Vibrational Spectra of Iron Carbonyl Complexes
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Darensboure and Darensboure ( 1 )described an exoeriment which illustrates the use of erouo theorv in determinine the aereorhemistr) tn metal complexes. The experiment distmguishes herween cis and trans-wmers of hlotCOJ,L1 romplt-xei un the hnsis d f the ohrewed mtrarrd bands in the CO-stretchmg regiun and the expected number ot intrnred actiw CO.stretching vibrntmns as deriv~dfrum group theoretical con~idernrions. A slightl) ditfrrrnt type vlatereochemistry is illuslrated hy F~I(.'O,.~L, where the ligand can he in an axial purition or equatorial pnr~rionuf a rrigonal hipyramid rnolerulr. To supplement the carlner experiment of the Darcnahourgs, I wish~todiscuss an erperimentkhich will cover this type of stereichemistry. An axially-substituted F e ( C 0 ) L belongs to a C8" point group and is expected to give three infrared absorptions in the CO-stretching region corresponding to 2A1 and E vibrations. An equatorislly-substitutedcomplex, on the other hand, belongs t o a C2, point group which should yield four fundamental CO-stretching vibrations corresponding to 2A1, B I , and B2 (2). One of the easier mono-substituted iron carhonyl complexes t o prepare is triphenylphosphine tetracarhonyl iron (O), F ~ ( C O ) ~ P ( C G HThis S ) ~ .compound is air stable and readily made by the reaction of FedC0)s and triphenylphosphine in tetrahydrofuram'. The infrared spectrum of a CCh solution of Fe(CO)4P(CsH& shows three very strong bands which would be consistent with an axial s u b s t i t u t i ~ n . ~ The infrared spectra of Fe(C0)s and F e d C 0 ) ~in the CO-region present additional application of group theory in determining stereochemistry. In the late 50'3, the strueture of Fe(C0)s was the subject of controversy. There were arguments in favor of a square pyramid strueture ( 5 ) and arguments for a trigonal bipyramid structure (2,6). Group theory predicts two infrared active fundamental CO-stretching vihrations (E' and AS") for the trigonal bipyramid model, while three infrared active CO-stretching vibrations (2A1 and E ) are predicted for the square pyramid structure. The observation of two very strong infrared hands in the CO-stretching region supports the argument in favor of a trigonal bipyramid model which is the currently accepted strueture. The X-ray structure of Fe2(CO)g showed bridging CO-groups (7) in addition to terminal CO-groups
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Two infrared hands (E' and As") are exoected for terminal CO-stretchine vibration. and one infrared hand IE') for n hrldginp ('-0 uhra~iun.The ob&anre k t r u o v e q strong bands nt 2034 and 2080 cm-I. m nddithn r u s strong band nt 192n rm 1 are in agreement with rhcse rxpectalimi. The bands between 1900-2200 cm-' nre rhnracteristw of a lermrnnl CO-group ahilr the hand at 1#2J rm-I is didgnostie cjf nn aldehydic ur k e t r n c rarlumyl whicn would be the case for a bridging carbonyl group. The vibrational spectra of these iron carhanyl complexes serve t o strengthen the confidence which the student could have in soectroscopic methods which are so widelv used in a studv of oreanametallic compounds, thus suoplemeriting the efforts of the Darenshourgs in their earlier experiment. ~~
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Literature Cited
' A 70% yield using this reaction which is an extension of a work by Douglas and Ruff (3). Fe(CO)rP(CsHa)a has also been made in 15-34% .~ vield bv, the reaction of PICcHJ* . . .. .. and FedCO),n . . .. or F d .C O .L. 14). Ir a i nppwprinre t b stress that equatorinl wbmturirm cannot he ruled out wmpletrly ilsill~lnfrarod data alone cincc a lme may he umk ur ~~r.pr.>ppru and w r r p r derwtiun
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R. G. Montemayor
The University of Utah Salt Lake Citv 84112
16 1 Journal of Chemical Education
