J . Phys. Chem. 1990, 94,4315-4318
4315
Molybdenum-Carbon Bond Dissociation Energies in Mo(CO)6 Jane A. Ganske and Robert N. Rosenfeld* Department of Chemistry, University of California, Davis, California 95616 (Received: October 2, 1989)
The pressure dependence of the recombination rate constants for Mo(CO), ( n = 3,4, and 5) with CO has been studied by time-resolved infrared laser absorption spectroscopy. These data, in conjunction with an RRKM model for unimolecular decay of the activated molecules [Mo(CO),]*, [Mo(CO),] *, and [Mo(CO),]*, have allowed the determination of the bond dissociation energies for several of the Mo-C bonds in Mo(CO),. The first Mo-C bond dissociation energies for Mo(CO),, MO(CO)~, and Mo(CO)~are found to be DHo[(CO),Mo...CO] = 35 f 5 kcal/mol, DHO[(CO),Mo...CO] = 27 f 5 kcal/mol, and DHo[(CO)3Mo-.CO] = 31 i 5 kcal/mol.
Mo(CO),
Introduction Coordinatively unsaturated metal carbonyls have been shown to play a fundamental role in both heterogeneous and homogeneous catalysis.' In many cases, the catalyst precursor must lose one or more carbonyl ligands before substrate binding commences. Knowledge of all metal-ligand bond strengths is therefore crucial toward a better understanding of the catalytic process. Thermochemical data of this type are also useful when assessing the feasibility of other reactions involving organometallics. Although tabulations of thermodynamic data are common for organic compounds, little data exists for organometallics. Because metal carbonyls represent an important general class of these molecules, efforts have recently begun to acquire bond strengths for the metal-CO bonds. Several studies have measured first bond dissociation energies. Peters and co-workers2 have used photoacoustic calorimetry to measure the first bond dissociation energies of M(CO)6 (M = Cr, Mo, and W) in solution. Lewis et aL3 have determined the first bond dissociation energies of a variety of metal carbonyls in the gas phase using a laser pyrolysis technique. Limited data, however, are available on bond strengths in coordinatively unsaturated metal carbonyls. Lineberger and cow o r k e r ~have ~ , ~ investigated successive removal of carbonyl ligands from Fe(CO)5 and Ni(C0)4. Their technique of photoelectron spectroscopy determines electron affinities for species M(CO);, which, when coupled with available appearance potential data, yield bond dissociation energies for the corresponding neutrals. Their work has shown that the average bond energy of a metal carbonyl, Le., the value of A H o / n for (I), is a poor approximation M(CO),
-
M
+ nCO
M o ( C O ) ~+ C O Mo(CO),
+ CO
--
MO(CO)~
(2)
Mo(CO),
(3)
Mo(CO)~
(4)
ciation energy of Mo(CO),, DHO [(CO),Mo.-CO], has been measured? there have been no reports of such data for M O ( C O ) ~ or M o ( C O ) ~ . We discuss the application of our method for determining bond dissociation energies to eqs 2-4, and the details of our findings, in the following text.
Experimental Section The experimental apparatus and data collection p " s have been described in detail previ~usly,~ so only a brief summary is given here. Mo(CO), ( n = 3, 4, and 5) is generated via pulsed excimer laser photolysis of vapor-phase M o ( C O ) ~and detected by time-resolved carbon monoxide laser absorption spectroscopy. Photolysis wavelengths required for generation of the desired molybdenum carbonyl are given below.
-
Mo(CO)~ Mo(CO)~
Mo(CO)~
351 nm
248 nm
193 nm
+ M o ( C O ) ~+ 2CO Mo(CO), + 3CO MO(CO)~ CO
(5) (6)
(7)
The identity of the molybdenum carbonyl photoproducts is established with use of time-resolved infrared absorption spectroscopy, as previously d e ~ c r i b e d . ~ Recombination kinetics were determined by tuning the CO laser to a Mo(CO), absorption band and monitoring the signal amplitude versus time. Additionally, the kinetics of formation of the recombination product, Mo(CO),,+,, were also monitored. Both methods gave identical results, within experimental error. The absorptions used for M o ( C O ) ~ M , o ( C O ) ~ and , Mo(CO), were 1888 cm-' [vco(e)], 1914 cm-l [uco(bl)], and 1983 cm-' [uco(e)], respectively. Samples typically contained 0.01 5-0.035 Torr of Mo(CO),, 0.050-5.00 Torr of CO, and 0-200 Torr of He or Ar. Typically 50-100 transient absorptions were averaged to obtain a signal-to-noise ratio of 1 5 . The measured rise time of the InSb detector is ca. 200 ns. All gases were obtained from commercial vendors and used as supplied. M o ( C O ) ~was sublimed and stored under vacuum prior to use.
(1)
to the individual bond energies. We recently developed a procedure that has proven to be a powerful means for determining bond strengths of both coordinatively saturated and unsaturated metal carbonyls.6 This technique utilizes kinetic recombination data in conjunction with an RRKM model of the corresponding bond dissociation process. This approach to estimating bond dissociation energies is quite general and should be readily applicable to a variety of organometallic molecules. Here, we continue our investigation of the group VI metal carbonyls by examining the molybdenum carbonyl recombination reactions, eqs 2-4. While the first bond disso-
Results and Discussion Coordinatively unsaturated molybdenum carbonyls, Mo(CO), ( n = 3,4, or 5), may be generated in the gas phase by UV laser photolysis of Mo(CO)~.The extent of Mo(CO), fragmentation following UV photolysis depends on the energy of the incident photon and the bond dissociation energies (BDE's) of the corresponding unsaturated metal carbonyl. We have previously observed7 the formation of M O ( C O ) ~Mo(CO),, , and M O ( C O ) ~
(1) Collman, J . P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organorransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987. (2) Bernstein, M.: Simon, J. D.; Peters, K. S. Chem. Phys. Lett. 1983,100, 241. (3) Lewis, K . E.; Golden, D. M.; Smith, G. P. J . Am. Chem. SOC.1984, 106, 3905.
(4) Engelking, P. C.: Lineberger, W. C. J . Am. Chem. SOC.1979, 101, 5569.
(5) Stevens, A. E.; Feigerle, C. S.; Lineberger, W. C. J . Am. Chem. SOC. 1982, 104, 5026. (6) Fletcher, T. R.; Rosenfeld, R. N. J . Am. Chem. SOC.1988,110,2097.
0022-3654/90/2094-4315$02.50/0
+ CO
(7) Ganske, J. A,; Rosenfeld, R. N. J . Phys. Chem. 1989, 93, 1959.
0 1990 American Chemical Societv
8 -
4316
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
Ganske and Rosenfeld
-
TABLE I: Input Parameters for RRKM Calculations,' M o ( C O ) ~ M O ( C O )+ ~ CO
vibrational frequency, cm-'
5
0
15
10
20
25
Pressure He (torr) Figure 1. Pressure dependence of the rate constant for the recombination of Mo(CO), with CO. Solid lines correspond to RRKM calculations where (a) Eo = 30 kcal/mol, (b) Eo = 35 kcal/mol, and (c) Eo = 40 kcal/mol. See text following the U V photolysis of Mo(CO),. The recombination reactions of these molybdenum carbonyls with CO have also been studied by time-resolved C O laser absorption spectroscopy (eqs 8-10),' By tuning to a Mo(CO), ( n = 3, 4,or 5) IR absorption Mo(CO), Mo(CO), Mo(CO),
+ CO
kl
Mo(CO),
+ CO M o ( C O ) ~ + CO A M O ( C O ) ~
(8)
moments of inertia, amu A*
k, + CO 7 [Mo(CO),+,]*
I?
-
Mo(CO),+,
= kcks[Ml/(ka + ks[MI)
867.44 (3)
-
reactant Mo(CO), vibrational frequency, cm-'
2024.8 (2) 2003.0 (3) 595.6 (1) 507.2 (3) 477.4 (3) 381.0 (2) 391.2 ( I ) 367.2 (2) 341.6 (3) 81.6 (13) 79.2 (3) 60.0 (3)
( 1 1)
moment of inertia, amu A*
867.44 (1) 625.94 (2)
(12)
reaction path degeneracy: L' = 4
(10)
(13)
The pressure dependence of kobeallows the BDE's to be determined. Because [Mo(CO),+,]* is created with an amount of energy approximately equal to DHO [(CO),Mo.-CO], it may decompose back to reactants, Mo(CO), CO, or be collisionally stabilized with bath gas, M, to the product, Mo(CO),+,. This competition between decomposition and collisional stabilization of [Mo(CO),+,]* is governed by the relative magnitudes of ka and k,[M]. The evaluation of each of these quantities is discussed in detail in the following text. Unimolecular Decomposition of [Mo(CO),+,]*. The rate constant, k,, for unimolecular decomposition of [Mo(CO),+,] * may be computed from the usual RRKM expression*
a
E-E,
(14)
Ef=O
where L* is the reaction path degeneracy; ( Q + / Q )is the ratio of (8) Robinson, P. J.; Holbrook, K . A. Unimolecular Reactions; Wiley-Interscience: New York, 1972.
transition state (CO),Mo.**(CO) 2143.0 (1) 2024.8 (2) 2003.0 (2j 595.6 (1) 507.2 ( I ) 477.4 (3) 381.0 (1) 391.2 (1) 367.2 (2) 341.6 (3) 90.0 (2) 69.2 (3) 50.0 (3) 32.0 ( I ) 1704.59 ( I ) 1462.16 ( I ) 625.01 (1)
Degeneracies indicated in parentheses.
-
TABLE 111: Input Parameters for RRKM Calculations,' M O ( C O ) ~ M o ( C O ) ~+ CO
reactant Mo(C0)A vibrational frequency, cm-'
+
k ( E ) = L*(Qi+/Q)[hN(E)I-' C f'(EV:)
2143.0 (1) 2024.8 (2) 2003.0 (3) 595.6 (3) 507.2 ( I ) 477.4 (3) 381.0 ( I ) 391.2 (2) 321.2 (2) 341.6 (3) 112.2 (2) 81.6 (3) 79.2 (2) 40.0 (3) 30.0 ( I ) 1721.13 (2) 867.44 ( I )
TABLE 11: Input Parameten for RRKM Calculations,' M O ( C O ) ~ M O ( C O )+ ~ CO
(9)
may be used to explain the observed pressure dependence. Here, n may be 3, 4, or 5, M indicates a third body, and the asterisk is used to denote internal (rovibrational) energy. Upon application of the steady-state approximation, where [Mo(CO),+,] * is the intermediate, an expression for the observed rate constant is obtained. kobs
2120.7 (1) 2024.8 (2) 2003.0 (3) 595.6 (3) 507.2 (3) 477.4 (3) 381.0 (1) 391.2 (2) 367.2 (3) 341.6 (3) 81.6 (3) 79.2 (3) 60.0 (3)
Degeneracies indicated in parentheses.
kJMI
[Mo(CO),+,I*
transition state (CO),M**.(CO)
reaction path degeneracy: L' = 6
frequency and monitoring the decay rate of the signal in the presence of CO, the rate constants k,, k2,and k3 were determined to be (2.0 f 0.2) X IO6, (1.6 f 0.4) X lo6, and (1.8 f 1.0) X 10, Torr-I s-I, respectively. These rate constants were measured with sufficient buffer gas to ensure high-pressure limiting conditions.' Here we report the pressure dependence of k , , k2, and k3. The pressure dependence for the recombination reaction of Mo(CO), with C O is shown in Figure I . This type of plot is suggestive of a Hinshelwood-Lindemann mechanism8 (eqs 11 and 12). which Mo(CO),
reactant Mo(CO),
moment of inertia, amu .A2
2024.8 ( I ) 2003.0 (3) 507.2 (2) 477.4 (3) 391.2 (2) 367.2 (2) 341.6 (3) 79.2 (2) 60.0 (3) 650.58 ( 1 ) 594.65 ( i j 377.79 ( I )
transition state (CO)~MO***(CO) 2143.0 (1) 2003.0 (3) 477.4 (3) 391.2 (2) 367.2 (2) 341.6 (3) 112.0 (2) 60.0 (3) 39.2 (2j 1362.86 (1) 1112.66i i j 617.24 (1)
reaction path degeneracy: L* = 4 'Degeneracies indicated in parentheses. the transition state and reactant adiabatic rotational partition functions; N ( E ) is the density of states of the reactant with internal energy, E ; P(E,,+) is the number of transition states at internal energy E,,'; and E , is the critical energy for the unimolecular decomposition reaction. Parameters necessary for the state counting and partition function evaluation are given in Tables 1-111 for Mo(CO),, Mo(CO),, and Mo(CO),, respectively. Sums and
Molybdenum-Carbon Bond Dissociation Energies in Mo(CO), The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4317 densities of states were estimated by the Whitten-Rabinovitch a l g ~ r i t h m . ~Frequencies'O for M o ( C O ) ~and Mo(CO), were assumed to be identical with those of Mo(CO), after removal of the appropriate number of stretching and bending vibrations. Transition-state parameters were determined in the following manner. Lewis and co-workers3 have reported the high-pressure Arrhenius preexponential factors for the thermal decomposition of several metal carbonyls. These A factors are similar from compound to compound; Le., for Fe(CO)5, A, = 10'5.s s-l; for s-l; and for Mo(CO),, A , = s-]. The W(CO)6, A, = magnitude of these A factors indicates that the dissociations involve "loose" transition states." We therefore consider Mo(CO), (n = 3,4, or 5) and CO to be loosely coupled in the transition state and have used the method of Waage and RabinovitchI2to calculate the interfragment distance, r * . Typical values of r * for these dissociations range from r* = 5.2 to 5.5 A. Frequencies for the transition states are determined, assuming Mo(CO), and CO to be isolated fragments, such that the experimental value of the A factor is reproduced with the statistical mechanical expressionss
AS* = R[ln Qlot++ T d In Qz,+/dT + T d In Qz,+/dT In Qlot- T d In Q2,/dT - T d In Q2,/dT] (15) In A = In ( L * e k T / h )
+ AS*/R
1.0
j C
V.V
, C
.
I
20
x k , [ M ] = kSHe[He] k,Co[CO] 4- k,Mo(C0)6[MO(C0)6](17) measured stabilization rate constants, k,, for a number of Mo(CO),/M collider pairs by monitoring the excited metal carbonyl's decay rate as a function of collider gas pressure. We find little variation in the values of ksHefor deactivation of Mo(CO), (n = 3, 4, or 5 ) ; Le., k, = (1-3) X lo5 Torr-] s-l. Stabilization rates when Ar is used are slightly larger, as expected on the basis of molecular mass.I4 Stabilization rate constants are measured under conditions where metal carbonyl clustering rates7 are comparatively slower. This is accomplished by adjusting the relative pressures of Mo(CO), and M (M = He or Ar). We find that 1/1000, the rate of Mo(CO), relaxation for [Mo(CO),]/[He] is over 1 order of magnitude faster than the rate of clustering. The addition of small amounts of C O (C0.275 Torr) to Mo(CO),/He mixtures changes these values of k, by less than their experimental uncertainty for [He] 1 1 Torr. Woodin and cow o r k e r ~ report '~ similar values of k, for the stabilization of
=
(9) Whitten, G. Z.; Rabinovitch, B. S. J . Chem. Phys. 1964, 41, 1883. ( I O ) Braterman, P. S. Metal Carbonyl Spectra; Academic: New York, 1975. (1 1) Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley-Interscience: New York, 1976. (12) Waage, E. V.; Rabinovitch, B. S. Chem. Reu. 1970, 70, 377. (13) Astholz, D. C.;Troe, J.; Wieters, W. J . Chem. Phys. 1979, 70, 5107. (14) Rabinovitch, 8. S.; Flowers, M. C. Q.Rev. Chem. SOC.1964.18, 122. (15) Bray, R. G.; Seidler, P. F.; Gethner, J. S . ; Woodin, R. L. J . Am. Chem. SOC.1986, 108, 1312.
60
Pressure Ar (torr) Figure 2. Pressure dependence of the rate constant for the recombination of M O ( C O ) ~with CO. Solid lines correspond to RRKM calculations where (a) Eo = 20 kcal/mol, (b) Eo = 27 kcal/mol, and (c) Eo = 34 kcal/mol.
photolytically prepared Mn(CO)5 by He, Ar, CO, and a number of other colliders. At low pressures of Mo(CO), (e.g.,
