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Transition Metals and Coordinationchemistry

Transition Metals and Coordinationchemistry

TRANSITION 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 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. Idtransition 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 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 I series 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 , 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 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 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 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. Usually, abilityto show variableoxidation states, state corresponds to the group nuntber. Some of the common oxidation states theIki$het oxidation are grven in Table 4.e of first ransltion series series (or 3d-series) Table 4. Oxidation states offirst transition tration Oxidation states Elclncnt At. No. Con 2 1 Scandium (sc) 21 [Arl 45 , 3d 2 Titanium (Ti) 22 [Arl 4s , 3d 3 Vanadium (V) 23 Chromium (Cr) 24 3d5 5 Manganese (Mn) 25 (+ 1), + 2, +3, (+ 4), (+ 5), (+ 16) Iron (Fe) 26 [Arl 4s2, 3616 7 Cobalt (Co) 8 Nickel (Ni) 28 1 10 Copper (Cu) 29 [Arl 4s , 3,1 2 1() Zinc (Zn) 30 [Arl 4s , 3d Reason : The variable oxidation states of these metals is due to theovailability of both (u —1) d and ultimate s-orbitals are and ns-electronsfor bond ormation, as the energies of penultimate d-orbitals nearly equa ost of them have two Its-electrons, so generally they exhibit + 2 oxidation state. In addition, they may also utilize one or more (n —1) d-electrons for bond formation, so oxidation states of + 3 and higher also occurs. The sunt total of IIS-and unpaired (n —1) d-electrons deternlines the highest oxidation state shown by a transition element. Examples : (i) The outer electronic configuration of scandium is 3dl, 4s2. It exhibits an oxidation state ofs+2by utilizing both 4s-electrons.It can also exhibit an oxidation state of + 3 when it utilizes its two 4s- and one 3d-electronS. (ii) Titanium (with outer electronic configuration 3d2,4s2) shows an oxidation state of +2, when it utilizes both 4s-electrons;of + 3, when it uses both 4s-electrons and one 3d-eIectron; and of + 4, when it utilizes both 4s-electron and the two 3d-electrons. (iii) Vanadium (with outer electronic configuration 3d3, 4s ) exhibits oxida ion states of + 2, +3, + 4 and + 5, depending upon the number of 3d-electrons utilized for bond f rmation. (i?) Chromium (with outer electronic configuration 3d5,4sl) shows oxiHation states equal to + 2, + 3, +4, + 5 and + 6, depending upon the number of 3d-eIectrons utilized for bond formation. 1056 valency exhibited by chromium is + 6, since The maximum the total number plus in 5 + 6. A few chromium

Contpound Cro crß)g cr2(S04h idalion state o! Cr Manganese (with outer electronic configuration 31, (o) 492)exhibits 2, + 3, + +6 and + 7. (vi) Elements iron (3116,4s2), cobalt (3117,452), nickel (3d , 'Is copper (31110 which 31/-electronsare more than five, exhibit (3,110, in maximum oxidation

(vii) If an element exists in more than one oxidation state, their relati , predicted from their standard electrodepotentials. For example : Cuf ((111)+ e - Cu (s) ; = 0.52 V 2 Cu (s) ; Eo = 0.344 Hence, Cu2f is ntore stable than Cu+. In other words, Cu is reduced moreeasily to Cu. (viii)The oxidation state of a transition is dependent on the nature atonts.Thus, compounds of transition metals with F and O exhibit the highestoxidation of F and O are very Iligh. (ix) The oxidation state of a transition metal depends on the nature of thesolvent For example, Fe is unstable in aerated (vater, since it undergoes oxidationin it. Similarly, unstable in (Oilter,since undergoes oxidation ; while Cro is stable in water. (x) Transition metals also exhibit + 1 and () oxidation states. For example, oxidation Ni in is zero. It may be pointed that bonding in compounds like Ni(CO)4is not it will be discussed in next chapter.

(xi) Oxidation state increases vertically downwards with atomic number in a groupd transition metals. For example, common oxidation states for Fe are + 2 and + 3; whilefor Os of the same group are + 4, +6 and + 8.

(xii) Ilighest oxidation state increasedown the group. For example, highestoxidationstlt: shown by Fe, Ru and os are + 2, + 3, (+ 6) ; + 2, + 3, +4, (+ 5), (+ 6), (+ 7), (+8) ;+2, respe.ctively. (8) Paramagnetism : A 'parantagnetic' substance is one, which is attractedäntoa Para mogne'tism is associated with the presence of unpairedelectrons in the Most 01 the compounds of transition elements are parmnagnetic in nature.This is due toll presence. of unpaired electrons in the (n —1) d- or (n —2)f-orbitals. Thus, onl!LJJlQ$L'Jt0111i.lCJJß Itavrng unpaired electrons show parantagnetistn. The greater the nuntber of unpaired electrons (atonl or• ions), the "tore strongly parantagnetic it is. Thus, Cu+ (3d10, 450) anci diantagnettc(i.e., repelled when placed in a magnetic field) and parantagneticrespectively, they contain zero and one unpaired electron respectively. Similarly, Fe2+(3d6 (Ad5,(so) having respectively •4 and 5 lunpaired electrons are both Pdrmnagnetic. is "lore strongly paranmgneticthan Fe +, because the former possesses greater elections than the latter. gALS ANC C*QESTRY a usexpressed as +2) Bm and rs tFæ bekariour: A e+ctnpt (a charged part«le) well as orE'1talmotxw a due a paramagnettc substance a held, then (t.e„ havutg magnetr moment (due to electron vs sum-rat to sptn and orbital the magnetic moment tnduv•d bv the apt'tted Cc•nseguently, such a suEstance expenenees attrxtra• tn the magnettc Oeld. of 3å-orS1taLsm tons of Nl•settes ate as follows

2 3 4 4 3 2

paramagnetic moment (or paramagnetic behauour) t'tcreasesfrom to Ntn and to Zn:- ; and only Z.n:• is dutma€netrc. Formation of complexes : Tae cations of transttion metals ate almost untqtte in thett ctgbendertcy to form complexes mth molecules(such as water, ammonta, cat+on ntonovde, et

•acc, ; ; . Reason : zoa formation transuhonelement tons is because trattsttten"tetats vteld .. t'.•achha:v co.-ant (n — I) d-.rFrtals 'f aprrcvmatelv the approprtate CttCtSV to 'lectrors, donated ether Stoups or "'telecules such as evantåc avtcr titelectde, monordz' molecules or tons which add themselves to the of metals, are called For example, ferrous and ferrtc tons torm follow Ing ions With 3 amde ions : 3 (ii - femxyarude ion. (u) FetCN)0- -s ferrucyanrdeton, the central ',ons. re-* and Fe are surrounded respecttvelv and six cyanide ions. Sm.larly, the central , mckel rs surrounded by CO molecules. ( 10j Formation of coloured ions : cemplcx (Ytrattsttiort "teta.lsan', itsuallv, m form and m T'rv. coloured posttive ions itt Reason : TYEcolour trans.'tn•t metal assuateti tnc0'rpletelvfilled (n —1) '1-01+1tals. d-ek•ctmnsundergo electronic transittons front enc 'c another tlns d-d-transitton process. absorb certat't radtattons (from the light and emit the remat*der as coloured Se the of an Jen is cornplt"tcntary Oftlte are colour lt- Hence. coloåred •orts formed due to transitions,C"luch fall V?ltht'tthe For example, elements I'lac tn colour, because It absorbs excitation) from the vvslble colou.c (reedd for light, leavtng the complementary blite coloured avewngth to pass exhibited by a trawsttton metal 'On depends upon the evutati0't state ot thei (blue); (dark (pale prnk); Mn (red); Feb •joeo that the ltott-ntetalhcprt (WI/ott').ft may be pointed associated With (black), Cu(OH2) the_go/our, e.g. Cuo (pale blue), CuC1 the (blue), etc Different colours exhibited by hydrated CUSO,S transition

Note : on white colour, yellow complimentary and purple, 01)orange and blue. and indigo. Table 5. Colours of different hydrated transilton metal ions ()utercon uratton Ion No. o un aired electrons (111) Nil cot 3rd) Nil White One Purple v (111) 3112 Two v (11) Green 31 Three Cr (111) Violet Three Mn (111) • Violet 3<14 Four Mn 01) Violet 3d5 Five Pink Fe (Ill) 3d5 Five Yellow Four Green co (Il) 3,17 Three Pink Two Green cu (11) One Blue 3,110 Nil White (colourless) Zn (Il) 3,110 Nil White (colourles) (Il) Catalytic properties : Many transition metals and their compoundsare found catalysis. For e:xample :

(i) finely@ivided ntckel is used as a catalyst in the hydrogenationof oil tvfgt. duricf manufacture of vegetable ghee ; (n) Iron is used as a catalyst in Haber's_processfor the manufactureof intenaction of nityggen andYdyggen (lit) platmtg_tn and vanadium pentoxide are employed as catalysts in the manufacturecir phur1C acid by contactprocess ; (IV) Mn02 is used as a catalyst, during decompositionof hydrogenperoxide solution; (v) ztnc chromite is employed in catalyst in the synthesisof methylalcohol; (VI) decomposition of bleaching powder solution is catalysed by cobalt salts. Reason for catalytic power of transition metals .6•Thecatalytic powerof transibonö probably though the use of the (n —1) d-orbitals due to the formation due to zvlltcll adsorb and activate the reacting stibstances, Thus, they acÜas catalyst", COORDINATION AND CHEMISTRY 1061 of presence of incomplete Because d-orbitals,they can form gnstableintermediate Transition métatcompounds the reactants. are believedto operateas catalys' byprotndmg pathways, either by txtriatiotiin energy the oridati«mstate of transition métal -E/ttfcrhlåatcs. during the F conversGoRR31to the catalyst V202is ability of vanadium to to the have Ytrral oxidationstates. The : probablemec)nntsm , F action is as follows Otølyoc a SC)2molecule on its v}Eadsorbs surface and gives it an oxyge•t atom to form rtrøxde.

tetroxide is then converted d,vanadium back to by the reaction with oxygen 2 V204 + 02 ---...-..-..-*2 vz05 thatfinct the transition metal used, . It nay be pointed triter is its catalytic activity, stnce finely divided and more nunvtrr cffrre sufføcearea aalencu•sat Its surface for the odxrpf•on rectant

Formationof interstitial compounds : Jnterstitial compounds are thosein snull H, C, N, B, etc) occupy the Interstitial sites (or empty spaces or touts) in tir crystal tattre. metalsform a number of interstitial compounds With small-sized hke H. B, C, N,

Example: Steel IS an antersutia) compound, in which voids among iron atcmtsare

: Atoms of transition metals are quite large m st:-c and they posg•ss smatt txnds in thetr So atoms of H, B, C, N, etc. can easily psgtjms m rx»åsprewnt lattues of transition metals, thereby Yielding nearly ally properties : ( I ) It makes the transition metals morr and nsul. (2) it fh•• trans:uonmetals. For example, steel a less malleable and ductlk th&i pure iron. but Its than that Iron. (13)Alloy formation : Alloy defined as the tue a rnctal. of alloys formed by transition metals : Tranutum t'*t.als jorrn tw : V)Interstitial alloys m h atontsof (hit 'f, B, C N' tntcrstJtutlspaces the mctal exampk•. an alloy carbon (a)Solid solution alloys an• formed Whenone as Nt. C', Ca. V, tmn. For example, sterl (an alloy of le. N/ and c t) solid allov : (an alloy oi and tut). Reøoøof alloy format"'" : Transition metab ionn alloys very easaly. *tab arr quite similar and they mutually substitute h" molten transition metals are mutually •n a maxtute 't gives an alloy. Fot evample. nn kel erm to torm in nickel to form art mangane« to vanadium in to form aik'ys am. and more cortoason-restst.uuvthan the m«ais. Truwt*M CF.V, W. co, etc, ate in the manuia•tum high-speed tool steely. etc.

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