PHYSICO-CHEMICAL STUDIES ON IRON CYANIDE COMPLEXES

Abstract

The eheaistry of eoaplex cyanides began with the discovery of Pruttlan Blue in 1704. The com? lex Ia[Fe(CR)i] 3RaO, prepared by the addition of an excess cyanide solution to Ft(II) solution and subsequent crystallisation,is known to bt txtremely stablt. Itt solutions do not givt ehamlcfil reactions of either Ft4** or CU~. It is very stable in alkaline solutions but is rapidly oxidised by air in &eid solutions. The structure of X* [Ft(CR) •] has been a subject of extensive investigations. The nature of the bond between the cyanide ion and iron had engaged the attention of aany workers,both from the experimental and theoretical point of view. Before reviewing the reactions of hexaeyanoferrates,It will toe worthwhile to discuss the toon-Ing properties of CH* ion and the electronic structure of the complex. The molecular orbital configuration of the OH" ion may be written asi (C-2S)1 <o-2$)« |(o-a©)* <w2p>* <w2p )° Froti tht obovt molecular orbital configuration it is that the cyanide Ion has two lone pslrs,one on the carbon and the other on nitrogen atom. These lone pairs project outwards and lie along the carbon-nitrogen axis. It Is these electrons^! eh overlap with the metal d-orbltals to give the L->M,o-coordinate bond, since nitrogen is mora electronegative,the donation of the carbon lone pair to the n vacant metal d-orbltals Is easier end hence a carbon-iron bond is formed, f'ince the electron density on carbon and nitrogen in CR" Is similar, It was not possible to distinguish the bonding between Fe and 0 or Fe end R by x-ray studies. Neutron diffraction and more refined x-ray studies1** show that CN* is bound to Iron through the carbon atom. This hat been further supported toy tht work of Ifrle*, who reported a Iana number of methyleUon reactions on complex cyanides. In each case metal alkyl lsocyanides were obtained,which on hydrolysis gave alkyl isocyanidtt and not alkyl nltrlles. This gave the chemical evidence for the metal-carbon bond In f errocyani dea. X-ray analysis* has shown that hexaeyeno ferreted I) molecule is octahedral with the six cyanide groups having a symmetric 1 spatial arrangtmtnt around the central Iron ttoa. Tht tix a- orbltals (one on eaoh CR* ) overlap with the M^ya, Sd^Ce^ ^sCti^) and 4px,4py,snd 4pJB(tHx ) orbltals of iron to give tht e- coordinate bond. Xht w2p orbltals on the CN* Ion are v&cant and hence gain electrons from the iron 3d f3d and 3dxg orbltals by back donation to constitute the v-coordinate bonds. The o- moleottlar orbital system in hexaeyeno ferreted I) • utilises the 3*xam y*,3dfa,4t,4pJt, 4py and 4pz metal (iron) orbltals with prorer linear combinations of the llgsnd(CN* ) er orbltals. Ttws there will be foraed 6 bonding and 6 an tl toondingo- orbltals. The pure v-c.o. art composed of the 3d ,3dxg and 'o&ygft*- ) metal orbltals and the t. b * combinations of * and v ligand suo. This there are forced three (degenerate) strongly bonding m.o. nalnly localised on the CR*, three virtually non bonding (which may be either weakly bonding or an tlbonding) m.o. mainly localised on the metal and three strongly antlbondlng m.o. a«inly idealised on the CR*. Although the (n* * )p metal orbltuls <**!! ) are expected to be primarily involved In bonding, they may also interact with the t^ ligand, r* and v* orbltnls to yield three bonding and three antitoon ding molecular orbltals mostly localised on the llg&nds, and thrte strongly antitoon-)lng m.o. with botho- and v character localise? on the Batata There are t,~ gnd ta- »** and v* ligand orbital combinations which do not interact with metal orbltals. The m.o. energy level scheme expected for the bonding situation described above Is shown In tht figurt. There will elways bt 36 electrons ( 6 from etch ligand) to pi see In the m.o. for the complex. In addition the metal may furnish as R&ny as 6 valence electrons. The ground state of the d* hexacyanlde may thus be written est \Vb)]* [*'«"b>]•[«.«(*>]• • •*« The m.o. scheme shown above has been utilized by Oray and Beach9 to interpret the ultraviolet spectre of hexecytnoferratedl). HoBBBalM and Cray* have in a similar manner prepotad t m.o. scheme for the nltroprusside ion end assigned Its ultraviolet srectrt. flaactiont of Votatslum '.'exrcyanoferratadl) and dllfr Haxtoytnoferrates are known to react with a large variety of compounds. The reactions have been reviewed at length by Orlfflth" and Ford-fmlth*. tudles on the reactions of hexaeyan©ferrates Baf be broadly divided Into thrtt eategeriesf (l) reactions involving replKCtrtnt of tht cations (11) rt-ctlont Involving displacement of the cyanogroups and (iii) photochemical decomposition reactions. (1) Reactions involving rfTl&car&ent of the Cntlonsi The earliest Investigations involving replacement of the oations began with studies on iron blues, and a large number of hef*vy Betel ferro and ferrlcyanidet havt been prepared *,nd their ©©repositions invesU. *ated. Kblthofftfl* for the first time determined the composition of these eornrlex cyanides by eonduotometry and potontlotretry. The constitution of iron bluos prat ante'? a controversial subject. The super complex concept was jostulated by ogger and Hllet11, who parol eved an infinite thrtt dimensional net work, with esch IBfJflla stolon surrounded by six sttatBtl cations and each e tion In turn by six F#(CR)« 3 roup t.