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Early atomic transition metal cations reacting with ammonia at room temperature: H2 elimination and NH3 ligation kinetics across and down the periodic table
International Journal of Mass Spectrometry 435 (2019) 181–187
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Early atomic transition metal cations reacting with ammonia at room temperature: H2 elimination and NH3 ligation kinetics across and down the periodic table夽 Voislav Blagojevic, Vitali V. Lavrov, Gregory K. Koyanagi, Diethard K. Bohme ∗ Department of Chemistry, Centre for Research in Mass Spectrometry and Centre for Research in Earth and Space Science, York University, Toronto, Ontario, Canada M3J 1P3
a r t i c l e
i n f o
Article history: Received 2 August 2018 Received in revised form 6 October 2018 Accepted 11 October 2018 Available online 22 October 2018 Keywords: Gas phase Reactions of early atomic transition metal cations Ammonia Periodic trends in ion kinetics Ligation Imide bond formation Di-imido bond formation Orbital filling ICP/SIFT technique
a b s t r a c t The kinetics of H2 elimination and ligation of ammonia reacting with early atomic transition metal cations and ensuing ammonia ligation reactions were measured across (Groups 3–9) and down (the three rows) the periodic table. The atomic cations were produced in an Inductively Coupled Plasma (ICP) source at ca. 5500 K and allowed to decay radiatively and to thermalize by collisions with argon and helium atoms prior to reaction. Kinetic measurements were performed using an ICP/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer with helium bath gas at 0.35 Torr and 295 ± 2 K. Our survey showed that bimolecular H2 elimination to produce MNH+ predominated with very early atomic transition metal cations: M+ = Sc+ and Ti+ in the first row, Y+ , Zr+ and Nb+ in the second row, and La+ , Hf+ , Ta+ , W+ , Os+ and Ir+ in the third row. The remaining atomic transition metal cations were observed to ligate ammonia (with He acting as a third body): M+ = V+ , Cr+ , Mn+ , Fe+ and Co+ in the first row, Mo+ , Ru+ and Rh+ in the second row, and Re+ and Ir+ in the third row. Higher order ligation reactions were observed with M+ to form M+ (NH3 )n with n up to 6 and MNH+ (NH3 )n with n up to 3. Experiments with three of the 3rd row MNH+ cations (M = Ta+ , W+ and Os+ ) revealed the rapid formation of a second imido bond to produce M(NH)2 + followed by ammonia ligation. Variations in the occurrence of imido bond formation and the extent of ligation and trends in measured rate coefficients are scrutinized across and down the periodic table in terms of available theoretical computations of potential energy surfaces as well as atomic orbital filling effects. © 2018 Elsevier B.V. All rights reserved.
1. Introduction The chemical behavior of the d-block transition metals is known to be driven by their electronic configuration or orbital occupancy which varies across and down the periodic table. Insight into this behavior can be ascertained by systematic reactivity measurements across and down the periodic table, ideally with a common neutral reagent that exhibits chemical bond redisposition. Here we systematically explore ammonia as a common neutral reagent and follow its reactions with early atomic transition metal cations at room temperature across (Groups 3–9) and down (the three rows) the
夽 Dedicated to Helmut Schwarz on the occasion of his 75th birthday and in appreciation of his constant friendship and superb contributions to ion chemistry. ∗ Corresponding author. E-mail address: [email protected] (D.K. Bohme). https://doi.org/10.1016/j.ijms.2018.10.021 1387-3806/© 2018 Elsevier B.V. All rights reserved.
periodic table. We focus on direct ligation, reaction (1a), and imide bond formation, reaction (1b), M+ (NH3 )n + NH3 → M+ (NH3 )n+1 +
+
M(NH)n + NH3 → M(NH)n+1 + H2
(1a) (1b)
as well as the ensuing ammonia ligation reactions (1c), M(NH)n+1 + (NH3 )m + NH3 → M(NH)n+1 + (NH3 )m+1
(1c)
and track variations in the efficiencies of these reactions with rate coefficient measurements. When available, observed variations in the efficiencies of these three reactions are interpreted in terms of d-orbital electron filling effects. Previously we have investigated the ammonia ligation of Fe+ and Ru+ (Group 8) for which we noted an influence of a spin-surface crossing on the kinetics of the higher-order sequential ligation of Ru+ [1,2]. Quite recently we discovered relativistic effects in reactions of ammonia with Group 10, 11 and 12 atomic transition metal cations that exhibited only ligation chemistry (except Hg+ which reacts by electron transfer) and were sensitive to orbital size and
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orbital occupancy [3–5]. Also, we previously observed H2 elimination in our room temperature measurements of reactions of 14 lanthanide atomic cations with ammonia [6]. Ligation was again found to be the predominant mode of reaction with the lanthanides, but La+ (5d2 ), Ce+ (4f1 5d2 ), Gd+ (4f7 5d1 6s1 ), and Tb+ (4f9 6s1 ) exhibited bond redisposition to form the protonated lanthanide imide LnNH+ . Apparently, only in these latter reactions is the electrostatic attraction between the atomic lanthanide cation and ammonia sufficiently strong to provide enough energy to achieve a 5d1 6s1 configuration by electron promotion at room temperature and so to overcome any intrinsic barriers to subsequent N-H bond insertion and H2 loss [6]. There have been previous experimental measurements by others of reactions of Group 3–9 atomic transition metal cations with ammonia. Early measurements with a Fourier Transform Mass Spectrometer (FTMS) and atomic transition metal ions produced by pulsed laser desorption/ionization were focused on the production of metal-imide MNH+ ions (by H2 elimination) which were seen to be produced by Group 3–5 (1st, 2nd and 3rd rows, except Hf+ ) atomic ions while Group 6–11 (1st row) atomic ions were seen to just add ammonia [7]. Laser ablation-molecular beam technology provided probably the first report of the formation of metal-diimide M(NH)2 + ions (by sequential H2 elimination) in the case of Nb+ and Ta+ with further ammonia ligation [8]. Also, Cr+ , Mn+ , Fe+ , Co+ , Ni+ , Cu+ and Ag+ were seen to form ammonia clusters M+ (NH3 )n (n ≤ 20) [8]. Later, guided ion beam tandem mass spectrometer experiments were reported that explored the role of electronic and translational excitation with Sc+ and Ti+ [9], V+ [10], Cr+ [11], Fe+ [12] and Co+ [12,13,11]. Ammonia reactions of La+ and several other lanthanoid cations have been investigated previously with a view to the analysis of nuclear fuels [14]. Quantum chemical computations of potential energy surfaces that provide insight into structural and energetic aspects of the mechanism of bond redisposition and the role of electronic spin have been reported for the reactions of ammonia only with the first row transition metal cations Sc+ [15], Ti+ , V+ and Cr+ [16], Mn+ [17], Fe+ [18], Co+ [19] as well as with La+ [14]. A partial overview of experimental and theoretical aspects of reactions of ammonia with atomic transition metal cations was provided in 2012 [20].
2. Experimental procedures The experimental results reported here were obtained with the ICP/SIFT tandem mass spectrometer that has been described in detail previously [21–24]. The atomic ions were generated within an atmospheric pressure argon plasma at 5500 K fed with a vapourized solution containing the metal salt. Solutions containing the metal salt of interest with concentration of ca. 5 g L−1 were peristaltically pumped via a nebulizer into the plasma. The nebulizer flow was adjusted to maximize the ion signal detected downstream of the flow tube. The sample solutions were prepared using atomic spectroscopy standard solutions commercially available from SPEX, Teknolab, J.T. Baker Chemical Co., Fisher Scientific Company, Perkin-Elmer and Alfa Products. Aliquots of standard solutions were diluted with highly purified water produced in the Millipore Milli-Qplus ultra-pure water system. The final concentrations were varied between 5 and 20 ppm to achieve suitable intensity of the resultant ion beam. A stabilizing agent was usually added to each solution in order to prevent precipitation: KOH for base-stabilized salts, HNO3 or HCl for acid-stabilized salts. Atomic ions emerge from the ICP at a nominal plasma ion temperature of 5500 K with the corresponding Boltzmann distributions [25,26]. After extraction from the ICP, the plasma ions may experience electronic-state relaxations via both radiative decay and
collisional energy transfer. The latter may occur by collisions with argon, as the extracted plasma cools upon sampling, and with helium in the flow tube (ca. 4 × 105 collisions with helium) prior to the reaction region. However, the exact extent of electronic relaxation is uncertain. Clues to the presence of excited electronic states of the atomic ions in the reaction region can be found in the product ions observed and in the shape of the semi-logarithmic decay of the reacting atomic ion upon addition of neutral reactants. Curvature will appear in the measured atomic-ion decay when the ground state and excited state react at different rates even when they give the same product ions. An excited-state effect can be hidden when the products and reaction rates are the same for both the ground and excited states, but in this case the measured atomic-ion decay defines the ground-state kinetics. On occasion the presence of excited states is revealed by a difference in product ion formation. The many collisions experienced by the atomic cations with the quite polarizable argon atoms as they emerge from the ICP and the ca. 4 × 105 collisions with helium atoms in the flow tube (the helium buffer gas pressure was 0.35 ± 0.01 Torr) appear to be sufficient to thermalize the excited states and to ensure that the atomic ions reach a translational temperature equal to the tube temperature of 295 ± 2 K prior to entering the reaction region. Reactions of atomic cations were investigated with NH3 at a helium buffer gas pressure of 0.35 ± 0.01 Torr and temperature of 295 ± 2 K. Reaction rate coefficients were determined in the usual manner using pseudo first-order kinetics. The highly pure NH3 gas was obtained commercially (Semiconductor Grade 99.999%, Matheson/Linde Canada), and introduced into the reaction region of the SIFT as a dilute (15%) mixture in helium. 3. Results and discussion Our survey of reactions of ammonia with atomic transition metal cations (Groups 3–9) showed that bimolecular H2 elimination predominated with very early atomic transition metal cations: Sc+ and Ti+ in the first row, Y+ , Zr+ and Nb+ in the second row, and La+ , Hf+ , Ta+ , W+ , Os+ and Ir+ in the third row. The remaining atomic transition metal cations were observed to ligate ammonia (with He acting as a third body): V+ , Cr+ , Mn+ , Fe+ and Co+ in the first row, Mo+ , Ru+ and Rh+ in the second row, and Re+ and Ir+ in the third row. As an indication of the quality of our experimental results, we provide our data for H2 elimination for the reactions of ammonia with the first-row cation Sc+ , the second-row cation Y+ , and the third-row cations Ta+ and Os+ . A composite of these results is provided in Fig. 1; we have reported previously experimental results for La+ [6]. All semi-logarithmic atomic ion decays for the primary H2 elimination reactions, reactions of type (1), were observed to be linear over more than a decade, so that there was no evidence for the presence of excited states which could be manifested by a change in slope. Ensuing higher-order ammonia ligation reactions of MNH+ with ammonia molecules, reactions of type (2), were observed in each case. Ligation was observed with atomic transition metal cations that did not react by H2 elimination. In two cases, the osmium and iridium experiments, NH4 + was observed to be formed (NH3 + quickly transforms to NH4 + in its reaction with ammonia); we attribute this formation to the occurrence of electron transfer from ammonia to excited Os+ * and Ir+ *, reactions (2) and (3). Os+ ∗ + NH3 → NH3 + + Os +
+
Ir ∗ + NH3 → NH3 + Ir
(2) (3)
The ionization energies of the Group 3–9 transition metals are quite low, with IE(Ir) = 8.967 eV being the highest [27]. Since IE(NH3 ) = 10.07 eV [27], ammonia should not transfer an electron
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Fig. 1. Composite of ICP/SIFT results for the reactions of ammonia with selected first, second, and third-row atomic transition metal cations that lead to H2 elimination in He buffer gas at 0.35 ± 0.01 Torr and 295 ± 2 K. Symbols represent experimental data and lines represent kinetic fits to that data. Only the 190 Os isotope is plotted, representing 60% of the overall Os+ ion signal. This exaggerates the intensity of NH4 + (as does mass discrimination), which does not have multiple isotopes.
to any of the Group 3–9 ground-state atomic transition metal cations that were studied. Os (IE = 8.438 eV) and Ir (IE = 8.967 eV) [27] are the two atoms with the highest ionization energies and our data indicate that excited states may be populated in Os+ and Ir+ that have sufficiently high energies to promote electron transfer. Os+ (4 D) provides an almost thermoneutral opportunity for electron transfer with Eav (4 D–6 D) = 1.62 eV [28] for about 5% of the Os+ ions generated in the ICP in the 4 D state. Electron transfer is also possible with Ir+ ions that are excited in the ICP at 5500 K of which 20% [28] have energy levels above 10.08 eV. With regard to higher order reactions of the MNH+ ions, experiments with three of the 3rd row MNH+ cations (M = Ta+ , W+ and Os+ ), revealed the rapid formation of a second imide bond with H2 elimination according to reaction (4), prior to ammonia ligation. MNH+ + NH3 → M(NH)2 + + H2
(4)
Di-imide bond formation was not observed with Nb+ , in contrast to the report of the observation of Nb(NH)2 + in very early laser ablation-molecular beam experiments, albeit only at very low levels (ca. 2% of the single imide) [8]. The di-imide cations M(NH)2 + were seen to sequentially coordinate further ammonia molecules, both in our experiments as well as in the beam experiments.
3.1. Experimental rate coefficients The measured kinetics of the ammonia reactions observed with first, second and third row atomic transition metal cations M+ (Groups 3–9) is summarized in Table 1. Also provided in Table 1 are the known ground-state electronic configurations of the atomic cations, computed values for the collision rate coefficients, kc , derived from collision theory [29] and the reaction efficiencies, k/kc .
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Table 1 Bimolecular and effective bimolecular rate coefficients, k (in units of cm3 molecule−1 s−1 ), for H2 elimination and ammonia addition, respectively, with first, second and third row atomic transition metal cations M+ (Groups 3–9) measured at 295 ± 2 K using the ICP/SIFT technique in our laboratory. The helium buffer gas pressure was 0.35 ± 0.01 Torr. Absolute uncertainties in the rate coefficients are estimated to be ±30%; relative uncertainties are estimated to be <15%. The collision rate coefficients, kc (see text, in units of cm3 molecule−1 s−1 ) and the reaction efficiencies, k/kc , are also provided. The ground electronic state configuration of the atomic transition metal cation is also given. M+
Electronic configuration
k
kc
k/kc
Primary product
Sc+ Ti+ V+ Cr+ Mn+ Fe+ Co+ Y+ Zr+ Nb+ Mo+ Ru+ Rh+ La+ Hf+ Ta+ W+ Re+ Os+ Ir+
3
3.2 × 10−10 2.5 × 10−10 1.1 × 10−11 5.0 × 10−12 2.1 × 10−12 1.7 × 10−11, [1] 5.0 × 10−11 1.2 × 10−9 4.3 × 10−10 5.6 × 10−10 2.7 × 10−12 7.7 × 10−12, [2] 9.4 × 10−12 3.2 × 10−10, [6] 1.1 × 10−9 1.6 × 10−9 9.6 × 10−10 1.0 × 10−11 6.5 × 10−10 4.1 × 10−10
2.3 × 10−9 2.2 × 10−9 2.2 × 10−9 2.2 × 10−9 2.2 × 10−9 2.2 × 10−9 2.2 × 10−9 2.1 × 10−9 2.1 × 10−9 2.1 × 10−9 2.1 × 10−9 2.1 × 10−9 2.1 × 10−9 2.0 × 10−9 2.0 × 10−9 2.0 × 10−9 2.0 × 10−9 2.0 × 10−9 2.0 × 10−9 2.0 × 10−9
0.14 0.11 5.0 × 10−3 2.3 × 10−3 9.5 × 10−4 7.7 × 10−3 0.023 0.55 0.20 0.27 1.3 × 10−3 3.7 × 10−3 4.5 × 10−3 0.17 0.55 0.80 0.48 5.0 × 10−3 0.33 0.21
ScNH+ TiNH+ V+ (NH3 ) Cr+ (NH3 ) Mn+ (NH3 ) Fe+ (NH3 ) Co+ (NH3 ) YNH+ ZrNH+ NbNH+ Mo+ (NH3 ) Ru+ (NH3 ) Rh+ (NH3 ) LaNH+ HfNH+ TaNH+ WNH+ Re+ (NH3 ) OsNH+ (95%)a IrNH+ (68%)b Ir+ (NH3 ) (22%)
a b
D (4s1 3d1 ) 4 F (4s1 3d2 ) 5 D (3d4 ) 6 S (3d5 ) 7 S (4s1 3d5 ) 6 D (4s1 3d6 ) 3 F (3d8 ) 1 S (5s2 ) 4 F (5s1 4d2 ) 5 D (4d4 ) 6 S (4d5 ) 4 F (4d7 ) 3 F (4d8 ) 3 F (5d2 ) 2 D (6s2 5d1 ) 5 F (6s1 5d3 ) 6 D (6s1 5d4 ) 7 S (6s1 5d5 ) 6 D (6s1 5d6 ) 3 F (6s1 5d7 )
Estimated 5% electron transfer due to excited 4 D state. Estimated 10% electron transfer due to excited states of Ir+ .
The secondary and higher order kinetics of ammonia ligation exhibited two features. Early ligation of primary product ions was often rate limited by the rate of formation, viz. early ligation reactions were faster than their rate of formation, and were followed by a transition to slower ligation kinetics. For example, in Fig. 1, this transition is seen to occur after four additions of NH3 to ScNH+ and YNH+ and after three additions of NH3 to Ta(NH)2 + . 3.2. Periodic trends A pictorial overview of the variation in the observed primary reaction product channels and the measured primary reaction kinetics (expressed as reaction efficiencies) down and across the periodic table is provided in Fig. 2. 3.3. Comparison with previous measurements and calculations In the first row, only Sc+3 D(d1 s1 ) and Ti+4 F(d2 s1 ), but not V+5 D (d4 ) nor the other atomic transition metal ions further along the row, were observed to react with ammonia exclusively by H2 elimination at room temperature and these two ions reacted with only about 10% efficiency. The observed transition from H2 elimination to ammonia ligation after Ti+ , as well as the low efficiency of the H2 elimination reactions with Sc+ and Ti+ , are both consistent with predictions of quantum chemical calculations of the potential energy surfaces for the reactions of Sc+ [15], Ti+ , V+ and Cr+ [16]. These calculations indicate energy differences of −95.9, −41.0, +1.7 and +119.3 kJ mol−1 , respectively, for reaction (1), viz. the H2 elimination reactions with Sc+3 D and Ti+4 F are exothermic while the reaction with V+5 D is slightly endothermic and that with Cr+6 D (and so for the 6 S ground state as well) much more so. H2 elimination with the remaining 1st row transition metal cations (Mn+ to Co+ ) apparently are also endothermic [18]. The case of V+ is quite interesting. The early FT [7] and guided ion beam [10] experiments pointed toward the occurrence of exothermic H2 elimination, while a flowing afterglow experiment at room temperature cited in [7] indicated ammonia ligation instead. This apparent discrepancy is likely to be a consequence of the very small endothermicity predicted for the H2 elimination with V+5 D (d4 ) which therefore may be induced to occur by above-thermal translational or internal energy.
Fig. 2. Partial periodic table showing the variation down and across the periodic table in the nature of the product channels and the overall reaction efficiencies measured for Groups 3–9 atomic transition metal cations reacting with ammonia at 295 ± 2 K in helium buffer gas at a pressure of 0.35 ± 0.01 Torr. Note trends observed across and down the periodic table. H2 elimination occurs early in a row and ammonia addition later.
The quantum chemical calculations also predict that the H2 elimination reactions of Sc+ and Ti+ to produce ground-state MNH+ are spin forbidden; a crossing of potential energy surfaces is required before metal cation N H bond insertion can take place with ensuing H2 elimination. Our observation of the low efficiencies (about 10%) for the Sc+ and Ti+ hydrogen elimination reactions is consistent with the requirement of the curve crossing that is predicted by theory.
V. Blagojevic et al. / International Journal of Mass Spectrometry 435 (2019) 181–187 Table 2 Rate coefficients for higher order reactions of atomic transition metal cations with ammonia in He buffer gas at 0.35 ± 0.01 Torr and 295 ± 2 K. Rate coefficients are provided in units of 10−10 cm3 molecule−1 s−1 for the formation of complexes M+ (L)n , where n = 1–6. For n = 1 and 2, L can be either NH or NH3 , depending on the metal cation. For the former the rate coefficient is underlined. For n > 2, only NH3 addition is observed for all cations. Metal ion +
Sc Ti+ V+ Cr+ Mn+ Fe+ [1] Co+ Y+ Zr+ Nb+ Mo+ Ru+ [2] Rh+ La+ [6] Hf+ Ta+ W+ Re+ Os+ Ir+
n=1
n=2
n=3
n=4
n=5
n=6
3.2 2.5 0.11 0.05 0.021 0.17 0.5 12 4.3 5.6 0.027 0.077 0.094 3.3 11 16 9.6 <0.1 6.5 3.1 1
6.7 5.8 19 5.2 3.7 10 9.7 3.1 a Poor data, rates not available 0.32 0.047 1.95 0.28 <0.001 10 0.082 <0.0005 2.6 0.47 9.8 17 21 9.2 0.15 5.4 6.8 11 3.7 0.62 −7.7 −10 0.11 <0.02 3.6 0.53 3.2 0.021 2.8 0.81 18 0.026 0.86 6.6 4.4 2.7 0.25 7.8 16 8.6 7 0.27 15 23 2 0.51 14 20 15 6.4 Poor data, rates not availablea 18 23 4 0.17 Saw 4 adducts with NH3 a 14 0.59 Saw 2 more adducts with NH3 a
a Secondary ion signals were too low to allow reliable kinetic fits to the product ion data.
Quantum chemical computations of potential energy surfaces seem not to be available for reactions of second row and third row transition metal cations with ammonia, with the exception of the reaction of ammonia with La+ [14]. We see from Fig. 2 that H2 elimination persists from Y+ to the Group 5 cation Nb+ in the 2nd row and from La+ all the way to the Group 9 cation Ir+ in the 3rd row, with a gap at the Group 7 cation Re+ . In the 2nd row, Zr+ and Nb+ react with very modest efficiencies, again perhaps because of required crossings in the potential energy surfaces (no theoretical predictions appear to be available). In contrast, the Group 3 cation Y+1 S (5s2 ) reacts with a substantially higher efficiency, perhaps because ground-state Y+ is a singlet and the H2 elimination reaction in this case can produce singlet YNH+ in a manner that is not spin forbidden! The third-row transition metal cations are all quite reactive, except La+ and Ir+ , and they all eliminate H2 . The reaction with La+ is inefficient, likely again due to a curve crossing, in this case predicted by theory [14]. Re+ is exceptional in that it only undergoes slow ligation with ammonia. Ground-state Re+7 S(6s1 5d5 ) is not expected to form the nitride because the half-filled orbital configuration is spherically symmetric. This will increase the activation energy for the insertion because it will weaken the metal cationligand interaction, but ligation is still an option as there is no barrier for the collisional stabilization of the Re+ (NH3 ) adduct. 3.4. Higher order ligation and H2 elimination Not shown in Fig. 2 are the secondary and higher-order reactions observed with ammonia. As indicated above, all primary product ions, the MNH+ ions that result from H2 elimination and the adduct ions M+ NH3 that are formed by ligation of the atomic ions that do not form MNH+ , both reacted further by ammonia ligation. But the 3rd row cations Ta+ , W+ and Os+ first formed a second nitride bond with H2 elimination to produce M(NH)2 + and these also reacted further by ammonia addition. Table 2 provides a summary of rate information that we were able to acquire by fitting measured ion profiles as a function of ammonia addition.
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3.5. Ligation kinetics and orbital filling As regards atomic transition metal cations, the spatial expansion of empty d-orbitals facilitates coordinate covalent (dative) bonding with the lone pair of electrons in :NH3 . Once d-orbitals are filled, the binding may proceed through the higher energy p-shell, but if this ligation reaction becomes endothermic, the interaction becomes purely electrostatic. Generally speaking, coordinate covalent bonding through d-orbitals should enhance the rate of ligation compared to that through p-orbitals or electrostatic bonding which is expected to be weaker (since the rate of ligation is enhanced by a higher binding energy of the ligand [3,30]. Thus a transition from coordinate covalent to p-orbital or electrostatic ligation should be manifested by a change (a drop) in the rate of ligation, but of course compensated by an increase in rate due to an increase in degrees of freedom which acts to enhance the rate of ligation [3,30]. This transition to p-orbital or electrostatic bonding might be expected once empty dorbitals are no longer available. When an imide is formed first by N H bond insertion into the first ammonia molecules, fewer empty dorbitals will be available for dative bonding with subsequent ammonia molecules. When diimide formation occurs, even fewer dorbitals will be available for subsequent dative bonding with ammonia. These general considerations do not include system specific properties that influence the bond strength between a particular metal cation and an ammonia ligand, such as the symmetry of the complex. These can produce significant deviations from a more general analysis, as we’ll note in the ensuing discussion of our experimental data. A full understanding of the bonding in these systems of course requires a full theoretical analysis that includes symmetry considerations. We can view d-orbital filling by “bookkeeping” electrons using electron configurations as drawn in Fig. 3 for a hypothetical s2 d2 transition metal cation undergoing possible sequential ligation to form M+ (NH3 )n or sequential H2 elimination to form MNH+ and M(NH)2 + . The number of empty d-orbitals available for dative bonding with :NH3 can be predictive of the number of ammonia molecules added rapidly in sequential ligation reactions. Fig. 2 indicates that ammonia ligation of first, second or third row atomic transition metal cations may be preceded by one or two H2 elimination steps that form covalently bonded MNH+ and M(NH)2 + .
3.5.1. Group by group The trends that are apparent in Fig. 2 within each Group suggest the operation of both an orbital size and an orbital occupancy effect, as we go down and across the periodic table, respectively. Since the size of the metal cation increases down the periodic table, electrostatic bonding is expected to be diminished and covalent and coordinate covalent binding to be enhanced as we go down the periodic table [31]. Increased covalent binding should favour metal cation insertion into the N H bond and an increase in coordinate covalent binding will increase the rate of ligation. Group 3: With M+ = Sc+3 D (4s1 3d1 ), Y+1 S (5s2 ), La+3 F (5d2 ) only MNH+ is formed in the reaction with NH3 . Y+ and La+ require promotion into the s1 d1 electronic configuration for the formation of the double bond. The MNH+ ions all rapidly add 4 ammonia molecules, likely into the d-orbitals before adding a 5th more slowly, presumably through p-orbital or electrostatic bonding (see Table 2 as well as Fig. 1 and Fig. 1 in Ref. [6]). Group 4: Only MNH+ is formed with Ti+4 F (4s1 3d2 ), Zr+4 F (5s1 2 4d ), Hf+2 D (6s2 5d1 ) with electron promotion required with Hf+ . In this group, MNH+ ions all rapidly add 3 ammonia molecules (corresponding to the number of vacant d-orbitals), with further additions proceeding more slowly presumably through p-orbital or electrostatic bonding.
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Fig. 3. Illustration of orbital filling for a hypothetical s2 d2 transition metal cation undergoing reaction with ammonia by sequential H2 elimination and ligation to form M+ (NH3 )n . Black arrows denote metal electrons and gray arrows denote ammonia electrons. Promotion may be required to achieve a desired electronic configuration for double bond formation. Spin crossover may occur when a low-spin complex becomes energetically favourable compared to the high-spin complex.
Group 5: This group exhibits a trend in reactivity down the periodic table as the size of the metal cation increases: V+5 D (3d4 ) only adds ammonia whereas Nb+5 D (4d4 ) and Ta+5 F (6s1 5d3 ) both form MNH+ first. TaNH+ eliminates H2 from NH3 to form Ta(NH)2 + . Four ammonia molecules add to V+ quite rapidly, possibly requiring two spin-crossovers to facilitate dative bonding with 3 d-orbitals in addition to the empty 4s-orbital. A fifth ammonia adds more slowly, likely through p-orbital or electrostatic bonding. NbNH+ similarly requires a spin-crossover to vacate d-orbitals for the fast addition of three ammonias and then a fourth much slower. Ta(NH)2 + quickly adds two ammonias with a slower further addition. Groups 6 and 7: These Cr+6 S (d5 ), Mo+6 S (d5 ), Mn+7 S (4s1 3d5 ), Re +7 S (6s1 5d5 ) all have half-filled orbitals and so add ammonia slowly. These are not expected to form coordinate covalent adducts because of the so-called “half-filled shell effect”; the five d-orbitals are all singly occupied with electrons of like spin and so will be reluctant to accept an electron pair. Mn+ is the slowest reacting atomic transition metal cation in this study with k = 2.1 × 10−12 cm3 molecule−1 s−1 . W+6 D (d4 s1 ) is the exception as it does not have a half-filled orbital configuration. It forms WNH+ and W(NH)2 + by sequential H2 eliminations. Furthermore, the resulting d7 s2 configuration quickly adds 3 ammonia molecules to W(NH)2 + , more than expected from the available empty d-orbitals. Group 8: Fe+6 D (4s1 3d6 ), Ru+4 F (4d7 ), Os+6 D (6s1 5d6 ). Fe+ and Ru+ add the first ammonia slowly, the second one more rapidly, and the third one again slowly. Slow first binding likely involves an energy cost (exchange energy) associated with pairing up the electrons and freeing up the d-orbitals. Spin-crossover at the first step will lower the binding energy for the first adduct. An increase in the rate of ligation for the second addition is expected from the increased degrees of freedom. Trends in Fe+ and Ru+ ammonia kinetics were discussed elsewhere [1,2]. Os+ forms OsNH+ and Os(NH)2 + rapidly, followed by three ammonia additions with decreasing rates (23, 4.0 and 0.17 × 10−10 cm3 molecule−1 s−1 ), similar to the trend observed for W+ , but shifted by one ligand (consistent with the extra 2 electrons in Os+ ). Group 9: Co+3 F (3d8 ), Rh+3 F (4d8 ), Ir+3 F (6s1 5d7 ) follow the same trend as the Group 8, Co+ reactions mirror those of Fe+ and Rh+ reactions those of Ru+ . Ir+ , in addition to H2 eliminations, also exhibits
NH3 addition (22% abundance). The addition channel shows the same trend and Co+ and Fe+ : slow first, fast second and slow third addition. Thus, a view of the d-orbital filling appears to provide a surprisingly simple account of the H2 elimination and ligation kinetics that we observed in our experiments for partially occupied dorbitals, viz. d-orbital occupancy up to and including d5 . Once ligation starts involving partially occupied d-orbitals (Groups 8 and 9), the cost in energy required for spin crossover (exchange energy) appears to make p-orbital binding competitive and additional ligation of ammonia, exceeding that predicted by d-orbital occupancy, is observed. Again, quantum chemical calculations are necessary to fully understand the bonding in each of these many systems.
3.5.2. Row by row Fig. 2 shows the trends in reactivity observed across the periodic table. H2 elimination occurs early in a row and ammonia addition later. We see only ammonia ligation under our experimental conditions for V+ to Co+ with rate coefficients in the range from 2.1 × 10−12 to 2.8 × 10−11 cm3 molecule−1 s−1 across the first row (see Table 1). As the d-orbitals are increasingly occupied by electrons as we go across the periodic table, the capacity for coordinate covalent ligation diminishes. In a separate publication we have shown that a strong correlation is observed across the entire first row of the periodic table (transition metal cations) between the measured rate coefficients for ammonia ligation and the ligand field stabilization energy (LFSE) [32]. Higher rates were observed for those atomic transition metal cations with higher LFSEs. The extent to which H2 elimination occurs along a row increases as we go from row to row down the periodic table, changing from 2 to 3 to 6 (with Re+ being an exception) for rows 1, 2 and 3, respectively. When H2 elimination occurs, it does so exclusively, with only Ir+ , which also shows a minor addition channel, being an exception. There is some structure in the variation of the efficiency of H2 elimination: a small decrease in row 1 for Sc+ and Ti+ , a relatively high efficiency for Y+ in row 2, and a peak in efficiency at Ta+ in row 3. Also, the efficiency of H2 elimination increases as we go down a group (with Y+ being anomalous in Group 3).
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4. Conclusions
References
Product identification and kinetic measurements performed with the ICP/SIFT tandem mass spectrometer at York University have revealed periodic trends in the nature and rates of reactions of ammonia with early 1st, 2nd and 3rd row atomic transition metal cations (Groups 3–9). With very few exceptions, imide ion MNH+ formation accompanied by H2 elimination prevails early in a row and extends further across a row as we move down the periodic table; in the third row all the Groups 3–9 atomic transition metal cations, with the exception of Re+ (Group 7), form imide ions. In the first two rows, imide formation gives way to coordinate covalent ligation as we move across the periodic table. Higher order chemistry is dominated by ammonia ligation except with Ta+ , W+ and Os+ which form a di-imide, M(NH)2 + , first. Minor contributions due to electron transfer to excited states were observed with Os+ and Ir+ . The magnitudes and trends in the measured rate coefficients (or reaction efficiencies) provide insight into reaction energetics and effects of d-orbital filling on the extent of ligation. The failure to observe imide formation with V+ is consistent with the endothermicity predicted by theory. Furthermore, the observed low efficiencies for imide formation with M+ = Sc+ , Ti+ and La+ are supported by previous quantum chemical computations that predict a requirement of electron spin inversion/crossing of potential energy surfaces for these spin forbidden reactions. The measured higher order ligation kinetics generally are consistent with the effects of d-orbital electron filling, at least for d-orbital occupancies up to and including d5 , but p-orbital binding may become competitive for higher d-orbital occupancies. Symmetry effects are likely to play a significant role in the binding and formation of higher-order complexes, but necessary theoretical calculations are beyond the scope of this study.
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Acknowledgments Continued financial support from the Natural Sciences and Engineering Research Council of Canada is greatly appreciated. As former holder of a Canada Research Chair in Physical Chemistry, D.K.B. (now retired) thanks the contributions of the Canada Research Chair Program to this research.
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International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
Early atomic transition metal cations reacting with ammonia at room temperature: H2 elimination and NH3 ligation kinetics across and down the periodic table夽 Voislav Blagojevic, Vitali V. Lavrov, Gregory K. Koyanagi, Diethard K. Bohme ∗ Department of Chemistry, Centre for Research in Mass Spectrometry and Centre for Research in Earth and Space Science, York University, Toronto, Ontario, Canada M3J 1P3
a r t i c l e
i n f o
Article history: Received 2 August 2018 Received in revised form 6 October 2018 Accepted 11 October 2018 Available online 22 October 2018 Keywords: Gas phase Reactions of early atomic transition metal cations Ammonia Periodic trends in ion kinetics Ligation Imide bond formation Di-imido bond formation Orbital filling ICP/SIFT technique
a b s t r a c t The kinetics of H2 elimination and ligation of ammonia reacting with early atomic transition metal cations and ensuing ammonia ligation reactions were measured across (Groups 3–9) and down (the three rows) the periodic table. The atomic cations were produced in an Inductively Coupled Plasma (ICP) source at ca. 5500 K and allowed to decay radiatively and to thermalize by collisions with argon and helium atoms prior to reaction. Kinetic measurements were performed using an ICP/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer with helium bath gas at 0.35 Torr and 295 ± 2 K. Our survey showed that bimolecular H2 elimination to produce MNH+ predominated with very early atomic transition metal cations: M+ = Sc+ and Ti+ in the first row, Y+ , Zr+ and Nb+ in the second row, and La+ , Hf+ , Ta+ , W+ , Os+ and Ir+ in the third row. The remaining atomic transition metal cations were observed to ligate ammonia (with He acting as a third body): M+ = V+ , Cr+ , Mn+ , Fe+ and Co+ in the first row, Mo+ , Ru+ and Rh+ in the second row, and Re+ and Ir+ in the third row. Higher order ligation reactions were observed with M+ to form M+ (NH3 )n with n up to 6 and MNH+ (NH3 )n with n up to 3. Experiments with three of the 3rd row MNH+ cations (M = Ta+ , W+ and Os+ ) revealed the rapid formation of a second imido bond to produce M(NH)2 + followed by ammonia ligation. Variations in the occurrence of imido bond formation and the extent of ligation and trends in measured rate coefficients are scrutinized across and down the periodic table in terms of available theoretical computations of potential energy surfaces as well as atomic orbital filling effects. © 2018 Elsevier B.V. All rights reserved.
1. Introduction The chemical behavior of the d-block transition metals is known to be driven by their electronic configuration or orbital occupancy which varies across and down the periodic table. Insight into this behavior can be ascertained by systematic reactivity measurements across and down the periodic table, ideally with a common neutral reagent that exhibits chemical bond redisposition. Here we systematically explore ammonia as a common neutral reagent and follow its reactions with early atomic transition metal cations at room temperature across (Groups 3–9) and down (the three rows) the
夽 Dedicated to Helmut Schwarz on the occasion of his 75th birthday and in appreciation of his constant friendship and superb contributions to ion chemistry. ∗ Corresponding author. E-mail address: [email protected] (D.K. Bohme). https://doi.org/10.1016/j.ijms.2018.10.021 1387-3806/© 2018 Elsevier B.V. All rights reserved.
periodic table. We focus on direct ligation, reaction (1a), and imide bond formation, reaction (1b), M+ (NH3 )n + NH3 → M+ (NH3 )n+1 +
+
M(NH)n + NH3 → M(NH)n+1 + H2
(1a) (1b)
as well as the ensuing ammonia ligation reactions (1c), M(NH)n+1 + (NH3 )m + NH3 → M(NH)n+1 + (NH3 )m+1
(1c)
and track variations in the efficiencies of these reactions with rate coefficient measurements. When available, observed variations in the efficiencies of these three reactions are interpreted in terms of d-orbital electron filling effects. Previously we have investigated the ammonia ligation of Fe+ and Ru+ (Group 8) for which we noted an influence of a spin-surface crossing on the kinetics of the higher-order sequential ligation of Ru+ [1,2]. Quite recently we discovered relativistic effects in reactions of ammonia with Group 10, 11 and 12 atomic transition metal cations that exhibited only ligation chemistry (except Hg+ which reacts by electron transfer) and were sensitive to orbital size and
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orbital occupancy [3–5]. Also, we previously observed H2 elimination in our room temperature measurements of reactions of 14 lanthanide atomic cations with ammonia [6]. Ligation was again found to be the predominant mode of reaction with the lanthanides, but La+ (5d2 ), Ce+ (4f1 5d2 ), Gd+ (4f7 5d1 6s1 ), and Tb+ (4f9 6s1 ) exhibited bond redisposition to form the protonated lanthanide imide LnNH+ . Apparently, only in these latter reactions is the electrostatic attraction between the atomic lanthanide cation and ammonia sufficiently strong to provide enough energy to achieve a 5d1 6s1 configuration by electron promotion at room temperature and so to overcome any intrinsic barriers to subsequent N-H bond insertion and H2 loss [6]. There have been previous experimental measurements by others of reactions of Group 3–9 atomic transition metal cations with ammonia. Early measurements with a Fourier Transform Mass Spectrometer (FTMS) and atomic transition metal ions produced by pulsed laser desorption/ionization were focused on the production of metal-imide MNH+ ions (by H2 elimination) which were seen to be produced by Group 3–5 (1st, 2nd and 3rd rows, except Hf+ ) atomic ions while Group 6–11 (1st row) atomic ions were seen to just add ammonia [7]. Laser ablation-molecular beam technology provided probably the first report of the formation of metal-diimide M(NH)2 + ions (by sequential H2 elimination) in the case of Nb+ and Ta+ with further ammonia ligation [8]. Also, Cr+ , Mn+ , Fe+ , Co+ , Ni+ , Cu+ and Ag+ were seen to form ammonia clusters M+ (NH3 )n (n ≤ 20) [8]. Later, guided ion beam tandem mass spectrometer experiments were reported that explored the role of electronic and translational excitation with Sc+ and Ti+ [9], V+ [10], Cr+ [11], Fe+ [12] and Co+ [12,13,11]. Ammonia reactions of La+ and several other lanthanoid cations have been investigated previously with a view to the analysis of nuclear fuels [14]. Quantum chemical computations of potential energy surfaces that provide insight into structural and energetic aspects of the mechanism of bond redisposition and the role of electronic spin have been reported for the reactions of ammonia only with the first row transition metal cations Sc+ [15], Ti+ , V+ and Cr+ [16], Mn+ [17], Fe+ [18], Co+ [19] as well as with La+ [14]. A partial overview of experimental and theoretical aspects of reactions of ammonia with atomic transition metal cations was provided in 2012 [20].
2. Experimental procedures The experimental results reported here were obtained with the ICP/SIFT tandem mass spectrometer that has been described in detail previously [21–24]. The atomic ions were generated within an atmospheric pressure argon plasma at 5500 K fed with a vapourized solution containing the metal salt. Solutions containing the metal salt of interest with concentration of ca. 5 g L−1 were peristaltically pumped via a nebulizer into the plasma. The nebulizer flow was adjusted to maximize the ion signal detected downstream of the flow tube. The sample solutions were prepared using atomic spectroscopy standard solutions commercially available from SPEX, Teknolab, J.T. Baker Chemical Co., Fisher Scientific Company, Perkin-Elmer and Alfa Products. Aliquots of standard solutions were diluted with highly purified water produced in the Millipore Milli-Qplus ultra-pure water system. The final concentrations were varied between 5 and 20 ppm to achieve suitable intensity of the resultant ion beam. A stabilizing agent was usually added to each solution in order to prevent precipitation: KOH for base-stabilized salts, HNO3 or HCl for acid-stabilized salts. Atomic ions emerge from the ICP at a nominal plasma ion temperature of 5500 K with the corresponding Boltzmann distributions [25,26]. After extraction from the ICP, the plasma ions may experience electronic-state relaxations via both radiative decay and
collisional energy transfer. The latter may occur by collisions with argon, as the extracted plasma cools upon sampling, and with helium in the flow tube (ca. 4 × 105 collisions with helium) prior to the reaction region. However, the exact extent of electronic relaxation is uncertain. Clues to the presence of excited electronic states of the atomic ions in the reaction region can be found in the product ions observed and in the shape of the semi-logarithmic decay of the reacting atomic ion upon addition of neutral reactants. Curvature will appear in the measured atomic-ion decay when the ground state and excited state react at different rates even when they give the same product ions. An excited-state effect can be hidden when the products and reaction rates are the same for both the ground and excited states, but in this case the measured atomic-ion decay defines the ground-state kinetics. On occasion the presence of excited states is revealed by a difference in product ion formation. The many collisions experienced by the atomic cations with the quite polarizable argon atoms as they emerge from the ICP and the ca. 4 × 105 collisions with helium atoms in the flow tube (the helium buffer gas pressure was 0.35 ± 0.01 Torr) appear to be sufficient to thermalize the excited states and to ensure that the atomic ions reach a translational temperature equal to the tube temperature of 295 ± 2 K prior to entering the reaction region. Reactions of atomic cations were investigated with NH3 at a helium buffer gas pressure of 0.35 ± 0.01 Torr and temperature of 295 ± 2 K. Reaction rate coefficients were determined in the usual manner using pseudo first-order kinetics. The highly pure NH3 gas was obtained commercially (Semiconductor Grade 99.999%, Matheson/Linde Canada), and introduced into the reaction region of the SIFT as a dilute (15%) mixture in helium. 3. Results and discussion Our survey of reactions of ammonia with atomic transition metal cations (Groups 3–9) showed that bimolecular H2 elimination predominated with very early atomic transition metal cations: Sc+ and Ti+ in the first row, Y+ , Zr+ and Nb+ in the second row, and La+ , Hf+ , Ta+ , W+ , Os+ and Ir+ in the third row. The remaining atomic transition metal cations were observed to ligate ammonia (with He acting as a third body): V+ , Cr+ , Mn+ , Fe+ and Co+ in the first row, Mo+ , Ru+ and Rh+ in the second row, and Re+ and Ir+ in the third row. As an indication of the quality of our experimental results, we provide our data for H2 elimination for the reactions of ammonia with the first-row cation Sc+ , the second-row cation Y+ , and the third-row cations Ta+ and Os+ . A composite of these results is provided in Fig. 1; we have reported previously experimental results for La+ [6]. All semi-logarithmic atomic ion decays for the primary H2 elimination reactions, reactions of type (1), were observed to be linear over more than a decade, so that there was no evidence for the presence of excited states which could be manifested by a change in slope. Ensuing higher-order ammonia ligation reactions of MNH+ with ammonia molecules, reactions of type (2), were observed in each case. Ligation was observed with atomic transition metal cations that did not react by H2 elimination. In two cases, the osmium and iridium experiments, NH4 + was observed to be formed (NH3 + quickly transforms to NH4 + in its reaction with ammonia); we attribute this formation to the occurrence of electron transfer from ammonia to excited Os+ * and Ir+ *, reactions (2) and (3). Os+ ∗ + NH3 → NH3 + + Os +
+
Ir ∗ + NH3 → NH3 + Ir
(2) (3)
The ionization energies of the Group 3–9 transition metals are quite low, with IE(Ir) = 8.967 eV being the highest [27]. Since IE(NH3 ) = 10.07 eV [27], ammonia should not transfer an electron
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Fig. 1. Composite of ICP/SIFT results for the reactions of ammonia with selected first, second, and third-row atomic transition metal cations that lead to H2 elimination in He buffer gas at 0.35 ± 0.01 Torr and 295 ± 2 K. Symbols represent experimental data and lines represent kinetic fits to that data. Only the 190 Os isotope is plotted, representing 60% of the overall Os+ ion signal. This exaggerates the intensity of NH4 + (as does mass discrimination), which does not have multiple isotopes.
to any of the Group 3–9 ground-state atomic transition metal cations that were studied. Os (IE = 8.438 eV) and Ir (IE = 8.967 eV) [27] are the two atoms with the highest ionization energies and our data indicate that excited states may be populated in Os+ and Ir+ that have sufficiently high energies to promote electron transfer. Os+ (4 D) provides an almost thermoneutral opportunity for electron transfer with Eav (4 D–6 D) = 1.62 eV [28] for about 5% of the Os+ ions generated in the ICP in the 4 D state. Electron transfer is also possible with Ir+ ions that are excited in the ICP at 5500 K of which 20% [28] have energy levels above 10.08 eV. With regard to higher order reactions of the MNH+ ions, experiments with three of the 3rd row MNH+ cations (M = Ta+ , W+ and Os+ ), revealed the rapid formation of a second imide bond with H2 elimination according to reaction (4), prior to ammonia ligation. MNH+ + NH3 → M(NH)2 + + H2
(4)
Di-imide bond formation was not observed with Nb+ , in contrast to the report of the observation of Nb(NH)2 + in very early laser ablation-molecular beam experiments, albeit only at very low levels (ca. 2% of the single imide) [8]. The di-imide cations M(NH)2 + were seen to sequentially coordinate further ammonia molecules, both in our experiments as well as in the beam experiments.
3.1. Experimental rate coefficients The measured kinetics of the ammonia reactions observed with first, second and third row atomic transition metal cations M+ (Groups 3–9) is summarized in Table 1. Also provided in Table 1 are the known ground-state electronic configurations of the atomic cations, computed values for the collision rate coefficients, kc , derived from collision theory [29] and the reaction efficiencies, k/kc .
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Table 1 Bimolecular and effective bimolecular rate coefficients, k (in units of cm3 molecule−1 s−1 ), for H2 elimination and ammonia addition, respectively, with first, second and third row atomic transition metal cations M+ (Groups 3–9) measured at 295 ± 2 K using the ICP/SIFT technique in our laboratory. The helium buffer gas pressure was 0.35 ± 0.01 Torr. Absolute uncertainties in the rate coefficients are estimated to be ±30%; relative uncertainties are estimated to be <15%. The collision rate coefficients, kc (see text, in units of cm3 molecule−1 s−1 ) and the reaction efficiencies, k/kc , are also provided. The ground electronic state configuration of the atomic transition metal cation is also given. M+
Electronic configuration
k
kc
k/kc
Primary product
Sc+ Ti+ V+ Cr+ Mn+ Fe+ Co+ Y+ Zr+ Nb+ Mo+ Ru+ Rh+ La+ Hf+ Ta+ W+ Re+ Os+ Ir+
3
3.2 × 10−10 2.5 × 10−10 1.1 × 10−11 5.0 × 10−12 2.1 × 10−12 1.7 × 10−11, [1] 5.0 × 10−11 1.2 × 10−9 4.3 × 10−10 5.6 × 10−10 2.7 × 10−12 7.7 × 10−12, [2] 9.4 × 10−12 3.2 × 10−10, [6] 1.1 × 10−9 1.6 × 10−9 9.6 × 10−10 1.0 × 10−11 6.5 × 10−10 4.1 × 10−10
2.3 × 10−9 2.2 × 10−9 2.2 × 10−9 2.2 × 10−9 2.2 × 10−9 2.2 × 10−9 2.2 × 10−9 2.1 × 10−9 2.1 × 10−9 2.1 × 10−9 2.1 × 10−9 2.1 × 10−9 2.1 × 10−9 2.0 × 10−9 2.0 × 10−9 2.0 × 10−9 2.0 × 10−9 2.0 × 10−9 2.0 × 10−9 2.0 × 10−9
0.14 0.11 5.0 × 10−3 2.3 × 10−3 9.5 × 10−4 7.7 × 10−3 0.023 0.55 0.20 0.27 1.3 × 10−3 3.7 × 10−3 4.5 × 10−3 0.17 0.55 0.80 0.48 5.0 × 10−3 0.33 0.21
ScNH+ TiNH+ V+ (NH3 ) Cr+ (NH3 ) Mn+ (NH3 ) Fe+ (NH3 ) Co+ (NH3 ) YNH+ ZrNH+ NbNH+ Mo+ (NH3 ) Ru+ (NH3 ) Rh+ (NH3 ) LaNH+ HfNH+ TaNH+ WNH+ Re+ (NH3 ) OsNH+ (95%)a IrNH+ (68%)b Ir+ (NH3 ) (22%)
a b
D (4s1 3d1 ) 4 F (4s1 3d2 ) 5 D (3d4 ) 6 S (3d5 ) 7 S (4s1 3d5 ) 6 D (4s1 3d6 ) 3 F (3d8 ) 1 S (5s2 ) 4 F (5s1 4d2 ) 5 D (4d4 ) 6 S (4d5 ) 4 F (4d7 ) 3 F (4d8 ) 3 F (5d2 ) 2 D (6s2 5d1 ) 5 F (6s1 5d3 ) 6 D (6s1 5d4 ) 7 S (6s1 5d5 ) 6 D (6s1 5d6 ) 3 F (6s1 5d7 )
Estimated 5% electron transfer due to excited 4 D state. Estimated 10% electron transfer due to excited states of Ir+ .
The secondary and higher order kinetics of ammonia ligation exhibited two features. Early ligation of primary product ions was often rate limited by the rate of formation, viz. early ligation reactions were faster than their rate of formation, and were followed by a transition to slower ligation kinetics. For example, in Fig. 1, this transition is seen to occur after four additions of NH3 to ScNH+ and YNH+ and after three additions of NH3 to Ta(NH)2 + . 3.2. Periodic trends A pictorial overview of the variation in the observed primary reaction product channels and the measured primary reaction kinetics (expressed as reaction efficiencies) down and across the periodic table is provided in Fig. 2. 3.3. Comparison with previous measurements and calculations In the first row, only Sc+3 D(d1 s1 ) and Ti+4 F(d2 s1 ), but not V+5 D (d4 ) nor the other atomic transition metal ions further along the row, were observed to react with ammonia exclusively by H2 elimination at room temperature and these two ions reacted with only about 10% efficiency. The observed transition from H2 elimination to ammonia ligation after Ti+ , as well as the low efficiency of the H2 elimination reactions with Sc+ and Ti+ , are both consistent with predictions of quantum chemical calculations of the potential energy surfaces for the reactions of Sc+ [15], Ti+ , V+ and Cr+ [16]. These calculations indicate energy differences of −95.9, −41.0, +1.7 and +119.3 kJ mol−1 , respectively, for reaction (1), viz. the H2 elimination reactions with Sc+3 D and Ti+4 F are exothermic while the reaction with V+5 D is slightly endothermic and that with Cr+6 D (and so for the 6 S ground state as well) much more so. H2 elimination with the remaining 1st row transition metal cations (Mn+ to Co+ ) apparently are also endothermic [18]. The case of V+ is quite interesting. The early FT [7] and guided ion beam [10] experiments pointed toward the occurrence of exothermic H2 elimination, while a flowing afterglow experiment at room temperature cited in [7] indicated ammonia ligation instead. This apparent discrepancy is likely to be a consequence of the very small endothermicity predicted for the H2 elimination with V+5 D (d4 ) which therefore may be induced to occur by above-thermal translational or internal energy.
Fig. 2. Partial periodic table showing the variation down and across the periodic table in the nature of the product channels and the overall reaction efficiencies measured for Groups 3–9 atomic transition metal cations reacting with ammonia at 295 ± 2 K in helium buffer gas at a pressure of 0.35 ± 0.01 Torr. Note trends observed across and down the periodic table. H2 elimination occurs early in a row and ammonia addition later.
The quantum chemical calculations also predict that the H2 elimination reactions of Sc+ and Ti+ to produce ground-state MNH+ are spin forbidden; a crossing of potential energy surfaces is required before metal cation N H bond insertion can take place with ensuing H2 elimination. Our observation of the low efficiencies (about 10%) for the Sc+ and Ti+ hydrogen elimination reactions is consistent with the requirement of the curve crossing that is predicted by theory.
V. Blagojevic et al. / International Journal of Mass Spectrometry 435 (2019) 181–187 Table 2 Rate coefficients for higher order reactions of atomic transition metal cations with ammonia in He buffer gas at 0.35 ± 0.01 Torr and 295 ± 2 K. Rate coefficients are provided in units of 10−10 cm3 molecule−1 s−1 for the formation of complexes M+ (L)n , where n = 1–6. For n = 1 and 2, L can be either NH or NH3 , depending on the metal cation. For the former the rate coefficient is underlined. For n > 2, only NH3 addition is observed for all cations. Metal ion +
Sc Ti+ V+ Cr+ Mn+ Fe+ [1] Co+ Y+ Zr+ Nb+ Mo+ Ru+ [2] Rh+ La+ [6] Hf+ Ta+ W+ Re+ Os+ Ir+
n=1
n=2
n=3
n=4
n=5
n=6
3.2 2.5 0.11 0.05 0.021 0.17 0.5 12 4.3 5.6 0.027 0.077 0.094 3.3 11 16 9.6 <0.1 6.5 3.1 1
6.7 5.8 19 5.2 3.7 10 9.7 3.1 a Poor data, rates not available 0.32 0.047 1.95 0.28 <0.001 10 0.082 <0.0005 2.6 0.47 9.8 17 21 9.2 0.15 5.4 6.8 11 3.7 0.62 −7.7 −10 0.11 <0.02 3.6 0.53 3.2 0.021 2.8 0.81 18 0.026 0.86 6.6 4.4 2.7 0.25 7.8 16 8.6 7 0.27 15 23 2 0.51 14 20 15 6.4 Poor data, rates not availablea 18 23 4 0.17 Saw 4 adducts with NH3 a 14 0.59 Saw 2 more adducts with NH3 a
a Secondary ion signals were too low to allow reliable kinetic fits to the product ion data.
Quantum chemical computations of potential energy surfaces seem not to be available for reactions of second row and third row transition metal cations with ammonia, with the exception of the reaction of ammonia with La+ [14]. We see from Fig. 2 that H2 elimination persists from Y+ to the Group 5 cation Nb+ in the 2nd row and from La+ all the way to the Group 9 cation Ir+ in the 3rd row, with a gap at the Group 7 cation Re+ . In the 2nd row, Zr+ and Nb+ react with very modest efficiencies, again perhaps because of required crossings in the potential energy surfaces (no theoretical predictions appear to be available). In contrast, the Group 3 cation Y+1 S (5s2 ) reacts with a substantially higher efficiency, perhaps because ground-state Y+ is a singlet and the H2 elimination reaction in this case can produce singlet YNH+ in a manner that is not spin forbidden! The third-row transition metal cations are all quite reactive, except La+ and Ir+ , and they all eliminate H2 . The reaction with La+ is inefficient, likely again due to a curve crossing, in this case predicted by theory [14]. Re+ is exceptional in that it only undergoes slow ligation with ammonia. Ground-state Re+7 S(6s1 5d5 ) is not expected to form the nitride because the half-filled orbital configuration is spherically symmetric. This will increase the activation energy for the insertion because it will weaken the metal cationligand interaction, but ligation is still an option as there is no barrier for the collisional stabilization of the Re+ (NH3 ) adduct. 3.4. Higher order ligation and H2 elimination Not shown in Fig. 2 are the secondary and higher-order reactions observed with ammonia. As indicated above, all primary product ions, the MNH+ ions that result from H2 elimination and the adduct ions M+ NH3 that are formed by ligation of the atomic ions that do not form MNH+ , both reacted further by ammonia ligation. But the 3rd row cations Ta+ , W+ and Os+ first formed a second nitride bond with H2 elimination to produce M(NH)2 + and these also reacted further by ammonia addition. Table 2 provides a summary of rate information that we were able to acquire by fitting measured ion profiles as a function of ammonia addition.
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3.5. Ligation kinetics and orbital filling As regards atomic transition metal cations, the spatial expansion of empty d-orbitals facilitates coordinate covalent (dative) bonding with the lone pair of electrons in :NH3 . Once d-orbitals are filled, the binding may proceed through the higher energy p-shell, but if this ligation reaction becomes endothermic, the interaction becomes purely electrostatic. Generally speaking, coordinate covalent bonding through d-orbitals should enhance the rate of ligation compared to that through p-orbitals or electrostatic bonding which is expected to be weaker (since the rate of ligation is enhanced by a higher binding energy of the ligand [3,30]. Thus a transition from coordinate covalent to p-orbital or electrostatic ligation should be manifested by a change (a drop) in the rate of ligation, but of course compensated by an increase in rate due to an increase in degrees of freedom which acts to enhance the rate of ligation [3,30]. This transition to p-orbital or electrostatic bonding might be expected once empty dorbitals are no longer available. When an imide is formed first by N H bond insertion into the first ammonia molecules, fewer empty dorbitals will be available for dative bonding with subsequent ammonia molecules. When diimide formation occurs, even fewer dorbitals will be available for subsequent dative bonding with ammonia. These general considerations do not include system specific properties that influence the bond strength between a particular metal cation and an ammonia ligand, such as the symmetry of the complex. These can produce significant deviations from a more general analysis, as we’ll note in the ensuing discussion of our experimental data. A full understanding of the bonding in these systems of course requires a full theoretical analysis that includes symmetry considerations. We can view d-orbital filling by “bookkeeping” electrons using electron configurations as drawn in Fig. 3 for a hypothetical s2 d2 transition metal cation undergoing possible sequential ligation to form M+ (NH3 )n or sequential H2 elimination to form MNH+ and M(NH)2 + . The number of empty d-orbitals available for dative bonding with :NH3 can be predictive of the number of ammonia molecules added rapidly in sequential ligation reactions. Fig. 2 indicates that ammonia ligation of first, second or third row atomic transition metal cations may be preceded by one or two H2 elimination steps that form covalently bonded MNH+ and M(NH)2 + .
3.5.1. Group by group The trends that are apparent in Fig. 2 within each Group suggest the operation of both an orbital size and an orbital occupancy effect, as we go down and across the periodic table, respectively. Since the size of the metal cation increases down the periodic table, electrostatic bonding is expected to be diminished and covalent and coordinate covalent binding to be enhanced as we go down the periodic table [31]. Increased covalent binding should favour metal cation insertion into the N H bond and an increase in coordinate covalent binding will increase the rate of ligation. Group 3: With M+ = Sc+3 D (4s1 3d1 ), Y+1 S (5s2 ), La+3 F (5d2 ) only MNH+ is formed in the reaction with NH3 . Y+ and La+ require promotion into the s1 d1 electronic configuration for the formation of the double bond. The MNH+ ions all rapidly add 4 ammonia molecules, likely into the d-orbitals before adding a 5th more slowly, presumably through p-orbital or electrostatic bonding (see Table 2 as well as Fig. 1 and Fig. 1 in Ref. [6]). Group 4: Only MNH+ is formed with Ti+4 F (4s1 3d2 ), Zr+4 F (5s1 2 4d ), Hf+2 D (6s2 5d1 ) with electron promotion required with Hf+ . In this group, MNH+ ions all rapidly add 3 ammonia molecules (corresponding to the number of vacant d-orbitals), with further additions proceeding more slowly presumably through p-orbital or electrostatic bonding.
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Fig. 3. Illustration of orbital filling for a hypothetical s2 d2 transition metal cation undergoing reaction with ammonia by sequential H2 elimination and ligation to form M+ (NH3 )n . Black arrows denote metal electrons and gray arrows denote ammonia electrons. Promotion may be required to achieve a desired electronic configuration for double bond formation. Spin crossover may occur when a low-spin complex becomes energetically favourable compared to the high-spin complex.
Group 5: This group exhibits a trend in reactivity down the periodic table as the size of the metal cation increases: V+5 D (3d4 ) only adds ammonia whereas Nb+5 D (4d4 ) and Ta+5 F (6s1 5d3 ) both form MNH+ first. TaNH+ eliminates H2 from NH3 to form Ta(NH)2 + . Four ammonia molecules add to V+ quite rapidly, possibly requiring two spin-crossovers to facilitate dative bonding with 3 d-orbitals in addition to the empty 4s-orbital. A fifth ammonia adds more slowly, likely through p-orbital or electrostatic bonding. NbNH+ similarly requires a spin-crossover to vacate d-orbitals for the fast addition of three ammonias and then a fourth much slower. Ta(NH)2 + quickly adds two ammonias with a slower further addition. Groups 6 and 7: These Cr+6 S (d5 ), Mo+6 S (d5 ), Mn+7 S (4s1 3d5 ), Re +7 S (6s1 5d5 ) all have half-filled orbitals and so add ammonia slowly. These are not expected to form coordinate covalent adducts because of the so-called “half-filled shell effect”; the five d-orbitals are all singly occupied with electrons of like spin and so will be reluctant to accept an electron pair. Mn+ is the slowest reacting atomic transition metal cation in this study with k = 2.1 × 10−12 cm3 molecule−1 s−1 . W+6 D (d4 s1 ) is the exception as it does not have a half-filled orbital configuration. It forms WNH+ and W(NH)2 + by sequential H2 eliminations. Furthermore, the resulting d7 s2 configuration quickly adds 3 ammonia molecules to W(NH)2 + , more than expected from the available empty d-orbitals. Group 8: Fe+6 D (4s1 3d6 ), Ru+4 F (4d7 ), Os+6 D (6s1 5d6 ). Fe+ and Ru+ add the first ammonia slowly, the second one more rapidly, and the third one again slowly. Slow first binding likely involves an energy cost (exchange energy) associated with pairing up the electrons and freeing up the d-orbitals. Spin-crossover at the first step will lower the binding energy for the first adduct. An increase in the rate of ligation for the second addition is expected from the increased degrees of freedom. Trends in Fe+ and Ru+ ammonia kinetics were discussed elsewhere [1,2]. Os+ forms OsNH+ and Os(NH)2 + rapidly, followed by three ammonia additions with decreasing rates (23, 4.0 and 0.17 × 10−10 cm3 molecule−1 s−1 ), similar to the trend observed for W+ , but shifted by one ligand (consistent with the extra 2 electrons in Os+ ). Group 9: Co+3 F (3d8 ), Rh+3 F (4d8 ), Ir+3 F (6s1 5d7 ) follow the same trend as the Group 8, Co+ reactions mirror those of Fe+ and Rh+ reactions those of Ru+ . Ir+ , in addition to H2 eliminations, also exhibits
NH3 addition (22% abundance). The addition channel shows the same trend and Co+ and Fe+ : slow first, fast second and slow third addition. Thus, a view of the d-orbital filling appears to provide a surprisingly simple account of the H2 elimination and ligation kinetics that we observed in our experiments for partially occupied dorbitals, viz. d-orbital occupancy up to and including d5 . Once ligation starts involving partially occupied d-orbitals (Groups 8 and 9), the cost in energy required for spin crossover (exchange energy) appears to make p-orbital binding competitive and additional ligation of ammonia, exceeding that predicted by d-orbital occupancy, is observed. Again, quantum chemical calculations are necessary to fully understand the bonding in each of these many systems.
3.5.2. Row by row Fig. 2 shows the trends in reactivity observed across the periodic table. H2 elimination occurs early in a row and ammonia addition later. We see only ammonia ligation under our experimental conditions for V+ to Co+ with rate coefficients in the range from 2.1 × 10−12 to 2.8 × 10−11 cm3 molecule−1 s−1 across the first row (see Table 1). As the d-orbitals are increasingly occupied by electrons as we go across the periodic table, the capacity for coordinate covalent ligation diminishes. In a separate publication we have shown that a strong correlation is observed across the entire first row of the periodic table (transition metal cations) between the measured rate coefficients for ammonia ligation and the ligand field stabilization energy (LFSE) [32]. Higher rates were observed for those atomic transition metal cations with higher LFSEs. The extent to which H2 elimination occurs along a row increases as we go from row to row down the periodic table, changing from 2 to 3 to 6 (with Re+ being an exception) for rows 1, 2 and 3, respectively. When H2 elimination occurs, it does so exclusively, with only Ir+ , which also shows a minor addition channel, being an exception. There is some structure in the variation of the efficiency of H2 elimination: a small decrease in row 1 for Sc+ and Ti+ , a relatively high efficiency for Y+ in row 2, and a peak in efficiency at Ta+ in row 3. Also, the efficiency of H2 elimination increases as we go down a group (with Y+ being anomalous in Group 3).
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4. Conclusions
References
Product identification and kinetic measurements performed with the ICP/SIFT tandem mass spectrometer at York University have revealed periodic trends in the nature and rates of reactions of ammonia with early 1st, 2nd and 3rd row atomic transition metal cations (Groups 3–9). With very few exceptions, imide ion MNH+ formation accompanied by H2 elimination prevails early in a row and extends further across a row as we move down the periodic table; in the third row all the Groups 3–9 atomic transition metal cations, with the exception of Re+ (Group 7), form imide ions. In the first two rows, imide formation gives way to coordinate covalent ligation as we move across the periodic table. Higher order chemistry is dominated by ammonia ligation except with Ta+ , W+ and Os+ which form a di-imide, M(NH)2 + , first. Minor contributions due to electron transfer to excited states were observed with Os+ and Ir+ . The magnitudes and trends in the measured rate coefficients (or reaction efficiencies) provide insight into reaction energetics and effects of d-orbital filling on the extent of ligation. The failure to observe imide formation with V+ is consistent with the endothermicity predicted by theory. Furthermore, the observed low efficiencies for imide formation with M+ = Sc+ , Ti+ and La+ are supported by previous quantum chemical computations that predict a requirement of electron spin inversion/crossing of potential energy surfaces for these spin forbidden reactions. The measured higher order ligation kinetics generally are consistent with the effects of d-orbital electron filling, at least for d-orbital occupancies up to and including d5 , but p-orbital binding may become competitive for higher d-orbital occupancies. Symmetry effects are likely to play a significant role in the binding and formation of higher-order complexes, but necessary theoretical calculations are beyond the scope of this study.
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Acknowledgments Continued financial support from the Natural Sciences and Engineering Research Council of Canada is greatly appreciated. As former holder of a Canada Research Chair in Physical Chemistry, D.K.B. (now retired) thanks the contributions of the Canada Research Chair Program to this research.