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Transition Metals and Coordinationchemistry TRANSITION METALS AND COORDINATIONCHEMISTRY knowledge, maturing into an in tegral (011011',become scientific knowledge. Science in an ordered theory. knowledge and qcthal JCTION ofmetals in the middle of the periodic table, whose inner d- orf-orbitals are not completely .ansitionelements (or d- or f-block) elements. Alternatively, a transition element is orat least one of its ions has incompletelyfilled d- or f- orbitals. Alternatively, a transition definedas "as elementwhQ$eat Icastpne simpleion 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 : transitionseries contain ten elements from scandium to zinc .(At. No. 30). 'liplete ,orbitais. Id transition series contain ten elements from yttrium cadmium (At. lave incomplete 4d-orbitals. transition series contain ten elementswith incomplete5d-orbitaIs and constitute 'htransition series has incompletelyfilledf-group element'. This consists of lanthanides hus, transition elements are : up elements : cries v Cr co cu ration4s 2 2 2 1 2 2 2 2 1 2 31.1 1 2 3 5 5 6 7 8 10 10 Iseries Y Zr Cd series La w Re 67 up elements : 'La' ce P! Nd Pm sm. 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 2 (nt. No.) sc (21) Y (39) 4611, 582 La (57) con 2 Ti (22) 3d , 42 Zr (40) 4d2, (72) 5di v (23) Nb (41) 4d4, 581 Ta (73) Cr (24) 5 451 Mo (42) 5 551 3d , 4d , W (74) Mn (25) 5 452 Tc (43) 5 3d , 4(1, Re (75) Fe (26) 6 4s 2 (44) 7 1 54 361 , Ru 4d , 55 os (76) 8 1 co (27) Rh (45) 4d , 5s (77) Ni (28) 3d8, I'd (46) Pt (78) 10 l 10 1 54, cu (29) 361 , 4s Ag (47) " , 55 Au (79) 0 2 Zn (30) Cd (48) 41 , 55 Hg (80) 5d10 CHARACTERISTICS OF TRANSITION ELEMENTS Some of the characteristics of transition elements are discussed below : (1) atomic radii lie in between those of s- Atomic radii : (i) Their and p- blockelements, ralues in a series,first decreasewith increase in atomic number, but the decreaseis smaller Table 2. Atonzic radii o/d-block elements (in pm) Sc Cr Mn Fe co cu •144 132 122 117 117 117 116 115 125 Zr 162 145 134 128 124 124 128 134 La w Re Ir Au 124 120 169 144 134 130 128 129 134 144 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 outermostelectrons lith the progressive increase in inner electrons, their screening effect counterbalancesthe 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 addedelectm the same d-orbitals become higher than the attractive force, due to increased nuclearcharge, k ereby resulting in the expansion of the electron-cloud and hence, the atomic radius increasesatth d of the series. us (iii) Atoniic radii of transition metals the group. For example, atomicradi transition series) = 117 pm ; MO (2nd transition series, below Cr) = 128 pm ; W (3rdtransition •ies, below MO) = IS(YÉm. transition Reason : This is due to addition ofa new shell down the group from 1st to 2nd to 3rd COORDINATION CHEMISTRY AND 1055 Ionic radii decreases in a series. For dii : (i) example : Cr Mn Fe co cu 90 88 84 80 76 74 72 69 to progressive increase in Thisis due thg.effec/@venuclear charge. transition decrease t increase in of oxidation state and vice versa. Thus, effective nuclear charge in MA+ ion is . The greater than that in M2+ of transition elements are smaller than the s- and p-block elements belonging of transition elements are high. atomic volumes of transition : Sincethe elements are low (due together with progressive to filling of the inner electrons, increase in nuclear charge), so their densities character : Transition elements possess (4)Metallic all the characteristic properties of metals, or two ns or ns electrons under loseone appropriate conditions and are changed topositive solid (except Hg, which is Thus,they are a liquid at room temperature), hard, lustrous, good conductor of heat able,Juctile and as well as electricity and possess high tensile igth, metals have 1 or 2 electrons 1-2 : Transition in tlleir outermost orbit (ns ), and their ionization so they form relativelylow, ntetallicbonds, having hcp or ccp or bcclattices. Moreover, cause the d.electronsalso formation of metallic bonds. Consequently, greater the number stronger is the metallic mpairedIl-electrons, bonding, becauseof overlappingof unpaired electrons Hence, Cr, MO and atonts. W having illaximunlnumber unpaired d-electrons (= 5) Cd and ; while Zn, Hg having no unpairedelectrons are low melting and soft metals. ISIt(lllid, dile to the absence of unpaired d-electrons. Note: Due to low value of its elasticity, copper possesses lotvyield-point (or crushing point). Consequently, starts flowing and hence, pressure exhibits ductility and lnalleability.Since zinc possesses comparatively tv,llueofelasticity, so brass (an alloy of copper and zinc) is tenacious. (5)Melting and boiling points of transition metals The melting point, how- visesto a maximumvalue and thenfalls 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 lgthof metallicbond depends upon the number of unpaired-electrons (or half-filled d-orbitals). ethe number of unpaired electrons increases upto d5 configuration (e.g.,Cr in 1st series) and decreases upto d10configuration (e.g., Zn in 1st series), consequently their melting points uptoCr and then decreaseto Zn. Hence, melting points of Zn, Cd and Hg itin (= 234 K) are theirrespective transition series. (6) Ionization energies : (i) The values of ionization energies of d-block element lie between s-blockelements on their left and p-block elements on their rightEThey are less thes-block electropositive elements and more electropositive than p-block elements. (ii)The ionizationenergies increases, but irregularly with increase in atomic number of the ele- infirst transition series (see Table 3). ENGINEERING 1056 CHEMISTRY series. Table 3. First ionization energies offirst transilion Element Cr Mn co cu 717 762 758 IE (kJ mol -I ) 631 656 650 652 745 905 to copper, following Reason : As the atomic number increases from scandium twoopposing forccs increase sinniltaneously : the nucleus and (1) Due to increase in nuclear charge, the attraction between the -innerelectrons increases. electrons in 3d-orbitals, (ii) Due to screening effect, caused by the addition ofntore the outer electrons due to However, the screening effect is nearly equal _toattractiveforces, nucleus on the inner electrons, so there is marginal and irregular variation in their ionization-energies. those of3d- alid (iii) First ionization energies of5d-ele111entsare higher than 4d- elentents. Reason : Due to the comparatively tveakershielding (or screening) effectof the nucleuson 4f-electrons in case of 5d-elements, there is a greater förcedattraction between the nucleusand charge the valence electrons. In other words, there is greater effectivenuclear acting on outer valence electrons in case of 5d-elements. Hence, the first ionization energies of 5d-elements are higherthan 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 changefrom 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 sufficientlyhigher 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 thestability of their conipounds.For example, first four ionization energies of nickel given below : (Ell + E12)MJ mol (E13+ MJ mol -1 Total 2.49 8.80 11.29 MJ mol -1 2.66 6.70 9.36 M] mol -1 From the above, it is clear that : (i) Second ionization energy of 'nickel is less than that of platinum, so N12+ compoundsare 1/101?stable that Pt compounds. (ii) Fourth ionization energy of platinum is less thall that of nickel, so Pt4+ compoundsare 4 Illore stable than N1 + compounds. It may be pointed that stability of contpounds depends upon their electrode potentials (EO),which depends upon sum of the enthalpy of sublimation, ionization energy and hydration energy. Ell sublimation hydration M (s) Mg (g) M+ (g) M+ (aq) —e AND COORDINATION N METALS CHEMISTRY 1057 EOvalues (M2+/M)for 1st series of transition metals is given below : Cr co cu Element —0.44 -1.18 -0.91 -1.18 -0.28 + 0.34 ECM"/M (V) in EOM2+/Mvalues as compared to (IEI + Thus, there is no regular trend 1%). Moreover, subli- exhibit any regular trend. energies of these elements also do not Hence, the of idea regarding the stabilities of their compounds. ionizationenergies give a rough important characteristics of transition elements (7)Oxidation states : One of the most is their which are related to their electronic configurations.
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