction is unfavorable but its reverse reaction is favorable.

    o The voltages are standard potentials (E) which would be measured at standard conditions, i.e., 25 C, 1 M concentration for aq. ions, and 1 atm. pressure for gases (as per the Nernst equation). These reactions are referred to as ‘half-cells’ because each is only half of a reaction. Two half cells must be combined in order for a chemical reaction to occur. As reduction occurs, oxidation must also be occurring simultaneously. As one chemical gains electrons, another must be providing (losing) them.

     Add two half cell potentials (one for oxidation and one for reduction). If the combined voltage is a ‘+’ value, we can expect the reaction to proceed as written.

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Ammine Complexes:

    Most representative metal hydroxides and transition metal hydroxides are insoluble in water.

    +2-e.g., Mg + 2 OH Mg(OH) ( 2

    Exceptions are the Group 1A metals and Sr(OH) and Ba(OH). 22

    Some insoluble metal hydroxides are amphoteric; i.e., in addition to acting as bases, they can act as acids dissolving in an excess of strong base.

    -e.g., Al(OH) + NaOH [Al(OH)] (a soluble hydroxo complex). 34

    Ammonia is a weak base (pKb = 4.8) and would not be expected to be able to produce a high -enough OH concentration to dissolve insoluble amphoteric metal hydroxides and form soluble hydroxo complexes. However, several metal hydroxides do dissolve in an excess of aqueous ammonia to form ammine complexes.

    +2-e.g., Cu(OH) ( + 4 NH [Cu(NH)] + 2 OH 2334

    +2-e.g., Co(OH) ( + 6 NH [Cu(NH)] + 2 OH 2336

    Interestingly, all metal hydroxides that exhibit this behavior are derived from the 12 metals of the Co, Ni, Cu, and Zn groups. All common cations of these metals form soluble complexes in the presence of aqueous ammonia.

    +2-Co(OH) ( + 6 NH [Co(NH)] + 2 OH 2336

    +3-Co(OH) ( + 6 NH [Co(NH)] + 3 OH 3336

    +2-Ni(OH) ( + 6 NH [Ni(NH)] + 2 OH 2336

    +1-CuOH ( + 2 NH [Cu(NH)] + OH 332

    +2-Cu(OH) ( + 4 NH [Cu(NH)] + 2 OH 2334

    +1-AgOH ( + 2 NH [Ag(NH)] + OH 332

    +2-Zn(OH) ( + 4 NH [Zn(NH)] + 2 OH 2334

    +2-Cd(OH) ( + 4 NH [Cd(NH)] + 2 OH 2334

    +2-Hg(OH) ( + 4 NH [Hg(NH)] + 2 OH 2334

    CuOH and AgOH are unstable compounds and decompose to CuO and AgO, but in the 22

    presence of aqueous NH they dissolve as shown above. 3

Problem 7: Write a balanced equation for the reaction of 6 moles NH with each of the 3

    following

    a) Au(OH) 3

    b) Pt(OH) 4

    bb/CH306/Transitionmetals.doc 10

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