COORDINATION CHEMISTRY - University Institute of ... COORDINATION CHEMISTRY 1055 dii : (i) Ionic radii

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  • TRANSITION METALS AND

    COORDINATION CHEMISTRY

    knowledge, maturing into an in tegral (011011', become scientific knowledge. Science in an ordered

    qcthal knowledge and

    theory.

    JCTION ofmetals in the middle

    of the periodic table, whose inner d- orf-orbitals are not completely

    .ansition elements (or d- or f-block) elements. Alternatively, a transition element is

    or at least one of its ions has incompletelyfilled d- or f- orbitals. Alternatively, a transition

    defined as "as elementwhQ$e at Icastpne simple ion contains 1 to 9 electrons in d-orbitals

    in f-orbitags

    : (i) Iron, chromium, nickel, zinc and copper ; (ii) Gold, silver, copper and platinum.

    series : There are in allfour transition series :

    transition series contain ten elements from scandium

    'liplete ,orbitais.

    Id transition series contain ten elements from yttrium

    lave incomplete 4d-orbitals.

    to zinc .(At. No. 30).

    cadmium (At.

    transition series contain ten elements with incomplete 5d-orbitaIs and constitute

    'h transition series has incompletely filledf-group element'. This consists of lanthanides hus, transition elements are :

    up elements :

    cries v ration 4s 2 2 2

    31.1 1 2 3

    I series Y Zr series La

    67 up elements :

    'La' ce P! Nd Pm sm.

    Cr co cu 1 2 2 2 2 1 2

    5 5 6 7 8 10 10

    Cd

    w Re

    Dy Ho Er Lu 'Ac' ThPA U NP Pu Am cm Bk Cf Es Fm Md No Lr.

    (1053)

  • 3d (or •rst) 4d (or second)

    Elejnent (At. No.) Con uration Element (At. No.) Con ration Element (nt. No.) consc (21)

    2 Y (39) 461 1 2

    Ti (22)

    v (23)

    Cr (24)

    Mn (25)

    Fe (26)

    co (27)

    Ni (28)

    cu (29)

    Zn (30)

    3d2, 42

    3d5, 451

    3d5, 452

    361 6

    , 4s 2

    3d8,

    361 10

    , 4s l

    Zr (40)

    Nb (41)

    Mo (42)

    Tc (43)

    Ru (44)

    Rh (45)

    I'd (46)

    Ag (47)

    Cd (48)

    , 58 La (57) 4d2,

    (72)

    4d4, 58 1 Ta (73)

    4d5, 551 W (74)

    4(15, Re (75)

    4d 7 , 551 os (76)

    4d8, 5s1 (77)

    Pt (78)

    " 10, 55

    1 Au (79)

    41 0 , 55

    2 Hg (80)

    CHARACTERISTICS OF TRANSITION ELEMENTS Some of the characteristics of transition elements are discussed below :

    5di

    54

    54,

    5d10

    (1) Atomic radii : (i) Their atomic radii lie in between those of s- and p- block elements, ralues in a series,first decrease with increase in atomic number, but the decrease is smaller

    Table 2. Atonzic radii o/d-block elements (in pm)

    Sc Cr •144 132 122 117

    Zr 162 145 134 128

    La w 169 144 134 130

    Mn 117

    Re 128

    Fe 117

    124

    124

    co 116

    124

    Ir 120

    115

    128

    129

    cu

    125

    134

    Au 134 144

    Reason : Initial radius is due to increase in nuclear charge. Since the electron

    Iters the penultimate shell, thereby the added electron shields (or screens) the outermost electrons

    lith the progressive increase in inner electrons, their screening effect counterbalances the opposing

    (ect of increased nuclear charge, thereby the atomic radii remains almost same after Cr.

    (ii) At the end of the series (or period), there is a slight increase in the atomic radius. For example

    omic radius of Zn (= 125 pm) is higher than that of Cu (= 117 pm).

    Reason : Near the end of the period (or series), the repulsions between the added electm

    the same d-orbitals become higher than the attractive force, due to increased nuclear charge,

    ereby resulting in the expansion of the electron-cloud and hence, the atomic radius increases atthk

    d of the series.

    (iii) Atoniic radii of transition metals the group. For example, atomic radi us

    transition series) = 117 pm ; MO (2nd transition series, below Cr) = 128 pm ; W (3rd transition

    •ies, below MO) = IS(YÉm.

    Reason : This is due to addition ofa new shell down the group from 1st to 2nd to 3rd transition

  • AND COORDINATION CHEMISTRY

    1055

    dii : (i) Ionic radii decreases in a series. For example :

    Cr Mn Fe co cu 90 88 84 80 76 74 72 69

    This is due to progressive increase in thg.effec/@ve nuclear charge.

    of transition decrease t increase in oxidation state and vice versa. Thus,

    . The effective nuclear charge in MA+ ion is greater than that in M2+

    of transition elements are smaller than the s- and p-block elements belonging

    of transition elements are high.

    : Since the atomic volumes of transition elements are low (due to filling of the inner

    electrons, together with progressive increase in nuclear charge), so their densities

    (4) Metallic character : Transition elements possess all the characteristic

    and are

    properties

    changed

    of

    topositive

    metals, lose one or two ns or ns electrons under appropriate conditions

    Thus, they are solid (except Hg, which is a liquid at room temperature), hard, lustrous, able, Juctile and good conductor of heat as well as electricity and possess high tensile

    igth,

    : Transition metals have 1 or 2 electrons in tlleir outermost orbit (ns 1-2), and their ionization relatively low, so they form ntetallic bonds, having hcp or ccp or bcc lattices. Moreover,

    d.electrons also cause the formation of metallic bonds. Consequently, greater the number mpaired Il-electrons, stronger is the metallic bonding, because of overlapping of unpaired electrons

    atonts. Hence, Cr, MO and W having illaximunl number unpaired d-electrons (= 5) ; while Zn, Cd and Hg having no unpaired electrons are low melting and soft metals.

    IS It(lllid, dile to the absence of unpaired d-electrons.

    Note : Due to low value of its elasticity, copper possesses lotv yield-point (or crushing point). Consequently, pressure starts flowing and hence, exhibits ductility and lnalleability. Since zinc possesses comparatively

    tv,llue of elasticity, so brass (an alloy of copper and zinc) is tenacious.

    (5) Melting and boiling points of transition metals The melting point, how- vises to a maximum value and then falls as the atomienumberincreases in a series. Reason : High melting and boiling points of transition elements is due to strong metallic bonds

    ir to . This is clear from their high values of enthalpies of atomization. Now the lgth of metallic bond depends upon the number of unpaired-electrons (or half-filled d-orbitals). e the number of unpaired electrons increases upto d5 configuration (e.g., Cr in 1st series) and decreases upto d10 configuration (e.g., Zn in 1st series), consequently their melting points

    upto Cr and then decrease to Zn. Hence, melting points of Zn, Cd and Hg (= 234 K) areit in their respective transition series. (6) Ionization energies : (i) The values of ionization energies of d-block element lie between s-block elements on their left and p-block elements on their rightEThey are less electropositivethe s-block elements and more electropositive than p-block elements.

    (ii) The ionization energies increases, but irregularly with increase in atomic number of the ele- infirst transition series (see Table 3).

  • ENGINEERING CHEMISTRY1056

    Table 3. First ionization energies offirst transilion series.

    Element Cr Mn co cu

    IE (kJ mol -I ) 631 656 650 652 717 762 758 745 905

    Reason : As the atomic number increases from scandium to copper, following two opposing

    forccs increase sinniltaneously :

    (1) Due to increase in nuclear charge, the attraction between the nucleus and the -inner electrons

    increases.

    (ii) Due to screening effect, caused by the addition ofntore electrons in 3d-orbitals, the outer electrons

    However, the screening effect is nearly equal _to attractiveforces, due to nucleus on the inner

    electrons, so there is marginal and irregular variation in their ionization-energies.

    (iii) First ionization energies of5d-ele111ents are higher than those of3d- alid 4d- elentents.

    Reason : Due to the comparatively tveaker shielding (or screening) effect of the nucleus on

    4f-electrons in case of 5d-elements, there is a greater förcedattraction between the nucleus and

    the valence electrons. In other words, there is greater effective nuclear charge acting on outer valence

    electrons in case of 5d-elements. Hence, the first ionization energies of 5d-elements are higher than

    those of 3d- and 4d-elements.

    (iv) Second ionization energies of Cr and Cu are exceptionally high.

    Reason : The electronic configuration of : Cr+ = [Arl 3d5, and Cu + = [Arl 3d1() . Thus, both

    Cr+ and Cu + have stable configuration of exactly half-filled [3d5] and fully-filled [3d10] d-orbitals.

    Consequently, removal of one electron form these, to give Cr and Cu Ions, means change from

    a Illore stable state to less stable configuration, [Ar] 3014 and [Ar] 3d9 respectively. Since such a change

    is quite difficult, so second ionization energies of chromium and copper are sufficiently higher than

    those of their neighbours on the left as well as right.

    (v) The Illagnitude of ionization energies of the transition metals is inversely linked to the stability

    of their conipounds. For example, first four ionization energies of nickel given

    below :