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The reaction of H atoms with Ni(CO)4: EPR spectrum of (OC)2NiCHO in krypton
Volume 111.
number 6
CHEIlICAL
PIIYSICS
I6 November
LETTERS
1984
THE REACTION OF H ATOMS WITH Ni(C0)4: EPR SPECTRUM OF (OC),NiCHO IN KRYPTON” J.R. MORTON and K.F. PRESTON Division of Chemistry. Received 3 1 August
is
National Rcsearcl~ Council of Gmada. Ottawa, Onmario. Canada
h-IA OR9
1984
The dg species (OC)2NiCH0 is formed by the rcnction of hydrogen atoms not thermally interconvcrtible with its isomer HNi(CO),.
1. Introduction
with Ni(CO)a
in n krypton matrix at 77 IL
It
We recently began a systematic study of the EPR spectra and structures of transition-metal carbonyl radicals trapped in a krypton matrix_ One aspect of this
The EPR spectrometer was a Varian E 12 system equipped with the usual field-and frequency-measuring devices, and with an Oxford Instruments Ltd_ ESR 9 liquid-helium cryostat_ The spectra were analysed by computerized diagonalisation of the spin matrix.
work concerns the reaction of H atoms with metal carbonyls, and is of relevance to catalytic prbcesses and the reduction of carbon monoxide in particular_ Our
3. Results
studies of the reaction of H with Ni(C0)4 at 4 K, for example,showed [ I] that the exclusive mode of attack at that temperature was carbonyl displacement resulting in the formation of the radical HNi(CO), _The presence ofonly traces of the formyl radical suggested that attack at the carbonyl function was of little importance_ The purpose of the present article is to discuss the spectra and structure of an isomer of HNi(CO)3, namely (OC)2NiCH0, observed as a product of the reaction between H and Ni(CO), at 77 K. The two isomers do not appear to interconvert thermally_
A sample of Ni(CO)4 and HI in krypton,y-irradiated or UV photolysed and examined at 77 K, revealed the spectrum of an axial species having an almost isotropic proton hyperfine interaction of 100 G_ The principal g-values were gzz= 2_0024(2),g,,=g,,=. 2_0207(2). On cooling to 4 K, the transitions corresponding to 2.0207 split, revealing principal g values in the _r)
9074.3 MHz
2. Experimental The samples were prepared by condensing into a 4 mm Suprasil tube 4 I.tmole of Ni(CO),, 2 pmole of HI and 2500 /.rmole of krypton. The sample tube was then sealed off under vacuum and annealed at -130°C to facilitate dissolution of the solutes in the krypton. In certain samples Ni(C0)4 enriched in 61Ni(86%) or 13C (99%) was used. The samples were irradiated at 77 and 4 K. *
NRCC
No. 23548.
Fig. L EPR spectrum of (OC)zNiCHO
(995
t3C) in krypton
at 77 I(.
611
\‘
1 11. number 6
CHEMICAL
Table 1 Principle g factors and their hyperfiie interactionsa) (OC&NiCHO in krypton ar 77 and 4 K
s OfI *Ni T-(l) =c(2)
T(E)
x
77 4 77 4 77 4 7;
2.0207 b) 2.0207 2.0181 2.0225 289.3 189.3 300.8 277.6 ==I2 112 =I5 =8 103.0 103.0 100 100 not resolved 5.5 5.5
77 4
Y
for
I
2.0024 2.0025 277.7 281.2 Ill 112 158.0 161 3.5 4.2
a) Ilypcrfinc interactions in MHz_ b) ,:_rrors * arc’ 12 in the last d$it given. plane of2.0225
and 2.01Sl
;gzz
was lI11affected.
of enriched samples (fig. 1) we were able to demonstrate the presence of a single nickel atom in the ~nolcculc plus rhrce carbon atoms, one of which was different from the other two. A list of rhc principal g factors and corresponding hypcrflne inrcractions is given in table 1_ The mw species (A) was not interconvcriblc with HNi(CO),: samples photolyscd at 4 K contain 11Ni(C0)3 and no A [ 1 ] _On warming to 77 K, I INi(COj3 disappears but A is not formed.Conversely, samples irradiated at 77 K contain A at 77 and 4 K, as WC have seen. No conversion to HNi(C0)3 occurs at 4 K. With
Ihe aid
4. Discussion Our lirst hypothesis, when we observed the specwhose parameters arc given in table 1, was that we were dealing with a conformer of HNi(C0)3 _This hypothesis was. however, destroyed by the fact that ~hc two species were not thermally interconvertible. Wr were therefore forced to seek a structure for A which was isorneric with HNi(C0)3, since A evidently also containsa proton,a nickel atom and three carbon atoms. One possibility is the following: II UIII
oc oc 612
>Ni-C
PHYSKS
LETTERS
16 Novcmbcr
1984
This structure offers a ready explanation of the fact that the rotation about z which is occurring at 77 K does not exchange all three carbons, but leaves C(1) more or less unaffected_ It also explains why the other two carbon atoms are equivalent at both temperatures, being connected by the _YZsymmetry plane. And finally, it offers an explanation of the proton hyperfine tensor, whose maximum principal value does not lie alongz. Indeed, we calculate from the proton hyperfine anisotropy and the C(Zpz) spin density that the angle between CH and the z axis is appro,ximately 68”, a not unreasonable value. The unpaired spin-population is shared between H Is (O-20), Ni3d,z (O-32), C(l) 2pz (0.19) and C( 1) 2s (0.03). These estimates [2] leave no doubt that we are dealing with a species belonging to the A’ representation in C, symmetry. A rationale for the occupation of an a’ orbital by the unpaired electron of a Ni*(dg) species in a C, environment may be found in the work of Evans [3] on the structure of Ni(CO)3. His C2,, structure for Ni(C0)3 has the electronic configuration a~b~b~a~a:. Lowering the symmetry to C, by replacingthe unique CO by an HCO ligand we obtain a”2a”2a’2a’2a’1 _The proximity of the three a’ levels and the relative remoteness of a” offers an explanation of the observed large Ag_,,,, and small Agzz _We assume that non-zero Ag,, is the result of incompletely quenched rotation about the z axis. The inefficiency of addition of H to the carbonyl functions at 4 K implies the presence of an energy barrier in that reaction mode. An activation energy of 1.7 kcal/mole has been reported for the addition of H to CO in the gas phase [4]. A barrier of that magnitude would certainly prevent addition at 4 K but not at 77 K. If H atom addition to the metal proceedswith little or no activation energy, that mode of reaction will be dominant at 4 K.
References [ 11 J.R. hlorton and K.F. Preston, J. Chcm. phys., to be published. 121 J.R. hlortonandK.F.Preston,J.Ma~n. 577. [3] J. Evans, J. Chem. Sot. Dalton [4] D.L. Baulch, D.D. Drysdale, J. Evahmted kinetic data for hi.@ Vol. 3 (Butterworths, London,
Rcson.30(1978) (1980) 1005. Dunbury and S. Grant, temperature reactions. 1976) p_ 327.
number 6
CHEIlICAL
PIIYSICS
I6 November
LETTERS
1984
THE REACTION OF H ATOMS WITH Ni(C0)4: EPR SPECTRUM OF (OC),NiCHO IN KRYPTON” J.R. MORTON and K.F. PRESTON Division of Chemistry. Received 3 1 August
is
National Rcsearcl~ Council of Gmada. Ottawa, Onmario. Canada
h-IA OR9
1984
The dg species (OC)2NiCH0 is formed by the rcnction of hydrogen atoms not thermally interconvcrtible with its isomer HNi(CO),.
1. Introduction
with Ni(CO)a
in n krypton matrix at 77 IL
It
We recently began a systematic study of the EPR spectra and structures of transition-metal carbonyl radicals trapped in a krypton matrix_ One aspect of this
The EPR spectrometer was a Varian E 12 system equipped with the usual field-and frequency-measuring devices, and with an Oxford Instruments Ltd_ ESR 9 liquid-helium cryostat_ The spectra were analysed by computerized diagonalisation of the spin matrix.
work concerns the reaction of H atoms with metal carbonyls, and is of relevance to catalytic prbcesses and the reduction of carbon monoxide in particular_ Our
3. Results
studies of the reaction of H with Ni(C0)4 at 4 K, for example,showed [ I] that the exclusive mode of attack at that temperature was carbonyl displacement resulting in the formation of the radical HNi(CO), _The presence ofonly traces of the formyl radical suggested that attack at the carbonyl function was of little importance_ The purpose of the present article is to discuss the spectra and structure of an isomer of HNi(CO)3, namely (OC)2NiCH0, observed as a product of the reaction between H and Ni(CO), at 77 K. The two isomers do not appear to interconvert thermally_
A sample of Ni(CO)4 and HI in krypton,y-irradiated or UV photolysed and examined at 77 K, revealed the spectrum of an axial species having an almost isotropic proton hyperfine interaction of 100 G_ The principal g-values were gzz= 2_0024(2),g,,=g,,=. 2_0207(2). On cooling to 4 K, the transitions corresponding to 2.0207 split, revealing principal g values in the _r)
9074.3 MHz
2. Experimental The samples were prepared by condensing into a 4 mm Suprasil tube 4 I.tmole of Ni(CO),, 2 pmole of HI and 2500 /.rmole of krypton. The sample tube was then sealed off under vacuum and annealed at -130°C to facilitate dissolution of the solutes in the krypton. In certain samples Ni(C0)4 enriched in 61Ni(86%) or 13C (99%) was used. The samples were irradiated at 77 and 4 K. *
NRCC
No. 23548.
Fig. L EPR spectrum of (OC)zNiCHO
(995
t3C) in krypton
at 77 I(.
611
\‘
1 11. number 6
CHEMICAL
Table 1 Principle g factors and their hyperfiie interactionsa) (OC&NiCHO in krypton ar 77 and 4 K
s OfI *Ni T-(l) =c(2)
T(E)
x
77 4 77 4 77 4 7;
2.0207 b) 2.0207 2.0181 2.0225 289.3 189.3 300.8 277.6 ==I2 112 =I5 =8 103.0 103.0 100 100 not resolved 5.5 5.5
77 4
Y
for
I
2.0024 2.0025 277.7 281.2 Ill 112 158.0 161 3.5 4.2
a) Ilypcrfinc interactions in MHz_ b) ,:_rrors * arc’ 12 in the last d$it given. plane of2.0225
and 2.01Sl
;gzz
was lI11affected.
of enriched samples (fig. 1) we were able to demonstrate the presence of a single nickel atom in the ~nolcculc plus rhrce carbon atoms, one of which was different from the other two. A list of rhc principal g factors and corresponding hypcrflne inrcractions is given in table 1_ The mw species (A) was not interconvcriblc with HNi(CO),: samples photolyscd at 4 K contain 11Ni(C0)3 and no A [ 1 ] _On warming to 77 K, I INi(COj3 disappears but A is not formed.Conversely, samples irradiated at 77 K contain A at 77 and 4 K, as WC have seen. No conversion to HNi(C0)3 occurs at 4 K. With
Ihe aid
4. Discussion Our lirst hypothesis, when we observed the specwhose parameters arc given in table 1, was that we were dealing with a conformer of HNi(C0)3 _This hypothesis was. however, destroyed by the fact that ~hc two species were not thermally interconvertible. Wr were therefore forced to seek a structure for A which was isorneric with HNi(C0)3, since A evidently also containsa proton,a nickel atom and three carbon atoms. One possibility is the following: II UIII
oc oc 612
>Ni-C
PHYSKS
LETTERS
16 Novcmbcr
1984
This structure offers a ready explanation of the fact that the rotation about z which is occurring at 77 K does not exchange all three carbons, but leaves C(1) more or less unaffected_ It also explains why the other two carbon atoms are equivalent at both temperatures, being connected by the _YZsymmetry plane. And finally, it offers an explanation of the proton hyperfine tensor, whose maximum principal value does not lie alongz. Indeed, we calculate from the proton hyperfine anisotropy and the C(Zpz) spin density that the angle between CH and the z axis is appro,ximately 68”, a not unreasonable value. The unpaired spin-population is shared between H Is (O-20), Ni3d,z (O-32), C(l) 2pz (0.19) and C( 1) 2s (0.03). These estimates [2] leave no doubt that we are dealing with a species belonging to the A’ representation in C, symmetry. A rationale for the occupation of an a’ orbital by the unpaired electron of a Ni*(dg) species in a C, environment may be found in the work of Evans [3] on the structure of Ni(CO)3. His C2,, structure for Ni(C0)3 has the electronic configuration a~b~b~a~a:. Lowering the symmetry to C, by replacingthe unique CO by an HCO ligand we obtain a”2a”2a’2a’2a’1 _The proximity of the three a’ levels and the relative remoteness of a” offers an explanation of the observed large Ag_,,,, and small Agzz _We assume that non-zero Ag,, is the result of incompletely quenched rotation about the z axis. The inefficiency of addition of H to the carbonyl functions at 4 K implies the presence of an energy barrier in that reaction mode. An activation energy of 1.7 kcal/mole has been reported for the addition of H to CO in the gas phase [4]. A barrier of that magnitude would certainly prevent addition at 4 K but not at 77 K. If H atom addition to the metal proceedswith little or no activation energy, that mode of reaction will be dominant at 4 K.
References [ 11 J.R. hlorton and K.F. Preston, J. Chcm. phys., to be published. 121 J.R. hlortonandK.F.Preston,J.Ma~n. 577. [3] J. Evans, J. Chem. Sot. Dalton [4] D.L. Baulch, D.D. Drysdale, J. Evahmted kinetic data for hi.@ Vol. 3 (Butterworths, London,
Rcson.30(1978) (1980) 1005. Dunbury and S. Grant, temperature reactions. 1976) p_ 327.