Thermal dissociation of iron carbonyls during growth of iron whiskers

Article abstract of DOI:10.1007/s11172-005-0048-3

Methods for the preparation of iron whiskers in chemical transport reactions of thermal dissociation of iron penta- and dodecacarbonyls and carbidocarbonyl clusters Fe₅C(CO)₁₅ were described. The morphology, structure, and chemical composition of the whiskers were studied. The main factors determining the growth rate and mechanical properties of the whiskers were revealed. A model for the mechanism of thermal dissociation of iron carbonyls was proposed. This process was shown to be a chain radical ion reaction initiated via the scheme of activating complex formation. Analogies between the thermal dissociation of iron carbonyls in the adsorption layer and the known radical ion processes in the liquid and gas phases were found.

Full text of DOI:10.1007/s11172-005-0048-3

Russian Chemical Bulletin, International Edition, Vol. 53, No. 9, pp. 1903—1913, September, 2004
1903
Thermal dissociation of iron carbonyls during growth of iron whiskers
V. N. Babin,a Yu. A. Belousov,aꢀ I. V. Dobryakova,b Yu. S. Nekrasov,a V. G. Syrkin,b† and A. A. Uel´skiib
aA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5085. Eꢀmail: belyur@ineos.ac.ru
bState Research Institute of Chemistry and Technology of Organoelement Compounds,
38 sh. Entuziastov, 111123 Moscow, Russian Federation.
Fax: +7 (095) 273 1323
Methods for the preparation of iron whiskers in chemical transport reactions of thermal
dissociation of iron pentaꢀ and dodecacarbonyls and carbidocarbonyl clusters Fe5С(СО)15
were described. The morphology, structure, and chemical composition of the whiskers were
studied. The main factors determining the growth rate and mechanical properties of the
whiskers were revealed. A model for the mechanism of thermal dissociation of iron carbonyls
was proposed. This process was shown to be a chain radical ion reaction initiated via the
scheme of activating complex formation. Analogies between the thermal dissociation of iron
carbonyls in the adsorption layer and the known radical ion processes in the liquid and gas
phases were found.
Key words: whiskers, iron carbonyls, thermal dissociation, reaction mechanism.
Metal whiskers are characterized by high ratios
(20—25) of the length to apparent diameter and represent
a peculiar type of inorganic material finding wide use in
the recent time due to its perfect crystal structure and, as a
consequence, a high mechanical strength, a considerable
resistance toward oxidation, and unique electrophysical
properties. In particular, they are used for the developꢀ
ment of new composite materials with metallic and polyꢀ
meric matrices and for the production of magnetoꢀ
dielectric cores and highꢀefficiency emitters.1
Different methods for growing metal whiskers in the
solid, liquid, and gas phases are known. Of these methods
the simplest and most promising is the gasꢀphase thermal
dissociation of metal carbonyls during a transport reacꢀ
tion in which the chemical nature of the starting compoꢀ
nents changes. The formation of small amounts of iron
whiskers (IW) from iron pentacarbonyl (IPC) and hydroꢀ
gen was observed for the first time in a chamber of an
electron microscope.2,3 However, lacking data on the
mechanism of metal carbonyl dissociation did not allow a
preparative method of αꢀiron IW preparation to develop.
The purpose of this work was to search for optimum conꢀ
ditions of preparing αꢀiron whiskers from iron carbonyls,
to reveal the main factors affecting the formation of the
structure, chemical and phase compositions of iron whisꢀ
kers, and to establish a scheme of thermal dissociation of
metal carbonyls.
Experimental
Iron pentacarbonyl Fe(СО)5, iron dodecacarbonyl
Fe3(СО)12, and carbidocarbonyl cluster Fe5С(СО)15 were used
as starting compounds for studying thermal dissociation.
Industrial IPC was purified by distillation.
Iron dodecacarbonyl was synthesized by refluxing IPC and
triethylamine in an aqueous solution followed by treatment with
hydrochloric acid and purified by washing with water, methaꢀ
nol, and hexane.4,5
Iron carbidocarbonyl cluster Fe5С(СО)15 was synthesized
according to a previously described procedure.6,7
All carbonyls used are volatile and easily decompose,
which makes it possible to significantly decrease the temꢀ
perature of thermal dissociation compared to that, e.g., of
oxides, to reject the use of explosive and corrosionꢀactive meꢀ
dia (hydrogen, chlorine, etc.), and to use supports of different
shapes. Wire and plates of different steels, Nichrome, and moꢀ
lybdenum, carbon rods, and ceramic tubes served as support
materials.
The temperatures of a support and a forming iron sublayer
were measured on a VIMPꢀ015 optical micropyrometer with an
accuracy of 50 °С.
The residual pressure in a decomposition system was meaꢀ
sured with a VTꢀ2AꢀP vacuum gauge.
Electron microscopic analysis was carried out by transmisꢀ
sion and scanning electron microscopy on SЕМꢀ505 (Philips)
and HUꢀI25 (Hitachi) microscopes. The accelerating voltage
was 20—35 kV.
The chemical composition of the metallic sublayer was moniꢀ
tored on an EDAX Xꢀray energyꢀdispersion spectrometer
mounted on an SЕМꢀ505 microscope.
† Deceased.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1826—1836, September, 2004.
1066ꢀ5285/04/5309ꢀ1903 © 2004 Springer Science+Business Media, Inc.

1904
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
Babin et al.
ESR spectroscopic study was carried out on an ERSꢀ221
instrument in the X range (~9.45 GHz) with a highꢀfrequency
field modulation of 100 kHz.
Results and Discussion
The thermal dissociation of IPC, Fe3(CO)12, and
Fe5С(СО)15 in the gas phase affords iron whiskers. Each
of the three starting substances is characterized by specific
regularities of IW growth. A single crystal character of the
obtained IW was established from the electron microꢀ
scopic and electron diffraction data. The diffraction patꢀ
tern obtained in an electron microscope (accelerating voltꢀ
age 100 kV) for a single whisker and its interpretation are
presented in Fig. 1.
Apparatus for IW synthesis. Thermal dissociation of iron
carbonyls affording iron whiskers was performed in reactors of
two types developed by us. The first reactor is horizontal with a
tangential supply of a metal carbonyl vapor; it was designed for
studying liquid iron pentacarbonyl. The second reactor is deꢀ
signed for studying solid metal carbonyls; it differs from the
firstꢀtype reactor by the presence of a sublimator (instead of an
evaporator) to vaporize iron carbonyls and by the mode of supꢀ
plying the vapors to the support (along the normal).
Since the thermal dissociation of iron carbonyls to form IW
is a chemical transport reaction in which a vapor of the subꢀ
stance under study is transported from a "cool" zone to a heated
support, both types of reactors were included into a technologiꢀ
cal scheme containing a system of valves and connections to
control a vapor flow of metal carbonyls, units of condensation
and additional decomposition of unreacted substances, devices
for combustion of evolving CO, instruments for the temperature
and pressure control during the process, a pump, and some
other units. The support was electrically heated.
<111>
<100>
<110>
Procedure for preparation of iron whiskers from Fe(СО)5,
Fe3(СО)12, and Fe5C(CO)15. The starting sample as a wire or a
plate was purified from contaminations, weighed with an anaꢀ
lytical balance, and mounted between electrodes of a vacuum
chamber, which was hermetically closed and evacuated with a
vacuum pump to a residual pressure of 9•10–3 Torr. Then the
sample was heated at a specified temperature for 1—1.5 h. When
an experiment was carried out with crystalline iron carbonyls,
the sublimator was simultaneously heated to increase the vapor
pressure of the substance under study. Iron carbonyl vapor was
transported to a heated support by a system of valves and deꢀ
composed on the support to form IW. The residual IPC vapor
and evolving carbon monoxide were utilized in additional deꢀ
composition and afterꢀburning systems. An observation window
of the reactor made it possible to monitor the course of experiꢀ
ment. After the end of experiment, the whole system was cooled
to room temperature, the chamber was opened, and the resultꢀ
ing sample was weighed and examined.
ESR procedure for studying the thermal dissociation of iron
carbonyls. An ampule 2—2.5 mm in diameter was preꢀevacuated
to 10–3—10–4 Torr, filled with a small amount of Fe(CO)5 or
small crystals of Fe3(CO)12 or Fe5С(СО)15. Then the ampule
was again evacuated and sealed (without thawing out the mixꢀ
ture). The thermal dissociation of iron carbonyls was carried out
for 1—10 s at ∼1000 °С until iron traces or an iron mirror
appeared on the ampule walls. A longer heating resulted in the
appearance of a considerable amount of a ferromagnetic phase,
which strongly impeded ESR spectra recording. Iron carbonyl
vapor in only one end of the ampule were subjected to decompoꢀ
sition. After heating, the ampule was rapidly frozen to the liquid
nitrogen temperature (–196 °C) and placed in a resonator of an
ESR spectrometer. The temperature of a standard sample of
Cr3+/MgO with g = 1.97965 was always maintained at 20 °С.
ESR spectra were recorded at temperatures from –196 °C
and higher. Signals were assigned on the basis of comparing
the data obtained in this work and published results for the
behavior of iron carbonyl radical anions in solution and adsorpꢀ
tion layers.
<111>
<100>
– –
<110>
334
– –
224
– –
332
– –
114
– –
222
–
330
–
004
– –
112
–
–
–
220
114
–
332
–
002
–
110
–
–
224
–
–
–
112
000
222
–
112
–
–
222
–
–
224
110
002
–
–
–
114
332
–
220
–
112
004
–
330
–
–
114
222
–
–
332
224
–
334
Fig. 1. Diffraction pattern of a particular αꢀFe single crystal
containing a twin in the plane (121) and interpretation of the
pattern (black points indicate reflections from the matrix, light
points show reflections from the twin, and sign "+" indicates
reflections caused by double reflection).

Thermal dissociation of iron carbonyls
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
1905
<111>
(110)
a
(011)
– –
(011)
(111)
1.5 µm
Fig. 2. Interpretation of the cutting and the scheme of planes of
an αꢀFe single crystal.
The single crystals can contain twins and other surface
defects, for instance, growth steps decreasing the strength
of the IW. The average tensile strength of αꢀFe whiskers is
3309 MPa.1
b
The main crystallographic directions of the growth of
αꢀiron single crystal are <100>, <111>, or <110>. Such
an ambiguity is due to a rotation of the diffraction pattern
with respect to the image of single crystals at an unknown
angle.
The microphotograph of a particular αꢀFe single crysꢀ
tal and the scheme of planes in a cubic lattice are preꢀ
sented in Fig. 2. In all cases, when the direction of IW
growth perpendicular to the most densely packed (in bcc
lattice) plane (111) is retained, the completion of the
planes of the family {110} occurs, transforming the crystal
into a pyramidal cube. This can be explained by a considꢀ
erable local and volume nonequilibrium character of crysꢀ
tallization.
Fig. 3. Microphotographs of the support surface: (a) high growth
rate of IW and (b) αꢀFe whiskers growing on the support surface.
The analysis of the microphotographs of the support
surface on which IW grow (Fig. 3, a, b) indicates that the
shape of IW "nuclei" composing the sublayer surface corꢀ
responds to the shape of the single crystals themselves.
All IW grown by us can be divided into two main
types: rectangular with an average length of 60—70 µm
and d = 0.5—1.0 µm (Fig. 4) and coneꢀlike IW achieving
a length of 120 µm and having the soꢀcalled "growth dropꢀ
let" at the vertex (Fig. 5).
The morphology and, hence, the mechanical and
electrophysical properties of IW depend substantially on
the conditions of synthesis.8—20 Of the whole variety of
technological factors that can affect the growth and forꢀ
mation of IW obtained by different methods (both physiꢀ
cal and chemical), the following factors are usually disꢀ
tinguished as main: temperatures of the support and carꢀ
rier gas, a saturated vapor pressure of the starting subꢀ
stance in the system, the presence and rate of the carrier
gas, the concentration of the starting substance vapor,
initiating additives (to supports or gas), the composition
of the support, and others.8—10,21—23
Fig. 4. Rectangular αꢀFe whiskers with different directions of
growth obtained at a temperature of the support of 1000 °С and
an IPC pressure of 1.5•10–1 Torr.
Let us consider the temperature effect on the thermal
dissociation of iron carbonyl vapor under its constant
partial pressure.

1906
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
Babin et al.
Fig. 5. Conic αꢀFe whisker with a "droplet" at the vertex obꢀ
tained at a temperature of the support of 1050 °С and an IPC
pressure of 5•10–1 Torr.
Fig. 6. Microphotograph of the surface of an iron coating obꢀ
tained upon the decomposition of Fe3(СO)12 at a temperature of
the support of 700 °С.
The decomposition of iron carbonyls on the support
and the formation of a metallic layer start at relatively low
temperatures. For example, IPC decomposes already at
200 °С. Iron dodecacarbonyl Fe3(CO)12 sublimes at 60 °С
and begins to decompose at 110 °С, while for Fe5С(СО)15
these temperatures are 80 and 250 °С, respectively.
solid iron carbonyls (unlike IPC). As a result, at similar
temperatures, a globular crystalline coating with particle
sizes of 25×60 µm is replaced by a purely crystalline layer
with pronounced, well formed, and distinctly cut crystals
We studied the decomposition of iron carbonyls in a
wide temperature range from 200 to 1150 °С. The results
obtained for the decomposition of liquid IPC in a lowꢀ
temperature interval (200—600 °С) agree completely with
the published data.24,25 When the temperature increases
from 200 to 600 °С, the globular character of the coating
is retained, a crystalline surface is formed, crystals grow,
and large spheroids transform into finer semispheroids,
which further transform into numerous crystals uniformly
distributed over the iron coating surface.
The decomposition of solid Fe3(CO)12 and
Fe5С(СО)15 under these conditions was studied for the
first time. As in the case of IPC, IW are not formed in a
lowꢀtemperature interval, and only the formation of a
metallic coating is observed. In the case of Fe3(СO)12, the
character of this coating changes with temperature in this
interval as follows: at first agglomerates of crystallites
12—15 µm in size appear and consist of crystals and parꢀ
ticles with a diameter of 2—3 µm, then spheroids resemꢀ
bling a single raspberry grain are formed, then trapezoid
crystals of base 3 µm and height 1 µm are observed, and
finally, laminated globules consisting of separate iron lamiꢀ
nated crystals (LC) 0.3—0.4 µm thick are formed (Fig. 6).
Similar compact iron coatings with crystals oriented
as parallel strips consisting, in turn, of finer semispheroids
are formed upon the decomposition of Fe5С(СО)15. The
further increase in the temperature (to 600—700 °С) of
the support covered with a metallic layer formed at lower
temperatures induces the growth of laminated iron crysꢀ
tals with an edge length of 3—5 µm and a thickness of
0.3—0.6 µm. At higher temperatures, αꢀFe whiskers grow
from the same globules on which the laminated crystals
are formed (Figs 7 and 8). This combined growth of lamiꢀ
nated whiskers is typical of the decomposition of only
Fig. 7. Parallel growth of whiskers and laminated crystals from
Fe3(СO)12 vapors at a temperature of the support of 850 °С.
Fig. 8. Growth of whiskers and laminated crystals from
Fe5C(СO)15 vapors at a temperature of the support of 750 °С.

Thermal dissociation of iron carbonyls
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
1907
Table 1. Temperature effect on the growth of whiskers and laminated crystals (LC) of Fe obtained from iron carbonyls
at 0.15 Torr
Starting
iron
carbonyl
TS/°C
Whiskers
Character of IW and LC
lmax/µm
d/µm
Fe(CO)5
850
—
—
Compact semispherical coating with discrete cupolaꢀlike
structures ~1 µm in size
950
1050
15—20
60
0.3—0.5
1—2
Rectangular IW with "growth steps"
IW with distinct cutting, direct and changing growth
direction
1150
700
10
—
4—5
—
Polycrystals, overgrown structures, absence of IW
LC of Fe of length 10 µm, thickness 0.3—0.4 µm,
and height 5 µm
Fe3(CO)12
800
10—15
0.5—1.0
(at base)
0.2—0.5
(at vertex)
1.0
IW of cone type with distinct cutting
900
1000
10—15
—
IW and LC of thickness 0.3—0.5 µm and length 15 µm
Isometric single crystals, 3 µm
—
Fe5C(CO)15 600
700
—
10—15
—
LC of length 5—6 µm and thickness 0.4—0.5 µm
IW of coneꢀtype
0.5 (at base)
0.2 (at vertex)
Note: TS is the temperature of the support, lmax is the maximum length, and d is the diameter.
with "growth steps" and then by a spheroidal coating with
particles virtually equal in size (3—4 µm). Finally, when
the temperature is increased by the next 50 °С, the sublayer
surface gains a strongly developed globular crystalline
character with the particle size from 5 to 60 µm, and these
particles are divided into finer cubic crystals, which probꢀ
ably act as nuclei of future IW.
Based on the results of studying the temperature effect
on the course of iron carbonyl decomposition in a highꢀ
temperature interval and on the growth of IW (Table 1),
we can draw the following conclusions.
The data on the influence of the partial pressure of
iron carbonyls (for IW as an example) on the growth of
αꢀFe whiskers at a temperature of the support of 950 °C
are presented in Table 2. The following specific features
of the growth of IW from IPC can be noted.
1. By analogy to the temperature range, there is a
narrow interval of IW vapor pressures in which IW grow.
At pressures <9•10–2 Torr, IW do not grow and only thin,
nonuniform coatings are formed. At pressures exceeding
the upper pressure threshold, IW do not appear either but
loosen coatings and coatings with laminated crystals or
1. Unlike the formation of metallic films and coatings,
the growth of IW occurs in a rather narrow temperature
interval (~50°). This interval is restricted both from the
top and bottom, i.e., the temperature plot of the IW growth
has a threshold.
Table 2. Pressure influence on the growth of whiskers of αꢀFe
and LC of Fe obtained from Fe(CO)5 at 950 °C
PFe(CO)
IW
Character of IW and LC
5
2. When the number of metal atoms in the starting
iron carbonyls increases, the lower boundary of the temꢀ
perature "threshold" of IW appearance decreases and the
temperature interval of IW formation and growth becomes
narrower.
/Torr
lmax
dmax
µm
9•10–2
—
—
Nonuniform coating of
crystalline type
3. The growth of IW occurs in two steps: at first a
sublayer is formed and then IW are formed.
1.5•10–1 15—20 0.3—0.5 Rectangular IW with growth
steps, distinctly cut
5•10–1
4. An increase in the support temperature in an interꢀ
val of IW formation results in the elongation of the crysꢀ
tals and their apparent diameter (from 10 to 60 µm and
from 0.5 to 3 µm, respectively) and in an increase in their
density on the support. The maximum length and denꢀ
sity of αꢀFe whiskers fall on a temperature range of
1000—1050 °С.
30
1.0—1.5 Rectangular IW with a "growth"
droplet at the vertex and
coneꢀtype IW
9•10–1
1.0
10—15 2.0—3.0 IW of dendrite type
—
—
Fe polycrystals of length 10 µm
and diameter 3 µm and
dendrites

1908
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
Babin et al.
irregular structures having secondary aggregates are
formed.
2. An increase in the pressure within the interval inꢀ
dicated (as well as the temperature increase) results in
the elongation of the IW themselves and their diameter
and an increase in the density of IW growth per surꢀ
face unit.
3. The partial pressure of IPC has a substantial effect
on the crystal morphology. Distinctly cut single crystals
of αꢀFe with growth steps at the vertex were obtained at
the lower limit of the partial pressure interval (Fig. 9, a).
The pressure increase changes the shape of cylinderꢀlike
IW, which become coneꢀlike with a spherical "droplet" at
the cone point (see Fig. 5). The crystals appear that exꢀ
hibit a turn of the IW axis or the beginning of side growth,
as well as different surface defects: steps and deflections
(Fig. 9, b) The further pressure increase results in the
growth of branched dendriteꢀtype crystals, the appearꢀ
ance of bridges and polycrystalline forms, and the comꢀ
plete disappearance of IW (Fig. 10).
Fig. 10. Whiskers with secondary formations. The temperature
of the support is 1150 °С, and the vapor pressure of IPC is
9•10–1 Torr.
patterns for many IW samples obtained under different
conditions indicates that one should distinguish the effect
of the support itself (on which iron carbonyl vapors deꢀ
compose) and the effect of a metallic sublayer, which is
formed directly on the initial support during iron carboꢀ
nyl decomposition beginning from 200 °C. The formation
of this sublayer is a necessary condition for the nucleation
and growth of IW. Whiskers grow only on discrete, cuꢀ
polaꢀlike (semispheroidal) structures of the sublayer ~1 µm
in size, which we call nuclei. In one of the experiments,
we succeeded to detect (Fig. 11) the beginning of growth
of a coneꢀlike crystal from this nucleus.
Effect of support (including the sublayer morphology)
on the IW growth. The analysis of electron microscopic
a
A coatingꢀsublayer is formed on both metallic (stainꢀ
less steel, steelꢀ3, Nichrome, molybdenum, tungsten, and
others) and nonmetallic supports (carbon rods, ceramic
tubes). Whiskers grow well on metallic supports, and their
modification by donor additives (for example, an eutectic
b
Fig. 9. Whiskers of αꢀFe obtained at temperatures of the support
of 950 °С (a) and 1100 °С (b) and an IPC vapor pressure of
1.5•10–1 Torr.
Fig. 11. Beginning of the growth of αꢀFe whiskers on cupolaꢀ
like (semispherical) structures of the sublayer.

Thermal dissociation of iron carbonyls
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
1909
(200—2000 V), IW stopped growing, whereas the change
in the electric field from 1 to 10 V had no substantial
effect on the IW length.
a
Additional studies are required to understand the pheꢀ
nomena of parallel crystallization of iron precipitates
forming a relief as strips for crystal growth from Fe3(СО)12
and Fe5С(СО)15 vapors and the retention of the parallel
orientation during laminated crystal growth or the formaꢀ
tion of groups of crystals oriented in one direction. It is
most likely that these phenomena are related to the ferroꢀ
magnetic nature of a precipitate.
ESR spectroscopic study of the thermodissociation of
iron carbonyls. The conditions of ESR spectra recording
were close to the conditions of IW preparation via the
technological scheme used. The data of ESR spectroꢀ
scopy for IPC are collected in Tables 3 and 4.
b
Table 3. Isotropic values of the g factor for
iron carbonyl radical anions (ICRA) detected
in experiments on IPC thermal dissociaꢀ
tion (183 K)
ICRA
g Factor*
•–
Fe(CO)4
2.0485
(2.0486 26
2.0394
)
•–
Fe2(CO)8
(2.0385 27
2.0493
)
)
)
)
•–
•–
•–
Fe3(CO)11
Fe3(CO)12
Fe4(CO)13
(2.0497 27
2.0018
(2.0013 27
2.0134
Fig. 12. Formation of whiskers on a support coated with an
eutectic FeS alloy at 750 °С.
(2.0134 27
* The data in Refs 26 and 27 are presented for
the THF and 2ꢀMeꢀsubstituted THF soluꢀ
tions at 193 K.
alloy FeS) results in an additional growth of IW with a
higher density on the treated regions. In addition, the
growth of the sublayer followed by that of whiskers at a
fairly short exposure (∼1 min) occurs only on these
surface regions. The lowꢀtemperature threshold of IW
growth decreases (for IPC, to 750 °С), and unusual agꢀ
gregates consisting of repeating structures (Fig. 12) are
observed along with standard whiskers. The unusual agꢀ
gregates resemble a toroidal pattern of magnetic field lines
of force around a magnet. These periodical structures are
formed, probably, due to a certain combination of the
rate of IPC decomposition on charged centers localized
along magnetic field lines of force and the rate of formaꢀ
tion of αꢀFe ferromagnetic domains below the Curie
point (768 °С).
Table 4. Anisotropic values of the g factor for iron carbonyl
radical anions detected in experiments on IPC thermal dissociaꢀ
tion
ICRA
Anisotropic values
T/K
Refs
of g factor
g1
g2
g3
•–
Fe(CO)4
2.0710 2.0710 2.0038
2.0707 2.0707 2.0039
2.0566 2.0523 2.0090
2.0557 2.0501 2.0094
2.0579 2.0538 2.0093
2.0540 2.0540 2.0092
183
77
183
103
77
77
183
77
*
28
*
29
30
27
*
27
31
•–
Fe2(CO)8
Effect of electric and magnetic fields on the IW growth.
The data indicating the influence of electric and magnetic
fields on the length and orientation of IW are pubꢀ
lished.8,11,12 Therefore, we studied the influence of a dc
electric field on the IW growth within intervals from 200
to 2000 V and from 1 to 10 V. At high electric field values
•–
Fe3(CO)11 2.0927 2.0314 2.0247
2.0921 2.0321 2.0243
2.0966 2.0297 2.0253
77
* Data of this work.

1910
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
Babin et al.
In experiments simulating different steps of dissoꢀ
ciation (variation of the time and temperature of heatꢀ
ing), we detected a family of signals with the g factor
in an interval of 1.9—2.1 corresponding to iron carboꢀ
nyl radical anions (ICRA) with the number of iron
atoms from one to four. Some groups of the signals
(which are not presented in the tables) can be attribꢀ
conditions of dissociative ionization under electron
impact.12,39
М+ is metal atom.
•–
uted to carbonyl—carbide radicals of the Fe5С(СО)14
type. Depending on the radical anions to surface bond
strength, all signals detected can be divided into two
groups: with isotropic and anisotropic values of the
g factor. The rigidly linked ICRA (chemosorbed) give
signals with anisotropic g factors, and the free (physiꢀ
cally adsorbed) ICRA are characterized by isotropic
g factors.
Prolongation of heating of a sample does not remarkꢀ
ably change the spectra. The only restraint is the growth
of the coating thickness, which does not allow the meaꢀ
surement of ESR spectra because of the formation of a
magnetic phase that decreases the resonator Qꢀfactor. The
repeated recording of the spectra of the same sample, i.e.,
elongation of the reaction time, either does not result
in significant changes in the spectra, indicating the staꢀ
bility of radical ion species (except for the shortꢀlived
Fe(CO)4•–) and a high rate of their formation and, hence,
a high rate of the IW growth. The temperature increase
leads to the broadening of the signals instead of their
weakening, which is related to electron exchange reacꢀ
tions in the adsorption layer.
Presently, several approaches are known8—10,32,33 to
explain the thermal dissociation of metal carbonyls in
general and under the conditions of IW growth in particuꢀ
lar. This is related to the complex character of the reacꢀ
tion under study including different physicochemical
steps: evaporation, mass transfer, adsorption, dissociaꢀ
tion, formation or growth of IW, and others. Using IPC as
an example, let us consider the chemical aspects of the
mechanism of metal carbonyl dissociation, i.e., a sequence
of elementary acts in the gas phase and on the growing
surface, resulting in IW formation.
The thermal dissociation of IPC is usually described
as a set of consecutive or singleꢀstep decarbonylations34—36
due to the thermal excitation of molecules with an active
involvement of the surface "adꢀatoms" in the formation of
a crystalline phase. Either intermediates of the Fe(СО)n
(n = 1—4) type or "naked" Fe atoms incorporate into the
lattice. These intermediate species were detected under
the hard photolysis of iron carbonyls in inert lowꢀtemꢀ
perature matrices.37
This approach seems to be quite substantiated, beꢀ
cause studies of the behavior of metal carbonyls in reacꢀ
tions with Lewis bases in solutions of different orꢀ
ganic solvents40—43 and of the gasꢀphase ionꢀmolecular
clusterization of metal carbonyls by highꢀpressure mass
spectrometry and ion cyclotron resonance44—45 showed
that these reactions can be considered as radical or radical
ion processes with the initiation of primary radical species
due to activating complexation and further cluster enꢀ
largement.41—43
Activating complexation implies the formation of an
active (with the properties of a reducing agent with reꢀ
spect to a nonactivated molecule of metal carbonyl) orgaꢀ
nometallic compound as a result of the reversible addition
of a nucleophilic molecule at one of the carbonyl ligands
Nu is nucleophile.
We propose the chain radical ion mechanism of therꢀ
mal dissociation of metal carbonyls with chain nucleation
via the scheme of activating complexation, which includes
all steps characteristic of chain processes: chain nucleꢀ
ation and the appearance of active iron carbonyl radical
anions, chain propagation, enlargement and mutual transꢀ
formations of ICRA, formation of an αꢀiron structure and
chain termination, decay of radical ion centers, and cesꢀ
sation of the fast IW growth in length.
The step of chain nucleation begins from the adsorpꢀ
tion of an IPC molecule from the gas phase on an adsorpꢀ
tion donor site (S) of the starting unchangeable support.
These sites are mainly admixtures or formed due to conꢀ
tact of the support with atmosphere. They exist in any
supports and interact with an IPC molecule.
The results of studying the IW growth in strong elecꢀ
tric and/or magnetic fields11,12 suggest the formation and
participation of charged ions and radical ions11,38 in crysꢀ
tal thermal dissociation and growth via the scheme of
successive decarbonylation of metal carbonyls under the
[M] are nodes of the crystalline lattice of supports of different nature.
Coordination is usually performed with the participaꢀ
tion of one of the carbonyl ligands via the hydroxyꢀ

Thermal dissociation of iron carbonyls
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
1911
carbenium type. Chemosorption of this type was demonꢀ
strated by the study of the adsorption of carbonyl comꢀ
plexes Cr(СО)6 and C5H5Mn(СО)3 on metallic surꢀ
faces46,47 and different carbonyls, including iron carbonꢀ
yls on oxide supports.48,49
than the temperature limit of the beginning of coating
formation, as well as the dependence of the nucleus size.
Whiskers are formed only on rather large nuclei (~1 µm),
and IW are completely absent on smaller nuclei. When
the temperature increases, highꢀlying unoccupied elecꢀ
tron levels in the conductivity band are populated to proꢀ
vide the electron transfer from a metallic nucleus to an
IPC molecule.
After adsorption, a radical ion center is nucleated,
which determines the radical nature of the process as a
whole. Two types of nucleation reactions can be disꢀ
tinguished: (1) on admixture atoms or groups of donor
atoms and on the starting support and (2) on the surface
of spherical nuclei of a metallic sublayer already formed
on the primary support of the metallic sublayer. In both
cases, the key role belongs to the oneꢀelectron transfer. In
the case of initiation on the starting supports, the redox
disproportionation of IPC molecules occurs due to elecꢀ
trons of donor sites of the support, due to which reacꢀ
tive species with an open electron shell appear on the
surface.
The formation of discrete structures of rather large
size also facilitates the electron transfer due to approachꢀ
ing and flexure of surface electron levels.
The decrease in the temperature of the beginning of
IW growth observed in experiments is parallel to a deꢀ
crease in the electrochemical reduction potential of iron
carbonyls in the series
Fe(CO)5, Fe3(СО)12, Fe5C(CO)15
–1.77 V 50, –0.32 V 50, –0.17 V 51
and agrees with the fact that the energy of electron transꢀ
fer from the metal surface to an adsorbate molecule deꢀ
pends on its electron affinity. It is also clear why the lowꢀ
temperature boundary of iron IW growth decreases when
hydrogen is introduced into the system (simultaneous inꢀ
let of Н2 and IPC),2,3 because the presence of hydrogen
facilitates the reduction of an adsorbate and the step of
IW growth initiation.
This process occurs at relatively low temperatures. For
instance, a metallic (in our case, iron) coatingꢀsublayer,
on which chain nucleation occurs in the scheme proꢀ
posed, is formed at temperatures from 250 °С. In addiꢀ
tion, coating formation is characterized by a broad temꢀ
perature interval (larger than 500 °C) and a high rate of
IW growth depending on the concentration of admixture
(donor) sites. In fact, the modification of a support of an
eutectic Fe—S alloy results in an increase in the density of
iron IW on donor sites Fe—S. In addition, at short
(~1 min) exposure times, when a cupolaꢀlike structure of
the sublayer necessary for the IW growth under normal
conditions had no time yet to form, an exclusive growth
of the sublayer and IW is observed only on the surface
regions treated by eutectic. All these data indicate a strong
influence of an initiator on the IW growth and profit of
donor admixtures.
•–
The formation of radical anions Fe(CO)4
by the
thermal dissociation of iron carbonyls confirms diꢀ
rectly redox processes during IW nucleation (see
Tables 3 and 4).
In the case of chain nucleation on the surface of spheriꢀ
cal nuclei of a metallic sublayer already formed on the
primary support, the surface group of iron atoms of cerꢀ
tain structure and size (~1 µm) is an electron donor. To
perform the electron transfer to an adsorbate molecule, it
is necessary to consume an energy for surmounting a relaꢀ
tively high work function for an electron withdrawal from
iron (1.5 eV). This results in a strong temperature depenꢀ
dence of the IW growth and the existence of a lower
temperature limit for IW formation, which is much higher
A possibility of two types of IW nucleation makes it
possible to understand why the induction period exists
before the beginning of IW growth and why two steps of
the process exist, where at first an iron sublayer is formed
and then IW growth begins in this sublayer.
At the step of chain propagation, adsorbed radical
anions formed in the previous step react rapidly with neuꢀ
tral iron carbonyl molecules to form a family of iron carꢀ
bonyl radical ion clusters with different numbers of metal
atoms (see Tables 3 and 4).

1912
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
Babin et al.
turn of a free electron of ICRA to the conductivity band
and is accompanied by the formation of a neutral adsorpꢀ
tion complex.
The radical anion adsorption complexes that are chain
carriers can decay with a temperature decrease or when
additives inducing ICRA "recombination" are added. The
inverse electron transfer can occur when the IPC concenꢀ
tration decreases, due to which mononuclear ICRA that
nucleated have no time to stabilize because of enlargeꢀ
ment. The IW growth becomes impossible but the coatꢀ
ings continue to grow due to admixture nucleation, which
is still possible under these conditions.
etc.
The temperature effect on the chain termination is
also indicated by the existence of an IW growth band
(40—50 µm) in the direction perpendicular to the supꢀ
port. Longer whiskers grow at an angle to the surface.
Thus, we proposed a model that provides a nonꢀconꢀ
tradictory explanation of the whole array of experimental
facts concerning the characteristic features of the compoꢀ
sition and morphology of IW and the kinetics of their
growth. The model agrees well with the models of chain
radical ion processes initiated by activating complexation
proposed for the liquid and gas phases.
Additives of small amounts of an inhibitor, for exꢀ
ample, ammonia that reacts readily with ICRA, to IPC
vapors cease the IW growth, which confirms the chain
mechanism of the reaction under study. With the addition
of new Fe(СО)5 molecules and during chain propagation,
the rapidly growing core of a cluster gains the structure of
an αꢀFe crystal and forms a ready surface unit that can
incorporate into the crystalline lattice. It can be assumed
that these are Fe4(СО)13•– radical anions (after their comꢀ
plete decarbonylation) that are incorporated due to a simiꢀ
–
lar structure of the [Fe4] fragment in the Fe4(СО)13
cluster and αꢀFe crystal. In addition, the catalytic
Bell—Boudoir reaction resulting in contamination of the
metallic surface can occur for heavier clusters, while the
IW that formed are characterized by high purity. Thus, in
the second step αꢀFe whiskers grow rapidly in length with
a rate of 1.5 µm s–1, which agrees with rather high rates of
This work was financially supported in part by the
Russian Foundation for Basic Research (Project No. 98ꢀ
03ꢀ3227).
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Received June 29, 2004