INORGANIC CHEMISTRY Coordination Compounds A Lewis base (e - donor) donates a pair of electrons to a Lewis acid (e - accepto r) formi ng a co-ordinate covalent bond. Ammonia is the Lew is base (elec tron dono r). H + and BF3are Lewis acids (electron acceptors). imilarl! " transition metals and their cations are Lewis acids. #he! have vacant orbita ls that can accep t ele ctr on pairs fro m donor ato ms (Le wi s bases ) formi ng coordination compou nds orcomplexes. ome e$amples are shown in the following table. Lewis Acid Lewis Base Comple!issociation Constants %r+3 + &H'→ %r(H')&* +3 ------------- %o +3 + &H3 → %o(H3)&* +3 '.' ×,-3i +' + %- → i(%) * -' ,.×,-3, Fe + /%→ Fe(%) /* ------------- Ag + + 'H 3 → Ag(H 3 ) ' * + &.3 ×,-0 Lewis bases (anions or molec1les) bonded to the central Lewis acid are called li"ands(Latin 2 tie or b ind). %harged coordina tion comp le$es also have counter ionsassociated with them to satisf! electrone1 tralit! . For e$ample" in Ag(H 3)'* + 3 - " a nitrate anion is the co1nter ion. %o1nter ion s are ionicall! bo nded. hen disso lved in water " the 3 - ion separates from the comple$" b1t the two H3ligands remain firml! bo1nd to the Ag + ion b! covalent bonds. #he n1mber of atoms bonded to the central ion4atom of a coordination comple$ is referred to as its coordination num#er. #he coordination n1mber of Ag(H 3)'* + 3 - is two. #he coordination n1mber of Fe( %)/* is five . %oordination comp le$ es occ1r with coo rdin atio n n1mber s of ' thro1gh ,'" however" coordination n1mbers of '" and & are most common. %omple$es s1ch as Fe(%) /* are 1s1all! insol1ble in water since the! are covalent compo1nds in which a n! dipole s cancel. %omple$ es s1ch as A g(H 3)'* + 3 - are generall! ver! sol1ble in water since the! are made 1p of cations" e.g." Ag(H 3)'* + and anions" e.g." 3 - . Importance o$ Coordination Complees% 5an! biologicall! important s1bstances are 6d6-transition metal coordination compo1nds that are made 1p of large organic molec1les bo1nd to the metal via coordinate covalent bonds" e.g." hemoglobin (blood protein) is a coordination comple$ involving Fe. vitamin B-,' is a cobalt comple$ (c!anocobalamin) phthaloc!aine bl1e is a %1 comple$ d!e 1sed for bl1e 7eans and in8 Coordination Complees , ammonia N H H H % & B ' ' ' N H H H B ' ' ' Boron (as a )aca nt *p +or#ital, Nitro"en (as a $ull sp -or#ital, A co.ordinate co)alent #ond is $ormed w(en nitro"en donates its lone e . pair to #oron/s empt0 *p +or#ital, N sp- or #ital s sp*or #ital s *p+ B H & H0dro"en ion (as a )acant 1s or#ital, Nitro"en (as a $ull sp -or#ital, A co.ordinate co)alent #ond is $ormed w(en nitro"en donates its lone e . pair to (0dro"en/s empt0 1s or#ital, ammonia ammonium ion N H H H % & H & N H H H H & N sp- or#itals 1s 2 H H H 1s1 1s1 1s1
