J. Phys. Chem. 1995,99, 7819-7828
7819
Ligand Effects in Organometallic Thermochemistry: The Sequential Bond Energies of Ni(CO),+ and Ni(NZ),+ (x = 1-4) and Ni(NO),+ (x = 1-3) Farooq A. KhanJ Dale L. Steele, and P. B. Armentrout" Department of Chemistry, University of Utah, Salt Luke City, Utah 84112 Received: July 30, 1994; In Final Form: November 23, 1994@
The sequential bond energies of Ni(CO),+ and Ni(N2),+ (x = 1-4) and Ni(NO),+ (x = 1-3) are determined by collision-induced dissociation in a guided ion beam tandem mass spectrometer. Values for the 0 K (CO),-INi+-CO bond energies (in eV) are found to be 1.81 f 0.11, 1.74 f 0.11, 0.95 f 0.06, and 0.75 f 0.03 for x = 1-4, respectively. The sum of these bond energies, 5.25 f 0.17 eV, is in excellent agreement with literature thermochemistry, but individual BDEs disagree with values obtained in photoionization and electron impact ionization experiments. We also observe electronically excited Ni(C0)2+ and NiCO'. We 'also report the following gas-phase bond energies (in eV at 0 K): 1.15 f 0.11, 1.15 f 0.11, 0.58 f 0.04, and 0.44 f 0.10 for (N2),-1Ni+-N2 (x = 1-4), respectively, and 2.36 f 0.08, 1.27 f 0.07, and 1.19 f 0.05 for (NO),-INi+-NO (x = 1-3), respectively. The trends in these bond energies are discussed in terms of a-donor and n-acceptor capabilities of the ligands.
TABLE 1: Summary of Values for D[(CO),-lNif-CO],
Introduction An important element in understanding organometallic reactions and homogeneous catalysis is the energetics of bondmaking and -breaking processes that occur at coordinatively unsaturated metal centers. Although a-donor and n-acceptor capabilities of various ligands provide a qualitative guide to relative transition-metal-ligand bond strengths, more quantitative measures are desirable. Some information comes from calorimetric measurements,' although only when stable precursors are available, notably carbonyl and alkyl compounds, and these data provide average rather than individual bond energies. The individual bond dissociation energies (BDEs) can vary significantly as the degree of ligation is varied, reflecting variations in geometric and electronic A versatile approach to determine accurate sequential bond energies for a multitude of metal-ligand systems is to generate individual ML,+ or ML,- species in the gas phase and to monitor their energy-dependent decomposition in a mass ~pectrometer.~-~ The ligands chosen in this study are of considerable interest in organometallic chemistry.6 Transition-metal carbonyls, of which nickel carbonyl is the oldest known? are often used as reagents in organometallic syntheses8 These syntheses incorporate metal carbonyl fragments, M(CO),, which are fundamental building blocks in organometallic ~ h e m i s t r y . ~Com.~ plexes of dinitrogen have been the subject of numerous studies, motivated in part by the search for synthetic routes to nitrogen activation.lO,ll N2 is generally considered both a weaker u donor and n acceptor than the isoelectronic CO molecule.12 This accounts for the observation that although carbonyl complexes such as Ni(C0)4 and Cr(C0)6 are stable at room temperature and atmospheric pressure, the corresponding N2 analogues are sufficiently unstable that they can be isolated only in lowtemperature mat rice^.'^.'^ NO is considered a poorer (7 donor but a better JC acceptor than CO and can be a one- or threeelectron donor ligand.I5 NO is generally found to be a more strongly bound ligand than CO based on ligand displacement reactions carried out in solution and photolyses in matrices,I6 although an exception is known."
' Present address: Chemistry Department, West Georgia College, Carrollton, GA 30118. * Abstract published in Advance ACS Abstracts, May 1, 1995. 0022-365419512099-78 19$09.00/0
eVa reference
x= 1
x=2
Distefano(21) Winters(22) Schildcrout(23) Bidinosti(24) Junk(25) Clements(26) theory(30)' this work'
1.9(0.1)* 2.5(0.4) 2.28(0.21) 2.1(0.3) 2.67 3.2 1.57 1.84(0.11)
1.55(0.1) 2.8(0.3) 1.96(0.21) 2.48(0.11) 2.21 2.0 1.45 1.75(0.11)
x=4
sumofBDEs
0.45(0.02) 0.72(0.21) 0.54(0.21) 0.65(0.14) 0.59 0.7
5.23(0.1)b 7.4(0.3) 6.10(0.21) 6.5(0.3) 6.76 7.0
0.98(0.06) 0.80(0.03)
5.36(0.17)
x=
3
1.33(0.1) 1.3(0.3) 1.32(0.21) 1.26(0.11) 1.29 1.1
Uncertainties are reported in parentheses. Based on the uncorrected AE for Ni+ of 13.55 eV. 298 K values. (I
A number of studies have been carried out in the gas phase to obtain sequential bond energies of ~ a t i o n i c ,neutral,'* ~,~ and a n i ~ n i c ~ ~metal-carbonyl '~.*~ species. Ni(CO),+ species have been the subject of several experimental studies in which the appearance energies (AEs) of Ni(CO),+ are measured upon photoionization21or electron impact ionization22-26of Ni(C0)4 in a mass spectrometer. The differences between the relative AEs are taken to equal the bond strengths and are listed in Table 1. These studies may provide accurate values for the ionization energy of Ni(C0)4; however, the AEs may be affected by kinetic shifts which will tend to increase with increasing degree of dissociation, rendering the BDEs thus determined inaccurate. Because kinetic shifts can depend on instrumentation and experimental sensitivity, this helps explain the large variations in the values listed in Table 1. Unlike their carbonyl analogues, dinitrogen and nitrosyl complexes of transition-metal cations have not received nearly as much attention in gas-phase studies. Experimental determinations of transition-metal cation-N2 bond energies are limited to D(Cr+-N2) = 0.61 & 0.04 eV, measured by Brucat and coworker~.~'In a study of ligand substitution reactions carried out in an ion cyclotron resonance mass spectrometer, Foster and Beauchamp found that NO molecules sequentially displaced COS from Fe(CO),+ (x = 1-5), Fe(NO)(CO),+ (x = 1-3), and Fe(N0)2(CO),+ (x = 1, 2).28 In a similar study, Ridge and coworkers showed that a single NO displaced up to two CO molecules from Co(CO)2+,CoNO(C0)2+, and C O N O ( C O ) ~ + . ~ ~ Both of these studies suggest that NO is more strongly bound to transition metal ions than CO, although this thermodynamic 0 1995 American Chemical Society
7820 J. Phys. Chem., Vol. 99, No. 19, 1995
Khan et al.
TABLE 2: Literature Thermochemistry (kJ/molyl information must be treated with caution for the following reason. In these experiments, the ions were produced by species OK 298 K electron impact ionization of Co(C0)3NO and Fe(C0)5, a co -113.81 f 0.17 -110.53 i0.17 process known to produce excited ions in the case of Fe(CO)j,2 Ni 428.1 f 8.4 430.1 i 8.4 and were reacted without further cooling. Ni+ 1165.0 i 8.4* 1173.2 f 8.4b In recent studies in our laboratory, we have demonstrated -601.6 f 10.5 Ni(C0)d -605.7 f 10.5 202.8 f 11.4' Ni(C0)dt 192.5 f 11.4' that guided ion beam mass spectrometry can be employed to obtain accurate sequential BDEs for Fe(CO),+ and Cr(CO),+ All species in the gaseous state. Unless otherwise stated, all data species.*s3 In the present work, we apply this technique to in this table are taken from ref 34. Ion heats of formation at 298.15 K correspond to the thermal electron convention. Calculated by using determine sequential BDEs for NiL,+ ions where L = CO, Nz, IE(Ni) = 7.6375 f 0.0012 eV taken from ref 50. Calculated by using and NO. There are several motivations to cany out the present IE[Ni(C0)4] = 8.273 f 0.046 eV; see text. study. First, we wished to determine accurate sequential BDEs for Ni(CO),+ ions as a part of an ongoing effort in our laboratory al.35 We thus obtain the heat of formation of Ni(C0)4+ as 192.5 to understand both the periodic trends in the bonding of metal f 11.4 kJ/mol at 0 K and 202.8 f 11.4 kJ/mol at 298.15 K in carbonyls and the possible changes in structure and bonding the thermal electron convention. Based on these values and that occur with variation in ligand around the metal ion. Second, the heats of formation listed in Table 2, the change in enthalpy we wanted to compare metal-ligand BDEs for ligands with for reaction 1 is calculated to be 517.24 f 14.2 kNmol(5.36 f different a-donor and n-acceptor capabilities. Third, experi0.15 eV) at 0 K and 528.24 f 14.2 kJ/mol (5.47 f 0.15 eV) at mentally determined sequential BDEs provide a benchmark for 298.15 K. comparison with theoretical models of structure and b ~ n d i n g . ~ - ~ ~ To compare individual bond energies measured here with those determined in the literature, we also need to convert from Literature Thermochemistry 0 to 298 K BDEs. Following the method outlined above, we Calculating an accurate value for the enthalpy of reaction 1, determine that the BDEs for (CO),-I Ni+-CO at 298 K are Ape( l), requires a knowledge of the heats of formation of Ni+ larger than those at 0 K by 2.74, 0.59, 3.29, and 4.38 kJ/mol for x = 1-4, respectively. The vibrational frequencies for the Ni(CO),+ Ni+ 4CO unsaturated nickel carbonyls needed for this calculation are listed (1) in Table 3 and determined as specified below. and CO, which are well established, Table 2, and of Ni(C0)4+. Experimental Section To obtain Afllo[Ni(CO)4+],we require AfW[Ni(C0)4, g], which is the sum of the heats of formation and vaporization of liquid General. Complete descriptions of the apparatus and exNi(C0)4 and the ionization energy (IE) of Ni(C0)4. The perimental procedures are given e l ~ e w h e r e . ~Production ~,~~ of JANAF tables34recommend a value of -631.78 f 8.4 kJ/mol ligated nickel ions is described below. The ions are extracted for AfH29g0[Ni(C0)4, 11 and 30.21 f 6.28 kJ/mol for the heat from the source, accelerated, and focused into a magnetic sector of vaporization of liquid Ni(C0)4 at 298.15 K. These values momentum analyzer for mass analysis. Mass-selected ions are lead to AfH298"[Ni(C0)4, g] = -601.6 f 10.5 kJ/mol, in slowed to a desired kinetic energy and focused into an octopole excellent agreement with the value cited in the critical compilaion guide that radially traps the ions. The octopole passes -598.3 k 4.2 kVmol. The heat of formation tion of Lias et through a static gas cell containing the neutral reactant at of Ni(C0)4 at 0 K can be derived by using the following relatively low pressures (0.05-0.25 mTorr). After exiting the relationship for polyatomic molecules:34 gas cell, product and unreacted beam ions drift to the end of
-
A.fH,o = 'fH298'
+
+ iHO0 - H298°]compound -
c[HO"
- HZ98°]elements (2)
where for a nonlinear polyatomic molecule
[H,"- H~o]compound-4RT
- RTxu/(e"
- 1)
(3)
and u = hvi/kBT. The 4RT term in eq 3 has contributions of 3RT/2 from translation, 3RT/2 from rotation, and RT from APV = AnRT for 1 mol of ideal gas. The summation in eq 3 is carried out over the vibrational frequencies of the polyatomic molecule, vi. The vibrational frequencies for Ni(C0)d are taken from ref 34 and lead to an enthalpy contribution of 8.30 RT. Thus, [Ho" - &go] for Ni(C0)4 is -12.30RT = -30.49 kJ/ mol. The enthalpy changes, [Ho" - H298'1, of the elements are -4.786, -4.204, and -17.366 kJ/mol for Ni(c), 4C(graphite) and 202, re~pectively.~~ Substituting these values in eq 2, we obtain AfHoo[Ni(C0)4,g] = -605.7 f 10.5 kJ/mol. The ionization energy of Ni(C0)4 has been measured in photoionization studies as 8.28 & 0.03,368.28 f 0.01,37 8.32 & 0.01,21and 8.21 eV38and in electron impact ionization studies as 8.64 f 0.15,228.35 f 0.15,238.57 f 0.10,248.75 f 0.07,*j and 8.8 eV.26 We adopt the value of 8.273 f 0.046 eV, the mean of the values obtained in photoionization experiments, essentially the value cited in the critical compilation of Lias et
the octopole where they are directed into a quadrupole mass filter for mass analysis and then detected. Ion intensities are converted to absolute cross sections as described p r e v i o ~ s l y . ~ ~ Absolute uncertainties in cross sections are about f20%; relative uncertainties are &5%. Laboratory ion energies are related to center-of-mass (CM) frame energies by E(CM) = E(lab)m/(M m), where M and m are the masses of the ion and neutral reactant, respectively. The absolute energy scale and the corresponding full width at halfmaximum (fwhm) of the ion beam kinetic energy distribution are determined by using the octopole as a retarding energy analyzer as described p r e v i o ~ s l y . The ~ ~ absolute uncertainty in the energy scale is f0.05eV (lab). The energy distributions are nearly Gaussian with a fwhm of 0.25-0.4 eV (lab). Ion Source. Ni(CO),+ ions are created in a 1-m-long flow tube, using microwave and dc discharge sources.4o For the microwave discharge source, He is used as a carrier gas (flow rate = 7000 sccm, resulting in a flow tube pressure of ca. 550 mTorr). As the He enters the flow tube, it passes through a quartz tube, where it is excited and ionized in a microwave discharge. Ni(C0)4 vapor is introduced through a leak valve into the flow tube -5 cm downstream from the discharge and ionized by charge transfer from He+ and possibly by Penning ionization with He*. Enough energy is present in the Ni(C0)4+ thus formed to cause fragmentation to form Ni(CO),+ ions, x = 1-3. An alternate method of producing Ni(CO),+ ions is
+
J. Phys. Chem., Vol. 99, No. 19, 1995 7821
Ligand Effects in Organometallic Thermochemistry
TABLE 3: Vibrational Frequencies and Average Energies at 298 Ka species
&br
(A)
NiCO+
(B) (A)
Ni(C0)2+
(B) (A) (B)
Ni(CO)3+ Ni(C0)4+ NiN2+
(A) (B) (A)
Ni(N2)2+
(B) Ni(N&+
(A)
(B) Ni(Nh+ NiNO+
(A)
(B) (A)
Ni(N0)2+
(B) (A)
Ni(NO)3+
(B)
eV
0.046 0.039 0.134 0.120 0.156 0.185 0.216 0.046 0.039 0.134 0.120 0.156 0.185 0.216 0.046 0.039 0.134 0.120 0.156 0.185
freq, cm-' (degeneracies)' 166,221(2), 2381 240,270(2), 2416 36(2), 149,225,231(2), 254(2), 2381,2385 52(2), 217,303,273(2), 299(2), 2417,2423 65(3), 250,350(6), 400(2), 2200(3) 45(3), 150(2), 250(3), 350(4), 2300(3) 2132.4, 2057.8(3), 458.9(3), 423.1(3), 380(2), 370.8, 300(3), 79(3), 62(2) 166,221(2), 2088 240,270(2), 2088 36(2), 149,225,231(2), 254(2), 2104,2106 52(2), 217,273(2), 299(2), 303,2104,2106 65(3), 250(1), 350(6), 400(2), 2140,2200,2230 45(3), 150(2), 250(3), 350(4), 2140,2200,2230 2174(2), 2246(2), 460(3), 420(3), 380(3), 300(3), 79(3), 62(2) 166,221(2), 1904.2 240,270(2), 1904.2 36(2), 149,225,231(2), 254(2), 1904.2(2) 52(2), 217,273(2), 299(2), 303, 1904.2(2) 65(3), 250(1), 350(6), 400(2), 1904.2(3) 45(3), 150(2), 250(3), 350(4), 1904.2(3)
Degeneracies in parentheses. (A) and (B)refer to two independent sets of estimated frequencies. Based on theoretically calculated vibrational frequencies for mono- and dicarbonyls of Cr and Cu in ref 30. The vibrational frequencies for Ni(C0)4+ are assumed to equal those for Ni(C0)4, ref 34.
ligation of Ni+ ions formed by a cold cathode dc discharge source.2 This source consists of a water-cooled cathode maintained at a high negative voltage, typically 1-3 kV, and covered by a cap made of tantalum with a hole for inserting a Ni rod. A mixture of He and 10% Ar at a total pressure of 500-600 mTorr flows over the cathode and is ionized by the dc field. Neutral and ionic metal atoms are sputtered off the cathode by accelerated Ar ions and entrained in the flow. CO is introduced downstream approximately 50 cm down the length of the flow tube, and Ni(CO),+ species are formed by threebody collisions. Both sources produce Ni(CO),+ ions with sufficient intensities (> lo5 ions/s) for the present experiments. Ni(N*),+ and Ni(NO),+ ions are generated in the dc discharge source by ligating Ni+ ions with N2 and NO, respectively. The intensity of the Ni(N0)3+ beam (> lo6 ions/s) was comparable to those of Ni(CO),+ beams, while those of Ni(NO),+ (x = 1 and 2) and Ni(N2),+ (x = 1-3) exceeded lo5 ions/s. Ni(N2)4+ was difficult to make, and the low intensity of the beam (5000 ions/s) is reflected by the signal-to-noise ratio of the cross sections for its dissociation products. While traversing the 1-m length of the flow tube, the NiL,+ ions undergo -lo5 collisions with the He (or He/Ar mixture) carrier gas. This environment should thermalize the internal energy distribution of the ions to 300 K, the temperature of the flow tube. Previous work on a number of systems is consistent with the production of thermalized ions under similar
condition^.^^^.^^ -44
collisions with the bath gases do not eliminate the possibility that the reactant species are electronically excited, as discussed later for the specific cases of NiCO+ and Ni(C0)2+. Second, effects due to multiple collisions with Xe are examined by performing the experiments at two or more pressures: -0.25 and 0.05 mTorr for most ions and -0.25, 0.10, 0.06, 0.03, and 0.015 mTorr for Ni(C0)4+. For all ions but NiNO+ and Ni(N2),+ (x = 3,4), the thresholds obtained at the higher Xe pressures are noticeably lower than those obtained at lower pressures. This pressure effect is eliminated, following a procedure developed previ0usly,4~by linearly extrapolating the cross sections to zero-pressure, rigorously single collision conditions. It is these extrapolated cross sections that are analyzed for their thresholds. Third, we showed in our study of Fe(CO),+ ions2 that a very important systematic effect on CID thresholds is the rotational and vibrational energy of thermalized ions. Because the rotational energy distribution is relatively narrow, we simply add the average rotational energy of the ions (kT = 0.026 eV for linear ions and 3kTl2 = 0.039 eV for nonlinear ions at 298 K) to the measured threshold. The vibrational energy of the ions is best handled by explicitly considering the entire distribution of populated vibrational states. The model used to reproduce the experimental cross sections is given by
+
a(@ = a o Z g i ( E E,, 4- Ei - Eo)"/E
(4)
I
Data Analysis. In our recent study of the collision-induced dissociation (CID) of Fe(CO),+ ions,2 we examined several systematic effects on deriving accurate thermodynamic information from CID thresholds. These effects include (a) intemal excitation of reactant ions above thermal, (b) multiple collisions with Xe, (c) thermal energy of the reactant ions that might contribute to the measured thresholds, and (d) the lifetime of the dissociating ions. Here, we account for each of the factors mentioned above as follows. First, the ions that traverse the 1-m flow tube are very likely thermalized by the los collisions they undergo, such that excess vibrational and rotational excitation is unlikely. This conclusion is further justified by our observations of identical results for Ni(CO),+ ions generated both by ligating atomic Ni+ and by dissociative ionization of Ni(C0)4. However, these
-
where E is the relative collision energy, Erotis the rotational energy of the reactants, EOis the reaction threshold at 0 K, and n is an adjustable parameter. The summation is over the vibrational states i having energies Ei and populations gi,where Zgi = 1. We assume that the relative reactivity, as reflected by a0 and n, is the same for all vibrational states. Details about our implementation of this equation are given elsewhereS2 Briefly, the Beyer-Swinehart algorithm46is used to evaluate the density of the ion vibrational states, and then the relative populations gi are calculated by the appropriate MaxwellBoltzmann distribution at 300 K. The vibrational frequencies of Ni(CO),+ ions used in this modeling are given in Table 3. We assume that the vibrational frequencies of Ni(C0)4+ are identical to those of Ni(C0)4, which
7822 J. Phys. Chem., Vol. 99, No. 19, 1995
are known.34 The vibrational frequencies for the remaining Ni(CO),+ ions, which are not known, are chosen as follows. We assume that the vibrational frequencies of mono- and dicarbonyl cations of Ni are intermediate between those of the Cr and Cu analogues, for which the vibrational frequencies have been c a l ~ u l a t e d .We ~ ~ find that CID thresholds obtained for NiCO+ and Ni(CO):!+ vary by less than 0.01 eV if the vibrational frequencies for their Cr vs Cu analogues are used. A comparison of the vibrational frequencies of Ni(C0)4+ and Ni(C0)2+ then reveals that the addition of CO ligands causes the bending and stretching vibrations involving the metal center to have larger frequencies, with a concomitant decrease in CO vibrational frequency. This allows us to make an educated guess for the vibrational frequencies of Ni(C0)3+, with upper and lower bounds given by those for Ni(CO):!+ and Ni(C0)4+. The vibrational frequencies chosen for Ni(N:!),+ and Ni(NO),+ are also listed in Table 3. We assume that the Ni-N bending and stretching vibrational frequencies in Ni(NZ),+ and Ni(NO),+ are identical to those for their Ni-C analogues in Ni(CO),+ for a particular value of x . Values for N2 vibrational frequencies are those measured for matrix-isolated neutral Ni(N2), c~mplexes.'~ The frequency for the stretching vibration of NO was varied by f200 cm-' of the value for free NO and found not to influence the average vibrational energy of Ni(NO),+ within 0.01 eV. We explicitly consider two sets of vibrational frequencies for Ni(CO),+, Ni(N:!),+, and Ni(NO),+ for all species where frequencies are guessed and find that the data analysis is insensitive to the choice of these frequencies such that the threshold energies vary by less than 5%. Finally, we explicitly examine lifetime effects on the thresholds by considering whether all ions with energies in excess of the dissociation energy dissociate within our experimental time window. The dissociation of NiL,+ species must occur during the flight time z from the gas cell to the quadrupole mass filter that is used for mass analysis. While z does depend on the s (as previously kinetic energies of the ions, it is roughly determined by time-of-flight measurements) in the threshold regions of the experiments described here. Dissociation of ions is expected to become slower as the size of the NiL,+ increases because this increases the number of vibrational modes where intemal energy can randomize. This lifetime effect has been examined in detail in our CID experiments of Cr(CO),+ ionsS3 On the basis of the analysis procedure described there, we find that lifetime effects are fairly small, 0.01 eV for Ni(C0)4+ and 0.015 eV for Ni(N0)3+, and negligible for Ni(C0)3+ and Ni(N&+. Lifetime effects should also be negligible for smaller ions in each homologous series. Before comparison with the experimental data, the model cross section of eq 4 (or its form that incorporates lifetime effect^)^ is convoluted over the ion and neutral translational energy distributions, as described p r e v i o ~ s l y .The ~ ~ parameters in eq 4, UO,EO,and n, are then optimized by using a nonlinear least-squares analysis to best reproduce the data. The optimized value of EO is taken to be the measured threshold for a given data set. Uncertainties in the reported thresholds are derived from the spread of EO values from different data sets acquired in experimental runs on two to four separate occasions, from the uncertainties introduced by the choice of vibrational frequencies of the NiL,+ ions, and from the absolute error in the energy scale. Data for NiCO+, Ni(CO):!+, NiN2+, and Ni(N2)2+ were acquired on only a single-experimental run, but at least three independent data sets were obtained for each of these species. We varied the value of EO above and below the value that best represents the data and reoptimized the values
Khan et al. ENERGY ( P V . Lab) 10.0
5.0
0.0
15.0
102 n
-
6 10-2 A
0.0
1.0
2.0
3.0
ENERGY
4.0 5.0 6.0 (oV. C M )
** 7.0
8.0
Figure 1. Cross sections for the reaction of Ni(C0)4+ with Xe at 0.20 mTorr as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). Sequential loss of CO ligands occurs to form Ni(C0)3+ (open squares), Ni(C0)2+ (filled circles), NiCO+ (open triangles), and Ni+ (filled diamonds). The solid line represents the total cross section for loss of CO ligands.
of a0 and n until these models no longer represent the data visually. This range of values for EO was taken as the uncertainty. In our analysis of Ni(C0)4+ and Ni(N:!)4+, we also use a modified form of eq 4 that accounts for a decline in the product ion cross section at higher kinetic energies due to further dissociation. This model has been described in detail previ0uslp7 and depends on ED, the energy at which a dissociation channel can begin, and p , a parameter similar to n in eq 4.
Results Collision-induced dissociation (CID) of NiL,+ species results in the sequential elimination of the ligand molecules. This is apparent in the data for Ni(C0)4+, shown in Figure 1, which is typical of the CID results for all the nickel-ligand cations. No ions with different numbers of C and 0 atoms in the CO system, N and 0 atoms in the NO system, or odd numbers of N atoms in the N2 system are observed. This observation is easily rationalized because individual CO, NO, and N:! bonds are substantially stronger than even the sum of the metal-ligand bonds in Ni(C0)4+, Ni(C0)3+, and Ni(N2)4+. The only other processes observed are the ligand-exchange reactions
+ Xe - Nixe+ + CO NiN2+ + Xe - Nixe+ + N, NiNO' + Xe - Nixe' + NO NiCO'
(5)
(6) (7)
Ligand exchange may also be taking place with Ni(CO),+, Ni(Nz),+, and NiNO,+ (x 2 2); however, we did not measure the cross sections for these products. Excited States of Ni(CO),+. Results for the CID of NiCO+ in three different experimental runs are shown in Figure 2. In the two runs having low-energy thresholds for production of Ni+, the ion beam was generated as described above. Comparable data were obtained for NiCO+ produced by ligation of Ni+ and by dissociative ionization of Ni(C0)4. The third set of data shown was obtained when 0 2 was added to the flow tube (at a pressure of 4-5 mTorr) and has an apparent threshold that is higher than the other two sets by about 1.3 eV. Note that the intermediate data set appears to be a superposition of
Ligand Effects in Organometallic Thermochemistry ENERGY (oV.
0.0
4 N "
NiCO*
+
Xo +
Ni*
J. Phys. Chem., Vol. 99, No. 19, 1995 7823
Lab) 5.0
0.0
1
CO + Xo
+
4.0
1
B
?!! 1
3.0
2 "
10.0
z
0 2
E U
~
N"
: "
Lab)
15.0
0
2
ENERGY (oV. 5.0
E I-
2.0
U
W VI
VI W
VI
:: 1.0
5.0
v)
VI
0
6
B
0.0I
0.0 f
0.0
~
'
'
'
1.0
I
'
'
'
2.0
'
I
'
'
'
3.0 ENERGY ( P V , CM)
ENERGY (oV.
0.0
'
I
'
4.0
'
'
'
~
5.0
'
'
'
+ Xo
4
NlXo*
+
CO
1 ' ' ~ ~ 1 ' ' ' * 1 ' ' 1 ' 1 ' 1 1 '
0.0
1.0
2.0 3.0 ENERGY CoV. CM)
4.0
~
1
0.0
1.0
~
~
~
2.0 3.0 ENERGY (0'4. CM)
~
'
4.0
'
'
I
5.0
~
'
Figure 3. Cross sections for the reaction of Ni(C0)2+ with Xe as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). Open squares (NiCO+, data extrapolated to zero Xe pressure) and solid squares (Ni+, increased by a factor of 50,0.20 mTorr of Xe) represent data acquired in an experimental run with the addition of 0 2 to the flow tube. Open circles (NiCO+) and solid circles (Ni+, increased by a factor of 50) represent data acquired at a Xe pressure of 0.20 mTorr when 0 2 was not added to the flow tube. The cross sections for Ni(CO)+ have been offset by 1 A2 for clarity. The dashed line is the model of eq 4 with the parameters in Table 4 for 0 K reactants. The solid line is this model convoluted over the translational, vibrational, and rotational energy distributions of the reactants.
Lab) 5.0
8.0
NiCO*
'
5.0
Figure 2. Cross sections for the formation of Ni+ (a, top) and Nixe+ (b, bottom) in the reaction of NiCO+ with Xe at a pressure of 0.05 mTorr as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). The circles and diamonds represent data obtained in two experimental runs without the addition of 0 2 to the flow tube, while the squares represent data when 02 was added. The dashed line in part a is the model of eq 4 with the parameters in Table 4 for 0 K reactants. The solid line is this model convoluted over the translational, vibrational, and rotational energy distributions of the reactants.
the other two cross sections. The cross section for the ligandexchange reaction to form Nixe+ shows parallel behavior, Figure 2b. When 0 2 is not present in the flow tube, the Nixe+ cross section exhibits an exothermic feature, while the presence of 02 in the flow tube eliminates this feature. Behavior similar to that shown in Figure 2 is also observed for loss of both one and two CO ligands from Ni(C0)2+, Figure 3, with apparent thresholds that again differ by roughly 1.3 eV. As for NiCO+, Ni(C0)2+ can be produced either by dissociative ionization of Ni(C0)4 or by ligation of Ni+. Both sources yielded cross sections exhibiting these bimodal features, and no systematic differences in the CID cross sections were observed for NiCOf and Ni(C0)2+ produced by using either of these methods. For Ni(C0)3+ and Ni(C0)4+, cross sections for the loss of CO exhibited small exothermic features in a few but not all of the experimental runs and only in those measured when 0 2 was not added to the flow tube. These low-energy features in the cross sections are clearly due to intemal excitation in the nickel carbonyl ions. It seems unlikely that the excess energy content of 1.3 eV is due to
vibrational energy because "hot" clusters would not exhibit the sharp bimodal distribution evident in Figures 2 and 3. We therefore attribute these features to electronically excited reactant ions and attempted to characterize this excitation by performing several diagnostic experiments. We explicitly examined the possibility that electronically excited Ni+ ions were abundant in the flow tube by carrying out a reaction of these ions (produced in the dc discharge source) with 0 2 . The cross sections for these reactions are consistent with a beam containing only ground-state ions as characterized previously$* although this experiment does not exclude the possibility that such excited states are present at the beginning of the flow tube. The key experiment is that the low-energy features in the CJD and ligandexchange cross sections for NiCO+ and Ni(C0)2+ can be eliminated by adding 0 2 early in the flow tube. 0 2 was chosen for this purpose because its diradical character allows it to react efficiently with various electronic states of atomic transitionmetal ions.48 We find experimentally that the presence of 0 2 either quenches electronically excited states of Ni+ more efficiently than Ar and He, preventing the production of excited Ni(CO),+ complexes, or possibly quenches the excited Ni(COX+ ions after production. The variations noted in the amount of these excited states, Figure 2, is consistent with some quenching by Ar and He, which apparently depends on the details of the pressure and excitation conditions in the flow tube. It is interesting that once formed these excited Ni(CO),+ complexes are not easily quenched by further collisions with Ar and He. This is discussed further below. CID of Ni(CO),+. In this section, we examine the variations in the CID behavior for nickel carbonyl cations that are generated in the presence of 0 2 in the flow tube and hence are believed to be in their ground electronic state. The interaction of NiCO+ with Xe is shown in Figure 2. The cross section for Ni+ has an apparent threshold of -1.6 eV. Ligand exchange to form Nixe+ exhibits a lower apparent threshold than the formation of Ni+, and a cross section that peaks as the cross section for the CID process increases. The mass resolution of
'
~
~
'
7824 J. Phys. Chem., Vol. 99, No. 19, 1995
Khan et al.
Lob)
ENERGY (oV.
ENERGY (oV.
5.0
0.0 N i KO),*
+
Xo
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Lab)
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(CO),*
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+
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+
30.0 N "
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z
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0.0
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ENERGY h V .
3.0
4.0
1 ' " ' 1.0 I " ' ' "2."0 ' " " 3.0 ' ~
Figure 4. Cross sections for the reaction of Ni(CO)3+to form Ni(C0)2+ (open squares, data extrapolated to zero Xe pressure) and NiCO' (solid circles, increased by a factor of 20,0.05 mTorr of Xe) as a function of relative kinetic energy (lower x axis) and laboratory energy (upper n axis). The dashed line is the model of eq 4 with the parameters in Table 4 for 0 K reactants. The solid line is this model convoluted over the translational, vibrational, and rotational energy distributions of the reactants.
the quadrupole mass spectrometer was set sufficiently low that the cross sections shown in Figure 2b should represent the product intensities for all isotopes of Xe. Results for the CID of Ni(C0)2+ with Xe are shown in Figure 3. The cross section for the NiCO+ product has a somewhat lower apparent threshold than that for the formation of Ni+ in Figure 2. Loss of two CO ligands from Ni(CO)Z+ is very inefficient and rises from an apparent threshold greater than 3 eV. The CID pattem in Ni(C0)3+, shown in Figure 4,is different from that of NiCO+ and Ni(C0)2+ in that the apparent threshold for the loss of a single CO is substantially lower and the cross section is much larger. This cross section remains relatively constant above 2.5 eV. The secondary NiCO+ channel has a cross section that is much smaller and rises slowly from its threshold. The CID pattem for Ni(C0)4+ is illustrated in Figures 1 and 5. The threshold for the loss of a single CO is fairly small, below 0.4 eV, and the Ni(C0)3+ cross section exhibits an appreciable decline at the apparent onset of Ni(C0)2+. This behavior is clearly due to the sequential nature of CO loss because the total cross section remains fairly constant at elevated energies, Figure 1. CID of Ni(N&+. CID of Ni(NZ),+ species, like their carbonyl analogues, results in the sequential loss of NZ molecules. Bimodal cross sections were observed for NiN2+ and Ni(N&+ when 0 2 was not added to the flow tube. We did not attempt to characterize the low-energy portions of these cross sections to the degree we did for Ni(CO),+, (x = 1, 2 ) . However, we attribute these features to electronically excited states, as is the case with the carbonyl analogues. These excited states appear to be efficiently quenched by adding 0 2 to the flow tube. For Ni(N2)3+ and Ni(N2)4+, we found that comparable results were obtained whether 0 2 was present in the flow tube or not. The general behavior of these cross sections is similar to that for the Ni(CO),+ complexes. The CID cross sections for the loss of a single NZligand from each of the four complexes are shown in Figure 6. For interaction of NiN2+ with Xe, the cross section for Ni+ has an apparent threshold near 1 eV, much less than that for loss of CO from NiCO+, and reaches a magnitude
4.0
0.0
ENERGY rev.
CM)
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Figure 5. Cross sections for the reaction of Ni(C0)4+ with Xe at 0.05 mTorr to form Ni(C0)3+ (open squares) and Ni(C0)2+ (solid circles) as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). The dashed line is the model of eq 4 with the parameters in Table 4 for 0 K reactants and includes the modification to account for dissociation at higher energies. The solid line is this model convoluted over the translational, vibrational, and rotational energy distributions of the reactants. 60.0
I
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Figure 6. Cross sections for loss of an NZ ligand in CID reactions of Ni(N2)4+ (open squares), Ni(N2)3+ (solid circles), Ni(N2)2+ (open triangles, increased by a factor of 2), and NiN2+ (solid diamonds, increased by a factor of 10) as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). The Xe presssure was 0.05 mTorr in all cases except Ni(N2)4+where it was 0.20 mTorr.
of 3 A2 by 3 eV. CID of Ni(N&+ with Xe exhibits a cross section for loss of N2 that is substantially larger than that from NiNz+, 16 A2 at 3 eV, and has a similar apparent threshold. The CID pattem in Ni(N2)3+ is different from those of Ni(Nz)+ and Ni(N2)2+. The apparent threshold for the loss of a single N2 is substantially lower (x0.2 eV) and the cross section is much larger, -40 A2, between 1.5 and 4 eV. The CID pattem for Ni(N2)4+ has an apparent threshold for the loss of a single N2 near zero. The magnitude of this cross section is -52 A2 at 0.5 eV and exhibits an appreciable decline at the apparent onset of Ni(N2)2+ (not shown in this figure) near 0.5 eV, due to sequential NZ loss. CID of Ni(NO),+. No bimodal features were observed in the CID cross sections for the Ni(NOX+ complexes. In addition, we verified that the results did not change when 0 2 was present in the flow tube. The cross sections for the loss of an NO ligand from each of the three Ni(NO),+ species (x = 1-3) are shown in Figure 7. For Ni(N0)3+, the cross section has an apparent
J. Phys. Chews., Vol. 99, No. 19, 1995 7825
Ligand Effects in Organometallic Thermochemistry 1
,
,
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,
1
,
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1
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.
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.
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TABLE 4: Summary of Parameters in Equation 4" species uo, A2 eV'-" EO,eV
.
+ Xo +
30.0
Ni+-CO (CO)Ni+-CO (C0)2Ni+-CO (CO)sNi'-CO Ni+-N2 (N2)Ni+-N2 (N2)2Ni+-N2 (N2)3NiC-N2 Ni+-NO (NO)Ni+-NO (NO)zNi+-NO
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Figure 7. Cross sections for loss of an NO ligand in CID reactions of Ni(NO)3+ (open squares, extrapolated to zero Xe pressure), Ni(N0)2+ (solid circles, extrapolated to zero Xe pressure, increased by a factor of 1.75), and NiNOf (open triangles, 0.05 mTorr of Xe, increased by a factor of 20) as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis).
threshold of about 0.8 eV and reaches a maximum of 26 A2 between 2.5 and 6 eV. The apparent threshold for the loss of NO from Ni(N0)2+ is somewhat higher, and the cross section is smaller by a factor of about 2. For NiNO+, loss of NO occurs at a substantially higher apparent threshold, -2.2 eV, than that observed for the loss of a ligand from any other reactant species reported in this work. In our experiments with the Ni(NO),+ species, an NO+ product was also observed, but the magnitudes of the cross sections for this product varied considerably (by over an order of magnitude from one data set to another). This variation was observed to correlate with the amount of NO added to the flow gases, suggesting that the Ni(NO),+ beams were contaminated with other ions containing NO that have similar masses, possibly N~(NO),+I+and (NO),+*+ for 58Ni(NO),+ and @"i(NO),+, respectively. To test this idea, we examined the CID of 58NiNO+(m/z = 88), @"iNO+ (m/z = go), (N0)3+ ( d z = go), a cluster ion that is easily formed at high partial pressures of NO, -10% of the total pressure in the flow tube, and (NO),' (x = 4,5), clusters that are more difficult to form. The (NO),+ clusters were characterized in the absence of Ni+ ions by using a microwave discharge source at the beginning of the flow tube. These studies verified that the NO+ product did not come from the Ni(NO),+ reactants and that the CID cross sections shown in Figure 7 are due exclusively to the nickel-containing products indicated. The results suggest that the level of contamination was typically a few percent for a NiNO+ beam and lower for Ni(NO),+ (x = 2, 3) beams. BDEs from Primary Thresholds. As concluded in our previous CID experiment^,^,^ our best measure of the bond dissociation energies for NiL,+ ions comes from analyses of the primary dissociation channels, NiL,'
+ Xe - NiL,-,+
-tL -I- Xe
(8)
Listed in Table 4 are the optimized parameters of eq 4 obtained from the analyses of reactions 8. In the case of NiL+ and NiL2+ species where L = CO and N2, only data sets where 0 2 was present in the flow tube were analyzed. In all other cases, the presence of 0 2 did not change the data enough to alter the parameters of eq 4 within experimental error. In all cases but Ni(N2)4+, the model of eq 4 accurately reproduces the magnitudes of the cross sections over a range of at least 2 orders of
~
~
4.0(0.6) 16(3) 44(3) 66(4) 4.1(0.8) 22(3) 56(6) 75(1) 1.2(0.3) 20(2) 460)
lAl(O.11) 1.74(0.11) 0.95(0.06) 0.75(0.03) 1.15(0.11) 1.15(0.11) 0.58(0.04) 0.44(0.10) 2.36(0.08) 1.27(0.07) 1.19(0.05)
n
1.2(0.1) 1.4(0.2) 1S(0.1) 1.2(0.1) 1.3(0.2) 1.4(0.3) 0.9(0.1) 0.8(0.1) 1,8(0,1) 1.4(0.1) 0.9(0.1)
Uncertainties in parentheses.
magnitude. In this single exception, the data can be reproduced nicely from 0.1 to 1.6 eV but not at the lowest energies ( D(Ni+-CO) > D(Nif-N2), Table 4, in agreement with the bond energy ordering anticipated in the Introduction. The same order is found for the third ligand BDEs, D(L2Nif-L), but not for the second where D[(CO)Ni+CO] > D[(NO)Ni+-NO] % D[(N2)Nif-N2]. This strong divergence from the expected pattem in BDEs is probably related to the total number of electrons being donated to the nickel ion. CO and N2 are both two-electron donors, whereas NO is usually a three-electron donor, although it can serve as a one-electron donor as well. Thus, if we consider the sequential NiL,+ BDEs, not as a function of the total number of ligands but as a function of the number of electron pairs donated by the ligands, we get the pattern shown in Figure 9 which shows parallel trends in the sequential BDEs and the anticipated BDE ordering of NO > CO > N2. The observation in Figure 9 that the sequential Ni(N&+ bonds are considerably weaker than their carbonyl analogues is in accord with the general perception that N2 is both a poorer a donor and n acceptor than CO. Further quantitative comparison can be made by considering the ratios of sequential bond energies of Ni(CO),+ and Ni(Nz),+. Values for D[(CO),-lNi+CO]/D[(N~),-IN~+-N~] are 1.57 f 0.18, 1.51 f 0.17, 1.64 f 0.15, and 1.70 f 0.39 for x = 1-4, respectively. These ratios demonstrate that changes in bonding patterns vary similarly for both ligands upon increasing the degree of ligation around Ni+. We note that these ratios are very similar to D[Cr+-CO]/ D[Cr+-N2] = 1.52 f 0.12, based on measurements of the CrCO+ and CrN2+ B D E s . ~ , ~ ~ A quantitative comparison of the sequential Ni(NO),+ BDEs to the CO and N2 analogues is somewhat more complicated. While CO and N2 have a dative interaction with transition-metal cations, NO can form a covalent bond with Ni+(*D,3d9). Thus, the Ni+-NO bond is substantially stronger than the Ni+-CO and Ni+-N2 bonds. The (NO)Ni+-NO bond is weaker than the (CO)Ni+-CO bond because 4s-3da hybridization is not advantageous in the former molecule and a second NO ligand cannot bind strongly to Ni+-NO, which presumably has a singlet ground state. However, (NO)Ni+-NO is a stronger bond than (C0)2Ni+-CO, presumably due to lesser steric crowding and the greater n-acceptor capabilities of NO in comparison with CO and N2. On similar grounds, (NO)ZNi+-NO is a stronger bond than (CO)3Nif-C0. The addition of a third NO to Ni(N0)2+ does not cause a decrease in bond energy, unlike
7828 J. Phys. Chem., Vol. 99, No. 19, 1995
in the cases of Ni(CO)Z+ and Ni(N2)2+, where the addition of a third ligand weakens the bond substantially. This observation may be rationalized by the fact that Ni(N0)3+ is an 18-electron species.
Conclusions We report gas-phase measurements of sequential Ni(CO),+ and Ni(N2),+ (x = 1-4) and Ni(NO),+ ( x = 1-3) BDEs by collision-induced dissociation. We also observe electronically excited NiCO+ and Ni(C0)2+ and measure their bond energies. Good agreement between the sum of the four sequential BDEs in Ni(C0)4+ and the literature value for A,W( 1) is found. The relative metal-ligand bond strengths for CO, N2, and NO are in good accord with the qualitative picture of bonding that is based on o-donor and n-acceptor capabilities of ligands and the number of electrons donated by each ligand.
Acknowledgment. This work is supported by the National Science Foundation, Grant No. CHE-9221241. F.A.K. thanks Prof. Tom Richmond, Dr. George Richter-Addo, and Dr. Brian Buffin for several informative conversations. D.L.S. thanks NSF and the University of Utah for support under the Research Experience for Undergraduates (REU) program. References and Notes (1) See, for example: Pilcher, G.; Carson, A. S. In Energetics of Organometallic Species; Simoes, J. A. M., Ed.: Kluwer Academic: Dordrecht, 1992. (2) Schultz. R. H.: Crellin. K. C.: Armentrout. P. B. J . Am. Chem. SOC.'1991, 113, 8590. (3) Khan, F. A.: Clemmer, D. E.: Schultz, R. H.: Armentrout. P. B. J. Phys. Chem. 1993, 97, 7978. (4) Schultz, R. H.; Armentrout, P. B. J. Phys. Chem. 1993, 97, 596. (5) Sunderlin, L. S.; Wang, D.; Squires, R. R. J . Am. Chem. SOC.1992, 114, 2788. (6) See, for example: Collman, J. P.; Hegedus, L. S. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1980. Yamamoto, A. Organotransition Metal Chemistrv: Fundamental Conceots and Aoolications:, Wilev-Interscience: , New Yo;k, 1986. (7) Mond, L.: Laneer, C.: Ouincke. F. J. Chem. SOC.1890. 749. (8) See, for exampi: Organ& Synthesis via Metal Carbonyls;Wender, I., Pino, P., Eds.; Wiley-Interscience: New York, 1977. Poliakoff, M. Chem. SOC.Rev. 1978, 7, 527. (9) Hoffmann, R. Angew. Chem., lntl. Ed. Engl. 1982, 21, 711. (10) See, for example: Allen, A. D.; Harris, R. 0.; Loescher, B. R.; Stevens, J. R.; Whiteley, R. N. Chem. Rev. 1973, 73, 11. (1 1) See, for example: Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Rev. 1978, 78, 589. (12) Pelikan, P.; Boca, R. Coord. Chem. Rev. 1983, 55, 55. (13) Huber. H.: Kundig. ", E. P.:, Moskovits., M.:, Ozin. G. A. J. Am. Chem. Soc~.1973, 95,' 332. (14) Devore. T. C. Inora. Chem. 1976, 15, 1315. (15) See, for example: Richter-Addo, G.; Legzdins, P. Metal Nitrosyls; Oxford University: New York, 1992. (16) See, for example: Clark, R. J. Inorg. Chem. 1967,6,299. Keeton, D. P.; Basolo, F. lnorg. Chem. Acta 1972, 6, 33. Crichton, 0.;Rest, A. J. J. Chem. SOC.,Dalton Trans. 1977, 536, 656. (17) Kubota, M.; Chan, M. K.; Boyd, D. C.; Mann, K. R. lnorg. Chem. 1987, 26, 3261. (18) Representative examples are: Bach, S. B. H.; Taylor, C. A,; Van Zee, R. J.; Vala, M. T.; Weltner, W., Jr. J. Am. Chem. SOC.1986, 108, 7104. Ray, U.; Brandow, S. L.: Bandukwalla, G.; Venkataraman, B.; Zhang, Z.; Vernon, M. J . Chem. Phys. 1988, 89, 4092. McQuaid, M. J.; Moms, ' 1
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