J . Am. Chem. SOC. 1989, 111, 4101-4103
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Table I.' Thermochemical Data for First-Row Transition-Metal Hydrates, Hydroxides, Oxides, and Hydrides M+ D(M+-0H2)* D(M+-OH)b E,C D,(MC=O)d D,(M+-H)' E(H-M+-OH)r E(H-M+=O)r K+ I7.0g Ca+ sc+ Ti' V+ Cr+ Mn* Fe+
29.0h 31.4 38.0 36.2 29.0 32.5 28.8
co+ Ni+
37.1 36.5 35.0 39.0
cu+ Zn+
106' 87.8 113 I07 74.3, 7 9 82 85.3, 76,k 77,' 73" 72.2, 71' 42.2
19.1 16.1 15.2 25.3 26.5 20.6
58' I59 161 131 85 51 69
45.9d 55.3 55.1 47.3 27.7 47.5 47.0
29.2 31.9
64 45
45.5 38.5 21.8 53.3
30.4
34 25 50 36 -16
3 113 115 77 12 3 15
12
14, 5, 6, 2
0
8 -18
-37
D(M+O-H)' 149 30 53 71 90, 89 127 117, 108, 109, 105 109, 108 98
'All values in kcal/mol. bThis work except where noted. Average standard deviation in about a dozen measurements for each ion, f 3 kcal/mol; this is also believed to be the accuracy of the method.2 C T h e promotion energy." dReference 8, with f3-7 kcal/mol uncertainty. 'Reference 3, f 2 - 4 kcal/mol. fAssuming reformation of the 0-H bond and using DJH-OH) = 118 kcal/mol and D,(O-H) = 101 kcal/mol from J A N A F Thermochemical Tables, 2nd ed.; N S R D S - N B S 37; National Bureau of Standards: Washington, DC, 1971. EReference 4a. Reference 4b. ' M u r a d , E. J . Chem. Phys. 1981, 75, 4080. 'Reference 8. kReference 9, f 4 . 6 kcal/mol. 'Reference 7, f 6 kcal/mol. "'Reference 7, f 3 kcal/mol.
Having established the hydroxyl bond energies, we can estimate the energies E(H-M+-OH) needed for removal of H 2 0 from H - M + - O H ions for all M+ except Ti+ (Table I). These disagree strongly with our measured H 2 0 binding energies, except in the case of V+ and possibly Ca+ and Sc'. W e conclude that our ions have the structure M+-OH,, with the possible exceptions of M + = Ca+ to V+. The hydroxyl binding energies of the transition-metal cations vary strongly and show a tendency to decrease for late transition metals. The water binding energies fall into two groups: the values for Sc', Cr', Mn', and Fe+ are near 30 kcal/mol, the others near 37 kcal/mol; this is hard to attribute to variations in the ionic radius. Detailed understanding of the D(M+-OH) and D(M+-OH2) values requires elaborate calculations similar to those publishedI2 for the M+-H bond. W e merely note the existence of an approximate linear correlation between the Mf-OH bond energies and promotion energiesI3 (Figure 1). The correlation suggests that early transition-metal ions have a propensity to use their 3d orbitals, whereas the late ones prefer to utilize the 4s orbital for bonding, as demonstrated previously12 for M+-H bonds. Our value of the hydration energy D(Fef-OHz) = 28.8 kcal/mol conflicts with the previous estimate6a Do(Fe+-CO) = 37.6 kcal/mol, since H 2 0 readily displaces CO in Fe'CO a t low pressure and room temperature.14 The reported value of Do(Fef-C2H4) = 34 f 2 k c a l / m ~ lalso ~ ~ contradicts the earlier estimate,6a since C2D4, too, displaces C O in Fe+C0.I4 The Cu+ ion binds a second water molecule somewhat more strongly (39 kcal/mol) than the first one (35 kcal/mol).2 W e have now measured the second hydration energies also for Fef (38 kcal/mol), Co+ (45 kcal/mol), and Ni+ (38 kcal/mol). These can be compared with the first hydration energies of 29, 37, and 37 kcal/mol, respectively. Monopositive ions of first-row transition metals thus may have a general tendency to attach a second water ligand a t least as strongly as the first one. Along with the variation in the first hydration energies across the series, this is indicative of some degree of dative bonding, distinct from the presumably purely electrostatic bonding between water and alkali4a or alkaline earth4bcations. In these, the first hydration energy is dictated by the ionic radius, and the second hydration energy is several kcal/mol smaller than the first one as a result of unfavorable ~
~~~~~~~~
(12) Schilling, J. B.; Goddard 111, W. A,; Beauchamp, J. L. J . Am. Chem. SOC.1986, 108, 582. Schilling, J. B.; Goddard 111, W. A,; Beauchamp, J. L. J . Phys. Chem. 1987, 91, 5616. Patterson, L. G . M.; Bauschlicher, C. W.; Langhoff, S. R.; Partridge, H. J . Chem. Phys. 1987, 87, 481. (13) ( i ) For Sc+-Cr+, the energy required to promote the ground state free ion to the lowest 3d" state plus a spin decoupling correction (Schilling, J. B.; Beauchamp, J. L. Organometallics 1988, 7 , 194). (ii) For Mn+-Cu+, the promotion energy of the lowest 3d"'4s state plus the average of the d-electron and s-electron spin decoupling energies. The free ion energy states were derived from ref IO. (14) Foster, M. S.; Beauchamp, J. L. J . Am. Chem. SOC.1975, 97, 4808. (15) Jacobson, D. B.; Freiser, B. S. J . A m . Chem. Soc. 1983, 105, 7492.
0002-7863/89/ 151 1-4l01$01.50/0
electrostatic ligand-ligand interactions.
Acknowledgment. This work was supported by a grant from the National Science Foundation, CHE-8421118. We are grateful to Prof. P. B. Armentrout for providing us with preprints of unpublished papers and to Prof. R . R. Squires for communicating his unpublished results. Registry No. Fe+, 14067-02-8; Co', H 2 0 , 7732-18-5.
16610-75-6; Ni',
14903-34-5;
Sequential Solvation of Atomic Transition-Metal Ions. The Second Solvent Molecule Can Bind More Strongly Than the First Peter J. Marinelli and Robert R . Squires* Department of Chemistry, Purdue University West Lafayette, Indiana 47906 Received October 31, 1988 W e wish to report the determination of sequential water and ammonia solvation energies for the series of first-row transition-metal atomic ions V', Cr', Mn', Fe+, Co', and Ni'. A remarkable trend is found wherein for most metals the second solvent binding energy significantly exceeds the first. This is a striking departure from the normal decrease in successive solvent binding energies of all other atomic ions for which data are available. I Atomic transition-metal ions are generated by dissociative electron ionization and Penning ionization of volatile metal carbonyl complexes in the helium flow reactor of a flowing afterglow-triple quadrupole apparatus.2 Termolecular association reactions of the metal ions with added H 2 0 or N H , vapor produce a kinetic mixture of thermalized M + ( H 2 0 ) , ( n = 1-3) and M+(NH3)n( n = 1-4) cluster ions. Unimolecular and bimolecular reactions of mass-selected cluster ions are carried out in the central quadrupole of the triple quadrupole analyzer, where both pressure and kinetic energy effects can be characterized. The thermochemistry for sequential solvation of the atomic metal ions by H 2 0 and N H 3 can be estimated from the translational energy thresholds for collision-induced dissociation (CID) of the corresponding cluster ions with argon target g a s 3 Figure ( I ) Keesee, R. G.; Castleman, A. W., Jr. J . Phys. Chem. Ref,Data 1986, 15, 101 1 and references therein.
(2) G r a d , S. T.; Squires, R. R . Mass Spec. Rec. 1988, 7 , 263. Standard experimental conditions: P(He) = 0.4 Torr, (He) = 9400 cm/s, T = 298 i 2 K. (3) P(Ar) 5 5 X lo-' Torr (single-collision conditions); reactant ion retarding potential analysis locates the energy origin and shows a typical kinetic energy distribution of 0.8-1.3 eV (fwhm, lab frame).
0 1989 American Chemical
Society
Communications
A 1 00
Fe'(H,O)
.. '
Table I. Water and Ammonia Binding Energies of Gas-Phase Ions" M+ln 1 2 3 4 '
. .~
. ...._.. . *
I
V+ Cr+
0.80
2
.
0.60
b
0
1
* Fe' x 20
0.40
Mn+ Fe+ co+ Ni+ cu+ Ag+ Na+ K+ H30+ NH4+ F OH-
D[M+(H@)~-I-OH~I~ 12.2 35.5 29.5 17.8 40.8 41.9 40.6 16.4' 15.0' 33.3c 25.4' 21.0, 24.0d 14.3, 19.8d 15.Sd 15.2, 17.9' 16.1f 13.21 17.6h 30.7, 31.Sh 15.2, 19.0h 13.4' 17.3' 14.7' 13.7j 20.4, 2 3 . 9 16.g 16.2h 22.8, 26.Sh 17.6h
35.1 21.9 26.5 32.8 40.1 39.7
COLLISION ENERGY ( l a b , eV -5.00
0.00
5.00
10.00
, ,15.0 , , $ , , , , , , ??.PO
V+ Cr+
Mn+
...
I
.*
I
Fe+(H20) x 20
I
0.20
0.00 -1.60
-0.60
0.40
4.40
5.40
Figure 1. Plot of reactant ion transmission curves ( X ) and normalized cross sections (*) for dissociation of F e + ( H 2 0 ) I , 2cluster ions versus center-of-mass collision energy (ev). Argon target gas at 4 X lo-' Torr; reactant ion energy distributions 1.0 eV fwhm (lab frame): (A) Fe+ from F e + ( H 2 0 ) , estimated threshold energy 1.48 eV; (B) F e + ( H 2 0 ) from Fe+(H20)2,estimated threshold energy 1.79 eV.
1 presents a plot of the normalized cross sections (u/umax)for two dissociation reactions of Fe+(H,O),,, cluster ions versus the center-of-mass collision energy. Estimates of the threshold dissociation energies were obtained from the appearance plots by a fitting procedure that is described in previous reports from this For the data sets shown in Figure 1, threshold energies of 1.48 eV (34.1 kcal/mol) and 1.79 eV (41.2 kcal/mol) are obtained for the first (Figure 1A) and second (Figure 1B)H 2 0 ligand dissociation energies, respectively. Table I provides a summary of results for the six transitionmetal ions, along with data for a few other atomic and small polyatomic ions. The first and second H 2 0 and NH, binding energies could be determined for each of the transition-metal ions; the third solvent binding energy is typically much lower than the first two and could be reliably estimated in only a few cases. The most outstanding feature in the transition-metal ion data is the relative magnitudes of the dissociation energies. For the hydrate series, the second H 2 0 binding energy is greater than the first in every case except Mn+ where a substantial decrease occurs (8.7 kcal/mol). The same type of behavior is displayed in the ammine series for all metals except V+ where a monotonic decrease in successive NH, binding energies occurs through the fourth cluster. It is of interest to note that while the Mn+ ion a l s o exhibits a decreased affinity for the second N H , molecule compared to the first, the difference is much less than that found with water. These results represent an unprecedented departure from the solvation energy trends established for all other atomic ions for (4) (5) (6) (7)
to the Editor
Graul, S. T.: Squires, R. R. I n l . J . Mass Spec. Ion Proc. 1987.81, 183. G r a d , S. T.: Squires, R. R. J . A m . Chem. S o t . 1988, 110, 607. Workman, D.B.; Squires, R. R. Inorg. Chem. 1988, 27, 1846. Graul, S. T.; Squires, R. R. J . A m . Chem. S o t . 1989, 111, 892.
51.9 37.4 36.9 38.5 58.8 51.2
D[M+(NHJn-i-NH31b 45.0 40.8 34.1 48.7 61.1 55.1
22.6
16.7' 14.9' 1 3.Sd 1l.Sr 11.5h 12.2' 13.9 12.2h
18.7
11.8 Fe+ co+ 17.8 Ni+ cu+ 14.OC 12.8' 36.9' 14.6c 13.0' Ag+ 14.7' 28.4, 29.1e 17.0, 22.9e 17.1e Na' 16.9, 20.19 16.39 13.59 K+ 1 1.69 23.8, 24.8' 15.7' 13.8' 12.5' NH4+ OAll values in kcal/mol. bThis work unless otherwise noted. The precision for at least five measurements of each value is typically better than 0.1 eV (2 kcal/mol); the estimated uncertainty is better than *0.2 eV ( f 4 kcal/mol). CReference 8. dDzidic, I.; Kebarle, P. J . Phys. Chem. 1970, 74, 1466. 'Castleman, A. W., Jr.; Holland, P. M.; Lindsay, D. M.; Peterson, K. I. J . A m . Chem. SOC.1978, 100, 6039. JSearles, S. K.; Kebarle, P. C a n . J . Chem. 1969, 47, 2619. gcastleman, A. W., Jr. Chem. Phys. Lett. 1978, 53,560. hMeot-Ner, M. J . Awl. Chem. SOC.1986, 108,6189. 'Payzant, J . D.; Cunningham, A. J.; Kebarle, P. Can. J . Chem. 1973, 51, 3242. 'Arshadi, M.; Yamdagni, R.; Kebarle, P. J . Phys. Chem. 1970, 74, 1475.
which data are available in the literature.' Alkali metal ions, rare earth metal ions, and halide negative ions all exhibit progressively decreasing energies of binding to water, ammonia, and other small molecules. Sequential H 2 0 binding energies for Ag+ are also known; these too display the "normal" ordering.* Simple electrostatic models satisfactorily account for this general behavior, Le., charge dispersal and/or intraligand steric effects accrue with the addition of each new solvent molecule, thereby reducing the successive interaction energies in the growing cluster ion.' W e consider three possible origins of the anomalous solvation energy orderings found for the transition-metal ions. First, it is important to recognize that ion appearance measurements are kinetic experiments and that our threshold energies are actually upper limits for the metal/ligand bond dissociation activation energies rather than true thermodynamic binding energies. It could simply be that the M+(H20), and M+(NH3)2cluster ions dissociate a t threshold a t a much slower rate than monoligated ions and that a greater collision energy is required to achieve detectable dissociation on the time scale of the experiment (i.e., a "kinetic shift").I0 This is definitely ruled out, however, by the fact that we can obtain the correct H 2 0 and N H , binding energy order (Le., second less than the first) for Na+, K+, H 3 0 + ,NH4+, F, and OH- with our measurement technique. While some of the absolute solvent binding energies we obtain for these ions are slightly lower than the established literature values' (ca. 3-4 (8) Hol!and, P. M.; Castleman, A. W., Jr. J . Chem. Phys. 1982, 4195. (9) Reviews: (a) Castleman, A. W., Jr.; Keesee, R. G.Chem. Reu. 1986, 86, 589. (b) Castleman, A. W., Jr.; Keesee, R. G. Acc. Chem. Res. 1986, 19, 413. (c) Kebarle, P. Ann Reu. Phys. Chem. 1977, 28, 445. (d) Kebarle, P. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O., Eds.; Plenum: New York, 1974. (10) Levsen, K. Fundamental Aspects of Organic Mass Spectrometry: Verlag: Weinheim, 1978; Chapter 4.
J. Am. Chem. SOC.1989, 111, 4103-4105
4103
molecules and the transition-metal ions. Since purely electrostatic models cannot possibly account for the observed trends, then variations in the electronic configuration of the metal ions must play an important role.I6 Moreover, the visible aberrations in the correlation between the water and ammonia binding energies (Figure 2) suggest that the observed effects are a function of not only the metal but the ligand as well. In view of the many low-lying electronic states in atomic transition metal ions1' and the likelihood of variable mixtures of s, p, and d bonding to H z O and N H , ligands by the different metals,I8 large basis M C S C F calculations will undoubtedly be necessary to account for these unusual solvation energy trends.
,
