JOURNAL O F T H E AMERICAN CHEMICAL S O C I E T Y (Registered in U. S. Patent Office)
VOLC‘ME78
(0 Copyright,
1050. by t h e American Chemical Society)
SEPTEMBER 3 , 1956
NLMBER17
PHYSICAL AND INORGANIC CHEMISTRY [CONTRIBUTION FROM THE CHEMISTRY
DEPARTMENT, PURDUE
UNIVERSITY]
The Infrared Spectrum of Cobalt Carbonyl Hydride’J B Y ~VALTER F. EDGELL,CHARLES MA4GEE3 AND GORDON GALLUP RECEIVED DECEMBER 10, 1955 T h e infrared spectrum of HCo(C0)4 has been determined from 2 to 33 p . The band at 704 cm.-’ is associated with a motion involving the hydrogen atom but no evidence was found for a n 0-H stretching vibration. The spectrum is that of the tetrahedron Co(C0)4 slightly perturbed by the presence of the hydrogen atom and a detailed assignment was made on this basis. The interpretation of the spectrum does not appear to be consistent with earlier models but is consistent with a model in which the hydrogen atom bridges three CO groups.
The infrared spectra of HCo(COj4 and DCoMetallic carbonyls have long been of interest to chemists inasmuch as they exhibit unique types of (CO)4 have been obtained in the interval between chemical bonding. Cobalt carbonyl hydride has 2 and 20 p.lov1l It was concluded that none of the intriguing chemical and physical properties, which observed bands were associated with motion of the include its catalytic effect in the oxo p r o ~ e s s . ~hydrogen atom but no attempt was made to make The structure of this substance, in particular the an assignment of the bands to the fundamental * ~ spec~~ location and the bonding of the hydrogen atom, has modes of vibration. Recent ~ t u d i e sof~ the been the subject of much speculation since its isola- tra of Ni(C0)4 and Co(COj3NO show that the tion.6a.b Hieber7 has proposed that the hydrogen stretching and bending vibrations of the CO units atom is bound as a proton within the core of the co- against the metal atom occur in these molecules balt atom forming a “pseudo-nickel” atom. In an between 17 and 33 p. Besides this region, absorpelectron diffraction study Ewens and Lister8 found tion due to fundamentals would be expected near 5 the four CO groups arranged tetrahedrally about p (observed) and beyond 100 p (not observed). the cobalt atom and suggested that the hydrogen These molecules are isoelectronic with HCO(CO)~. was bound to an oxygen atom in the linear arrange- Thus one expects the presence of infrared bands for ment COH. This view was disputed by Hieber, the hydride just beyond the limits of the earlier Seel and Schneiderg who maintained that the OH study. By extending the investigation to 35 1.1, it bond must make an angle of 90’ or less with the CO might be possible to obtain sufficient data to permit a detailed frequency assignment to be made, bond. which, in turn, could provide structural infornia( 1 ) Based in part upon t h e Ph.D. thesis of Charles Magee, Purduc tion relating to the hydrogen atom. ’This paper reC‘niversity, December, 1954. ports such a study. (3) Presented before t h e Division of Physical and Inorganic Chemist r y a t t h e Cincinnati Meeting of t h e American Chemical Society, April, Experimental 1955. (3) National Science Foundation Fellow, 1952-1954. (4) J. S. Anderson, Quart. Rev., 1, 331 (1947). (5) I. Wender, R . Levine and M. Orchin, THISJ O U R N A L , 72, 4375 (1950). (fi) (a) W. IIirber, Anpew. Chcm , 49, 463 (1936); (b) G . W. Coleniau a n d A. A. Blanchurd, THISJ U I , K N A I . , 58, 2160 (1936). (7) W Hirber, D i e Cheinie, 6 5 , 25 (19.42). See also t h e model fur E’e(CO)rFIs i n W Hit,lwr anct E’. I,eutrrt, Z . a i m r g . C h e m . , 204, 745 (1939) ( 8 ) R . V. G. Ewens and hl. \V. Liiter. Trans. F a r u d u y SOC., 36, 681 (103‘J). ( 0 ) W. Hiebcr. I;. Seel and H. Schneider, C h c m B E Y85, . , (347 (1032)
The HCo(C0)a was prepared by t h e rwctioiib
2coo
+ loco -+-
3CO2(CO)s f 12Py +
-~ _. ~ . -.. -
coz(co)s
+ 2co.
O[CO(I’Y)61 [CO(CO),l?
+ 8CO
(1)
(2)
( I O ) H. Sternberg. I . Wender, K . IJriadel aud h l . Orl.hiu, ‘ ~ H I S J O I I X N A I . . 76, 2717 (1953). ( 1 1 ) R . Friedel, I. Wenrler, S. Shufler and I1 Sternhrra. ihiri 77, 3051 (1965). (12) W. F . Edgell and E . S h u l l , J . Chcm. P h y s , tu be submitted. (13) W. ?J Edgell and C. Xiagee ibid , in p r e s .
ALTER F. EDGELL, CHARLES IUaGEE
41% [cO(py)61[co(co)4lZ
+ HzSO4
--f
2HCo(COh
+ [Co(Py)6lSOa
AND
TABLE I THEINFRARED SPECTRUM OF H C o ( C 0 h
(3)
P y = pyridine The dicobalt octacarbonyl was prepared by a modification suggested t o us by Brimm and Lynch.14 Carbon monoxide was added to a slurry of COO and hexane t o a pressure of 3300 p.s.i. The temperature was raised t o 170" and the autoclave vigorously shaken for 2 hours. The dark solution was decanted and filtered quickly t o remove particles. C o ~ ( C 0 ) gcould be precipitated immediately by cooling the solution to -80" but this gave small crystals which readily decompoyed to pyrophoric cobalt when dry. Larger crystals were grown by maintaining the solution a t 0" for a day. U'heii collected and dried, these crystals could be kept for months if not exposed to the air for long periods of time. Reactions 2 and 3 were carried out as indicated elsewhere.lj The cobalt carbonyl hydride was collected in a trap a t liquid nitrogen temperatures and stored at -80'. The infrared spectrum was obtained in .lO-cm. cells at pres,wres up to 40 mm. I n addition a one-meter cell was u5ed with pressures as high as 30 mm. for the region between 2 and 15 p . XaC1, CaFl and KBr prisms were employed in a Perkin-Elmer Model 21 spectrophotometer to study the region between 2 and 20 p . For wave lengths to 33 p , KBr and CsBr prisms were used in a spectrometer built around a Perkin-Elmer Model 99 monochromator. The envelopes of the bands near 5 p were examined with a LiF prism in this instrument. The frequency calibrations were made with the following reference gasesi6: H20, COP, CO, HC1, HBr and CHI. The hydride decomposed during the rutis, the reaction being rapid a t the higher pressures. Decomposition products, C ~ ( C O ) SC, O ~ ( C O )etc., ~ ~ ,deposited 011 the windows and mirrors OF the cells. To determine which bands properly belong to HCo(CO)4, a run was repeated at periodic intervals for each filling of the cell and the snectrum of the empty cell taken before and after each filling. The infrared spectrum i i ihown in Figs. 1 and 2 and the frequciicies are collected i i i Table I .
\Jd. 7s
GOKDON GALLUP
w(cm.-I)
I
4153 4107 4076 2746 2661 2615 2527 2453 2211 2123 2107 2067R 2062Q 2048R 2043Q 2038P 2013R 2007Q 2000P 1978
W
YZ
W
Uga
W
2vgb Vgb f
\V
w w w
Assign.
VP
+ + +
+ + + + v:, + LW uga
Ygb
Ysb Vi0 ~7
~7 v4
W-
YZ
MI
\\'
Y:,
Yj
s \V
Ygb
vs
~ g a
VVS
V8b
S-
m
Ua
uc13 - U vcI3 - 0
w(cm.-I)
I
1930 1863 1818 1631 1524 1242 1149 1081 1031 983 913 870 850 803 787 752 703 541 467 403 330
w w 15'
\V \V IV
W'
\v w \Y
w
Assign.
+
2~1u 2~10 2Y10 f 3w 2ug -1-
VI
+
+ + 2u; + 31J1 + 07
~4
Y6
YID
YIU
"Ye
u,
1.1
YIB
Yg
Y7
u1
17
ininw w
+ vgS+ 2vs +
\V-
Ygb
s
ulo or ulI
s
~ 7 a ,Y i t )
b1
Y,
~4
+
111
Y4
s
U6n3 b6h
111
1r1
Y;
thetical Co(C0)J skeleton in which all the CO groups are equivalent. The approximate description of the vibrations expected of such a group is found in Table 11. Only the Fz vibrations would be infrared active. When a hydrogen atom is brought up, it perturbs this structure. As a consequence inactive vibrations may become active and degenerate vibrations may split into two or more modes, depending upon the symmetry of the resulting structure. The results of this perturbation are shown in Fig. 3 for the models of Heibe~-,~
I1
.ot-
Fig. 1.-The
infrared spectrum of HCo(COj4.
Fig. :i.--Correlation o r vibrations of v;irioii.; models of I % C O ( C O )with ~ tliose of the C O ( C O ) ~ skeleton.
I l l .l l l d
all
IlY
Fig. 2.-The
I I I POI7
IO.*
I l l IO10.1
I
I l l ,oee
2013
LO07
I I IJ , . a
,mi>
w.-'
,,.I
infrared spectrum of H C O ( C O ) in ~ 5 obtained with LiF prism.
I
p
rcgion
Discussion In discussing the vibrations of cobalt carbonyl hydride it will be convenient to picture a hypo(14) E 0. R r i m m and hI. Lynch, Jr., p r i v a t e cornmiinici~tiun. (15) H . W. Sternberg, I. Weoder, R.A . Friedel a n d M O r c h i n , ?"IS
76, 2717 (1953). (16) A . Downie, M.Magoon, I.. Purcell and B. L. Crdwford, J . Oh! Sot. A n . , 4 3 , !bll (I!J53). JOVRNAI,,
1Swcns and Lister? and Hieber, See1 and Schneidcr9 (known as I, I1 and I11 hereafter). All vibrations are active in I , I1 and 111 except the APmodes. I n these models, as a rough approximation, the hydrogen atom will move with the atom to which it is attached in the skeletal vibrations. Besides these modes, there will be a stretching (Al) anti bending (E) of the Co-H group in I, a stretching (A,) arid bending (E) of the 0-H group in I1 and a stretching of (L41)and two bendings (A1, A") of the 0-13 group in 111. Ni(C0)4 serves as a rough experirnental iiioclel for the unperturbed Co(COj4 skeleton while Co(CO),NO shows the rcsults of a substantial CaVper-
INFRARED SPECTRUM OF COBALT CARBONYL HYDRIDE
Sept. 5, lS5G
4187
TABLE I1 APPROXIMATEDESCRIPTION OF VIBRATIONS C o ( C 0 ) r Skeleton E Fa FI
Ai
~co-(co)* vco-(CO) 6C-0
vc-0 WH(//) @H(
1)
v3
VI
V3
US
v1
V6
YP v2
H C o ( C 0 ) e B r i d g e model E AI E Az E
AI
Vl V8
V¶
Y4 V2
V3a
ISb
V6a
v6b
Vla
Vlb
VSS
V8b
V9n
V9b
v11 v10
6 = bending, v = stretching, (CO) = thiq group move? as a unit, w~ (11) = motion of hydrogen parallel to CSaxis, (1
etc.
WCOhNO
I
1 I I/
I
I
I I
A,
A, EEE
A,
A,
E
A,
turbation. Thus their f r e q u e n c i e ~ , lwhich ~ * ~ ~ are plotted in Fig. 4, provide a key to the skeletal Fig. 4.-Correlation of the vibrations of HCo(CO), with modes in HCO(CO)~.A comparison with the data those of Ni( CO)a and Co( CO)aNO. of Table I shows a striking correlation. The bands a t 330 and 403 em.-' correspond to v1 and sponding to the 703 cm.-' of the hydride. The 555 Y g (see Table 11) found a t 381 and 423 em.-' in Nicm.-l band is just what is expected from the above (CO)4. The bands a t 467 and 541 cm.-' cor- assignment since it corresponds to the Fz (infrared respond to the bending vibrations of the CO groups active) C-0 bending vibration v7 of the tetrahedral v4 and v7 found a t 461 and 540 em.-' in Ni(C0)d. ion. When compared with Co(C0)3NO, 330 and 467 No strong infrared band remains unassigned in cm.-l in the hydride go over to 309 and 467 em.-' the spectrum of cobalt carbonyl hydride. In parin the nitrosyl and 403 and 541 cm.-l split into 391 ticular no band was found which could be associplus 441 and 481 plus 564 cm.-l, respectively. These ated with an 0-H stretching vibration although the correlations are shown in Fig. 4. Those frequencies spectrum was examined from 2 to 15 p with a one in HCO(CO)~ which arise from infrared inactive vi- meter cell a t pressures up to 30 mm. brations in Ni(C0)4 are weak, while those correThis information may be compared with that sponding to active modes are strong. It can be said expected from the several models for cobalt carthat the perturbation of the tetrahedral c O ( c 0 ) 4 bonyl hydride. The fact that the hydrogen atom skeleton by the hydrogen atom in the hydride is great perturbs the tetrahedral Co(CO)4 skeleton enough enough to make all modes allowed by the symmetry to make inactive frequencies active but not to split active, but not to split the degenerate modes in this degenerate modes (observably) in the intermediregion enough to be observable under the moderate ate frequency region from 330 to 540 cm.-' is conresolution available here. sistent with all of the above models. But when the Although not apparent in Fig. 1 the strong band perturbation a t the same time results in a substana t 5 p is doubled. This is more clearly seen in the tial splitting of the C-0 stretching vibrations, one is trace of this region under the higher resolution ob- led to the conclusion that the hydrogen atom is astained with the LiF prism in the double-pass spec- sociated with one or more C-0 groups. This is contrometer, Fig. 2. Here the PQ and R branches are sistent with models I1 and 111but not with I. seen for the 2043 cm.-l band and the Q and R Gutowsky and co-workers18have measured the branches for the band centering a t 2062 cm.-'. proton magnetic resonance spectrum of this moleThese values correspond very closely to the Fz C- cule and found a chemical shift of - 1.55 with re0 stretching vibration found in Ni(CO)4 a t 2057 gard to the hydrogen in water. This means a high cm.-l. In addition the bands a t 2122 and 2043 electron density about the hydrogen and indicates have almost the same position and relative intensi- that vibrations of the hydrogen atom should give ties as the C-0 stretching vibrations in Co(C0)3NO. rise to intense infrared bands. This is the case for It is concluded that 2122, 2043 and 2062 cm.-l the band at 703 em.-'. Therefore the absence of a correspond to C-0 stretching vibrations in HCo- band in the 0-H region should be accepted a t face (C04). The remaining bands in this region have value; the data are also inconsistent with models relative intensities and frequency patterns similar I1 and III.ls to the bands found in Ni(C0)4 and Co(C0)3NO. The agreement is not too bad between the data There remains one strong infrared band a t 703 and the expectations of a model with a weak bond cm.-' which is unassigned. Comparison with the formed by d-s overlap between the cobalt and hyspectra of Ni(CO)a and Co(C0)3NO shows that all drogen atoms.20 When a hydrogen atom is placed the skeletal vibrations expected in this region have in such a position near a face of the Co(CO)4 tetraalready been assigned to observed frequencies. hedron, substantial overlapping also occurs beMoreover no skeletal vibration is expected with so tween its s-orbital and the p-orbitals of the three high a frequency.17 Therefore the 703 em.-' band (18) H . S. Gutowsky, private communication. must arise from a vibration involving the hydrogen (19) T h e bond would presumably be formed by t h e overlap of t h e atom! This assignment is strongly supported by hydrogen s-orbital and t h e oxygen (s-p)-orbital in model I1 and the recent infrared data" on the CO(CO)~-ion which hydrogen s-orbital a n d a n oxygen p-orbital in model 111. These be expected t o form reasonably strong covalent bonds. shows a strong band a t 55.5 cm.-' but none corre- would (20) T h e difference between this and Hieber's model (I) lies in the (17) T h e C-0 stretching vibrations near 2000 cm.-l are not reerred t o in this statement.
position of the hydrogen on t h e face of the C o ( C 0 ) r tetrahedron with little "penetration" of t h e cobalt electron cloud.
carbon atoms and to a smaller extent the p-orbitals of the three oxygen atoms. Thus, bonding should also take placewitli the atoiiir of the three CO groupS. In the general case such boritliiig would be tlescribetl by ati eight-centered Irmlecular orbital but it is coilvenient to speak of the hydrogen as forming a bridgc between the several atoms involved. The bridge i~iotlcl is shown i n Fig. 5 . Onc extreme (if this 0
C
The approximate description of the vibrations of the bridge model is given in Table IT and their correlation with those o f the Co(CO)4 tetrahedrori i i i Fig. 3 . - h i assignment of the observed fuiidanieiitals on this basiscan be found in Table 111. These serve to give a reasonably good representation of the overtone and combination bands in the spectrum, as seen in Table I, despite the fact that values for a t least four fundamental frequencies lie beyond thc region of observation. Thus, the infrared spectrum, taken a t face value, is not consistent with the previous models for cobalt carbonyl hydride. As now understood, it is consistent with a model in which the hydrogen f o r m a bridge (Fig. 5 ) . TABLE III THEFCNDAMENTAL FREQUEXCIES OF HCo(COJ,
Chi.
In
d63, I’ciil 403 330 ~:n, ~ ; b 541 2122 Y j r , Val, 2062, 2043 v3 (El)? VI 40i ~ g n ,v!,I> (309 21 1 )’ Y j 3 , J;i, (%I)? L : O or Y , , ’703 T.‘:ilues in p n r c n t h e ~ care ~ obtniiietl from c o i n t ~ i i ~ ~ t i o i i niitl overtone h i i d \ and are uncertairi.
VI
bridge model of H C O ( C O ) ~ (Bond ortier‘ Heavy lines indicate strong covalcnt bontling aii(1 dotted lines meak covalent bonding )
Fig 3 --The arc not qhown
model limits the covalent bonding of the hydrogen atom to bonding with the cobalt atom and the other extreme t o bonding with the CO groups. While the relative amounts of these two types of covalent bonding in the molecule is a matter for conjecture a t present, a simplified M. 0. calculation favors that involving the CO groups. I n any event the hydrogen is expected to be weakly bound to the Co(CO)4 .;keleton.
/COSTRIRUTION FROM THE
Acknowledgment.-Thanks are due to Drs. E. Brimm and AI. Lynch, Jr., for stimulating discussions and to Dr. R. Friedel and Prof. H. Gutowsky for information in advance of publication. One of us (C.M.) thanks the National Science Foundaticui for a fellowship during 1952-1954. The support o f the Atomic Energy Commission through contract AT(11-1)-1G4 with the Purdue Research Found:rtion is gratefully acknowledged. LAFAYETTE, IXDIASA
DEPARTMEXT O F CHEMISTRY, PURDVE
GSIVERSITY]
A Simple M. 0. Treatment of the Binding of the Hydrogen Atom in the Bridge Model for Cobalt Carbonyl Hydride BY LTTALTER F. EDCELL AND GORDON G.ILT.VP RECEIVED J A N C A R Y 25, 19% It was recently pointed out t h a t the infrared spectrum of cobalt carbonyl hydride was consistent with a model i l l which tlie hydrogen atom occupies a bridging position between the three CO groups and the cobalt atom. Such a model is examined here in terms of elemeiitary molecular orbital theory. A basis for the bonding exists and it should be corisidered as a plausible model. The hydrogen atom bears a negative charge and covalent bonding to the CO groups is stronger than t h a t t o the cobalt atom. I t is pointed out t h a t this model of cobalt carbonyl hydride gives agreement wit11 the known, substantixteti cxperiineiital facts.
Introduction Co(C04)His perhaps the best known example of an unusual class of substances-the metal carbonyl hylrides. Interest in this material is enhanced by the discovery of its catalytic role in the oxo process.’ Its chemical and physical properties? suggest unusual chemical bonding. The structure of this substance, in particular the location and the bonding of the hydrogen atom, has been the subject of much (1) I. Wender, R Levine and lf.O r c h i n , THISJ O I T R N I I . , 72, 437:
(i9m
(?I
1. S. Anclerson, O u o v l . R P W .1, , 931 (1047).
speculation. An early proposal was that of Hielicr:? who suggested that the hydrogen atom is bound as a proton in the core of the cobalt atom, formiiig ;I “pseudo nickel” atom. Ewens and Lister4 stuc!ied the electron diffraction of the gas and found the CO groups arranged tetrahedrally about the cobalt atom. For a structure they proposed that the hydrogen atom gives its electron to the cobalt (3) W. Hieber, Die Chemie, 66, 25 (1942). See also the model for Fe(C0)rHz b y W. Hieber and I-. Leutert, Z . n m ~ tChem., 204, 716 (1932). ( 4 ) R . V C,. llwens xncl \ I . W . I.ister, 7’1,oit.. F r r i i r i i n s , 4rir , 3 5 , i i Y l ‘ 1 !l3!1).
