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 t o 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 t h a t involving the CO groups. I n any event the hydrogen is expected to be weakly bound t o 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).
A BRIDGEMODELFOR COBALT C.\RBOXYL HYDRIDE
Sept. 5 , 19.56
atom, completing the 3d shell, and t h a t the proton is bonded linearly to a CO group through the lone pair of electrons of the oxygen atom. The CO groups are bonded via the hybridized 4s4p3 orbitals of the cobalt atom. This view was disputed by Hieber, See1 and S ~ h n e i d e r who , ~ maintained t h a t the OH bond must make an angle of 90" or less with the CO bond. Edgell, Magee and Gallup6 have interpreted the infrared spectrum6-s of cobalt carbonyl hydride as being inconsistent with these models. They point out t h a t the present knowledge of the spectrum is consistent with a model in which the hydrogen atom forms a bridge between three CO groups and the cobalt atom in a structure of CSvsymmetry. It is the purpose of this note to show t h a t this structure is plausible, ;.e., that a basis for it exists in valence theory and to clarify, qualitatively, the nature of the bonding of the hydrogen atom in such a model. The Starting Orbitals.-The beginning point for this treatment is the Cav symmetry of the model. The proposed spatial arrangement of the atoms' is shown in Fig 1. The four CO groups are bonded to the cobalt atom in part by a-bonds, utilizing the sp3 hybridized 4s4p orbitals of this atom and the (s-p) lone pair electrons of the carbon atoms. AS in other metal carbonyls these bonds are presumed to have some double bond character as a result of resonance with structures of the type Co=C=O. The r-bonds involve 3d orbitals of the cobalt and p orbitals of the carbon atoms.
C
we are primarily concerned with the bonding of the hydrogen atom, we shall restrict our attention to orbitals of this symmetry. The orbitals involved in a-bonding are considered localized and not available for the bonding of the hydrogen atom. However, the cobalt 3dZ2=4co orbital shown in Fig. 2 has the required symmetry. One N
bridge model of Co( CO)dH
The oqbitals may be classified in terms of the CaVsymmetry of the molecule. Those designated as AI are symmetric to rotation of 120" or 240" about the figure axis of the molecule and to reflection through the three planes of symmetry. Orbitals of type A2 or E are not symmetric to one or more of the above operations. This classification is important because only orbitals of the same symmetry may overlap to form bonds. The hydrogen Is orbital, hereafter called &,I is of type AI. Since (5) W. Hieber, F.See1 and H Schneider, Chem. Bey., 86,647 (1952). (6) W. F. Edgell, C. Magee and G. Gallup, THISJOURNAL, 78, 4185 (1956). (7) H.Sternberg, I. Wender, R. Friedel and M. Orchln, ibid., 7 6 , 2717 (1953) (8) R . Friedel, I. Wender, S. Shufler a n d H. Sternberg, ibid., 7 7 , 3951 (1955). Some implications of t h e short wave length spectrum were discussed in references (7)a n d (8) but no conclusions were reached regarding the location and bonding of the hydrogen atom.
N
-005
005
0 05
-0 0 5
Fig. 2.-The
cobalt 3dz2 = +c0 orbital
molecular orbital formed from a p orbital of each of the three symmetrically equivalent carbon atoms is of type XI. *Is seen in Fig. 3 the axis of greatest
1
V
Fig. 1.-The
4189
Fig. 3.-The
\
/
orientation of the carbon p orbital in +c.
electron density of each p orbital is oriented to intersect the figure axis of the molecule. A similar molecular orbital may be formed from the p orbitals of the oxygen atoms. I n the LCAO approximation these are
40
=
- (PO(1) + PO(2) + PO(3)I 43 1
If one electron is placed in each of the four AI orbitals, there are just enough electrons left to fill the bonding orbitals discussed above and the remaining atomic orbitals. The electronic configuration corresponding to single bonds between the cobalt and carbon atoms is [ C o : K L ( 3 ~ ) ~ ( 3 ~ ) ~ ([ 3Cd:)K* ] ~ [ O : K ( ~ S ) ~ ] ~ ( U l h 4 ) d U I 2 1 r 1 2 ) U * 8 dc+c+odlr
Vol. 78
L\-ALTER F. EDGELL AND GORDON GALLCP
4190
Here u1 and i r 1 refer to bonds between the carbon and oxygen atoms and uz and i r 2 to bonds between the cobalt atom and the carbon atoms. Resonance of the double bond u12ir12between the three symmetrically equivalent CO groups is implied. Similar configurations may be written corresponding to various amounts of double bonding between the cobalt and carbon atoms. For example with two double bonds one has
tributions due to bonding between carbon and oxygen atoms, cobalt and carbon atoms and hydrogcn and cobalt atoms. However, maximum covalent bonding between two orbitals takes place when the energy of the electron in each orbital is equal. As the energy difference of the two orbitals increases, the mixing of the orbitals and the shift in the energv levels decreases, and the bond is formed with increasing charge transfer. This factor is important [Co : I(L(3s)'( 3 ~ ) ~ ( 3 d[)Cl :]K]r[O :K( ZS)'] 2 1 0 :K ( 2 ~ ) * ( 2 p ) ~ ] a here. To obtain accurate energy levels is beyond the ( U I 2 T l 4 ) ( U I 2 T l 2 ) 3 UZ8TZ4 #Co#CdOdH scope of the present work. Approximate energies The problem of the bonding of the hydrogen atom suffice, however, for the purpose of obtaining a qualis thus reduced t o the question of how the four orbi- itative description of the hydrogen binding. The tals, +c~+c+&H, combine to form bonds. energy values for the hydrogen and cobalt electrons Overlap and Energy Considerations.-A cri- are taken from appropriate spectroscopic term valterion for the tendency of two orbitals + A and +B t o ues of the atoms. I n the spirit of the type of apform a bond is the degree of overlapping measured proximations being made here, the energy terms inby the integral volving carbon atoms or oxygen atoms of different s = f +*A #B dv CO groups are dropped. One is led t o approximate These have been calculated for the integrals of in- the energy of an electron in $C or $0 by that of the terest t o this problem. Slater atomic orbitals were p electron of the carbon or oxygen atom. These valused throughout and values taken from tables of ues are given in column one and two of Fig. 3. I t is overlap integralsgwhere possible. Corrections were seen that the small overlapping of 4~ with 40 will made for the fact t h a t the hydrogen atom does not be utilized to the fullest extent while the effect of lie on the symmetry axis of the p orbitals in evalu- the larger +H+C overlap will be reduced by that ating SHCand SHO. Figure 4 shows these quan- energy difference. I t is clear t h a t any tendency tities as a function of the Co-H distance along the to covalent bonding between the cobalt and the C3 axis. The salient feature of this diagram is the hydrogen atom must be weak. fact t h a t the hydrogen overlap integrals 911 have maxima a t a Co-H distance just short of 2 A. Thus one would expect the hydrogen to be located near this position. A second feature is the relatively large value of the +H+C overlap and the relatively 0 small value of the 4 ~ 4 overlap.
I
0.7
2 E. v,
Fig. 5.-The approximate location of the atomic, free radical and molecular orbitals in cobalt carbonyl hydride.
The Bonding Orbitals.-The molecular orbitals governing the binding of the hydrogen atom are of the form 4,
= 2 a,,
@,
where the + j are the four AI orbitals discussed above. Of course, there are four $, which are t o be classified as bonding, antibonding and possibly non-bonding. The coefficients ai, were chosen by the variational method, which leads t o the secular equation for the energies of the $,orbitals Fig. 4.-The
overlap integrals.
If all other things were equal, Fig. 4 would suggest that the primary bonding is between the hydrogen and the carbon atoms with smaller con(9) R . S. hfulliken. C. A . Rieke, D Orloff and H. Orloff, J . Chcm. P h y s , 17, 1248 (1949).
lHik
- ESjkl
= 0
I n carrying out the calculations, the following approximations were made H,, = E , , S,, = 1
A BRIDGEMODELFOR COBALT CARBONYL HYDRIDE
Sept. 5 , 1936
E j is the energy of an electron in the dj orbital and
-0.10
the S j k are the overlap integrals (both discussed abovej. R j k is the distance between an atom of the kind associated with + j and one associated with Cpk. All integrals between non-adjacent atoms were set equal to zero. Two ca!culations were made, one for a Co-H distance of 2 A. and the other with the hydrogen a t infinity. The latter gives the energies and orbitals of a cobalt carbonyl radical predisposed to bond with the hydrogen atom and which is written as Co(CO)4*. The orbital energies obtained in the latter case are shown in column two of Fig. 5 while the orbitals themselves are
n
,
I
4191
N
/
/
\
\
bonding:
$v = 0 . 3 5 6 4 ~f O.Si64o f 0 . 0 1 0 ~ ~ ~ antibonding : $VI = 0 . 8 6 7 4 ~- 0.558+0 -t 0.316& antibonding : +VII = 0 . 4 7 8 4 ~- 0.22340 - 0 . 9 6 5 4 ~ ~
In the state of lowest energy, two electrons are found in $V and one in $vl. The energies of the four orbitals of Co(C0)dH are found in column three of Fig. 5 while the corresponding orbitals are bonding: $1
+ 0.39640 -I- 0 . 6 1 2 4 ~f 0 . 0 0 5 4 ~ ~
H Fig. 6.-The
= 0.416@c
covalent bonding molecular orbital carbonyl hydride.
$1
in cobalt
non-bonding : $11 = 0.005+c - 1.00Oqbo f 0 . 6 2 5 4 ~- 0.001qh antibonding : $111 = 0.4134~- 0.209+0 - 0 . 3 9 0 + ~f 0 . 8 5 7 4 ~ ~ antibonding : $ 1 ~ = 1 . 0 0 0 4 ~- 0.074@0 - 0 . 6 3 3 4 ~- 0.548+co
The approximate nature of these values should be stressed; they serve to give a qualitative picture of the bonding. In the ground state of the molecule, two electrons are found in $1 and two in $11. Figures 6 and 7 show the contours of these orbitals on the CoCOH plane. Here one can see the sharing of the electron pair in $1 between the hydrogen atom and the carbon atoms, between the carbon and oxygen atoms, and, to some extent, between the hydrogen atom and the oxygen atoms. The electron pair in $11 is concentrated on the oxygen atoms and the hydrogen atom with a node between them. Such an arrangement may be described as a hydrogen bridge between the atoms involved. This qualitative calculation puts most of the covalent bonding of the hydrogen atom to the CO groups. The charge distribution may be calculated from the squares of the coefficients appearing in the equation for each occupied orbital. Of the two electrons in $1, slightly more than one is associated with the hydrogen atom while slightly less than 1/6 of an electron appears on each of the three carbon and oxygen atoms involved in the bonding. Of the two electrons in $11, slightly more than l/z an electron appears on the hydrogen atom while slightly less than ‘/z an electron is on each of the three oxygen atoms. While additional contributions to the charge on the carbon, oxygen and cobalt atoms are made from the rest of the bonding, the total charge on the hydrogen atom is given by the above. Thus 1.?I electrons is associated with the hydrogen atom, and it i s surrounded by a sheath of negative charge.
/
0.20
L O
Fig. 7.-The
non-bonding molecular orbital carbonyl hydride.
$11
in cobalt
The above picture would be more satisfying to some if it could be represented in the valence bond picture through a set of resonating structures. This is very awkward to do (as is CO itself) if the usual unhybridized meanings are given to the bonds and the formal charge distribution found in the customary manner. Structures I, I1 and I11 below are obtained by using the hybridization discussed by Coulsonloa and Moffit’Ob for CO. Charge transfer (10) (a) C . A. Coulson, “Valence,” Clarendon Press. Oxford, 1952. (b) W. Moffit, Proc. Roy. Soc. (London), 1968,624 (1949).
p. 212;
upon bond formation produces a charge on each atom of the sign which is circletl. Only one C'O group is shown. 1 is fornietl by puttinq the four C O groups tetrahedrally about the cobalt atom in its sp3 orbitals. The negative charge on the cobalt atom is reduced by the transfer of the d elcctrori to the hydrogen atom on the C3axis.
I
111
I1
Structure I1 may be obtained from I by removing an electron pair from one CO s bond and placing it in the p orbital of the oxygen followed by bond formation between the p orbital of the carbon and the s orbital of the hydrogen using the electron pair previously on the hydrogen. Of course there are three resonating structures like 11. Structure I11 enters into the resonance composite to counter-act the negative charge on the cobalt atom in structures I and 11. Comparison with Experiment.-The bonding found for the bridge model is in qualitative agreement with the substantiated experimental facts now known for CO(CO)~H. It predicts a diamagnetic molecule. The weak covalent bonding of the hydrogen atom agrees with the ease of decomposition t o form Hz and c o ~ ( C 0 ) ~T.h e large negative charge on the hydrogen atom calls for the large negative chemical shift found by Gutowsky and coworkers" in the proton magnetic resonance spectrum. And an interesting prediction may be made about the CO stretching vibration in CO(CO)~-, which may be described as having two electrons in $" and two in followcd by the delocali7ation of (11) H 3 ' Giltowsky, private report
the charge. This latter process will riot chniige the general c-hnractvr of tht: orbitals. Since, $1 1 is ; i i i tibonding will: rpg:tr(:s to the CO bond, cine exl'ccts the frequency oi the CO stretching vibration in Co(CO)1- to be less than those in C O ( C O ) ~ H .The exare6 2122, 2043 perimental values ior C O ( C O ) ~ H and 2062 em.-' while the value for C O ( C O ) ~is3 1S83 cm.-l, in striking agreement with theory. The question naturally arises as to how a 111olccule with a negative charged hytlrogen can tlissolvrin water to form a strong acid. This proccs.: in:iy he divided into the following steps Co(COj,H(gj +Co(CO),*--(g) Co(C0j;" -(g) -+ Co(cOl4-(g) H + ( g ) --+ I l + ( a q ) Co(COJ)-(gi --+Co(CO),-(ayj Co(CO)JI(g) --+ Co(C0j4-(,zqj
+ TWg)
f8.2 e . v . 11 )
-H
( 21 13)
-11.1 -1.7
(1)
- 4 . 9 -I
