The comparative chemistry of ammine and methylamine complexes of rhodium(II1) and cobalt(II1)

THOMAS WILSON SWADDLE Department of Chemistry, The University of Calgary, Calgary, Alra., Canada T2N IN4

Received April 12, 1977

THOMAS WILSON SWADDLE. Can. J. Chem. 55,3166 (1977). For the aquation of (CH3NH2)SRhC12+, the first order rate coefficients are represented by

AH,,* = 101.9 kJ mol-' and AS,,* = -50.2 J K-' mol-' in 0.1 M HCIO,, while for base hydrolysis the rate is first order in [(CH3NH2),RhC12+] and [OH-] at ionic strength 0.10 M and the rate coefficients (in M- ' s-') are represented by AHOH* = 108.6 kJ mol-' and ASoH* = 74.1 J K-I mol-'. Acid dissociation constants are reported for (RNH2),MOHZ3+ (R = H or CH,; M = Rh or Co), and these, combined with spectral data, show CH3NH2 to be a poorer electron donor than NH3 in complexes of this type, contrary to expectations. The com- parative kinetics of reactions of (RNH2),MC12+ support the assignment of an I , mechanism to aquation when M = Rh or Cr, Id to aquation when M = Co, and D,, for base hydrolysis in all these cases.

THOMAS WILSON SWADDLE. Can. J. Chem. 55,3166 (1977). Pour l'aquation de (CH3NH2),RhC12+, dans HC104 0.1 M, les coefficients de vitesse du

premier ordre sont reprCsentks par AHa,* = 101.9 kJ mol-' et ASa,* = -50.2 J K-' mol-', et pour l'hydrolyse basique a force ionique de 0.10 M la vitesse est du premier ordre en [(CH3NH2),RhC12+] et en [OH-] et les coefficients de vitesse (en M- ' s-') sont reprCsentCs par AHoH* = 108.6 kJ mol-I et ASoH* = 74.1 J K-I mol-'. On rapporte les constantes de dissociation acide pour (RNH2),MOHZ3+ (R = H ou CH3; M = Rh ou Co) et ces valeurs, combinkes avec des donnees spectrales, montrent que le CH3NH2 est un plus mauvais donneur dlClectron que le NH, dans des complexes de ce type et ceci contrairement aux anticipations. Les cinCtiques comparees des rkactions de (RNH2),MC12+ sont en accord avec l'attribution d'un mkcanisme I, pour l'aquation quand M = Rh ou Cr, d'un mkcanisme Id pour l'aquation quand M = Co, et Dcb pour I'hydrolyse basique dans tous ces cas.

[Traduit par le journal]

Introduction The hypothesis has been advanced (1, 2) that

simple ligand substitution reactions of cationic octahedral complexes in solution occur by an associative interchange (I,) mechanism for tri- valent transition metals in general, with the im- portant exception of cobalt(III), for which a dissociative interchange (I,) mechanism evi- dently operates. The term 'simple' implies exclu- sion of special cases, notably conjugate-base solvolysis pathways in which a dissociative (D,,) mechanism is considered to be generally applicable.

If this hypothesis is valid, then the effects of replacing R = H by R = CH, in (RNH2),- MC12' (hereinafter called 'N-methylation') on the rates of chloride aquation

[l] (RNHZ)5MC12+ + H z 0 + (RNH2),MOHZ3 +

+ Cl-

should be qualitatively different for M = Co relative to M = Cr, Rh, etc. Indeed the methyl- amine complex aquates 22 times more rapidly

than the ammine for M = Co, but 33 times more slowly when M = Cr, at 298 K (3-5). In the simplest view, this can be attributed to the dif- ferent roles of steric effects in the mechanistic dichotomy (1, 6-8), since it is known (9) that marked steric compression exists in both of these methylamine complexes, although there are fewer nonbonded contacts in the chromium one (as expected, since the ionic radius of Cr3' is 61.5 pm as opposed to 52.5 pm for Co3' (10)). We may therefore naively predict that (CH3- NH2),RhC12' should aquate more slowly than (NH3),RhC12+, if the mechanism is indeed I,, but that the rates should be more nearly equal than in the chromium(II1) analogues, since the relatively large radius of Rh3' (67 pm) (10) should reduce the number of nonbonded con- tacts in (cH,NH,),MC~~' still further.

On the basis of a common conjugate-base mechanism for the alkaline hydrolysis of (RNH2),MC12'

[2] (RNH2),MC12+ + OH- e (RNH,),M(NHR)Cl+ + H z 0

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SWADDLE 3 167

one can similarly predict a marked steric ac- celeration of reaction 3 in all cases on N-methyla- tion (6), but the decline in the number of non- bonded interactions in (CH,NH2),MC12+ on going from M = Co to Cr to Rh should reduce this effect rather sharply. Thus, if it can be shown that the effect of N-methylation on reaction 2 is not very much different for the various M, it can be predicted that the acceleration of base hydrol- ysis on N-methylation will fall in the sequence M = Co >> Cr > Rh.

The purpose of this study was to test these predictions experimentally, especially since Bu- chacek and Harris (1 1) have recently pointed out anomalies in the assignments of mechanism in octahedral rhodium(II1) complexes. The predic- tions are based on the more obvious steric con- sequences of N-methylation, such as have been considered by many authors both recently (12, 13) and in the formative years of inorganic mechanistic studies (14). Consideration must be given, however, to possible indirect influences of , N-methylation on reactivity such as through solvational and electronic effects, especially since the latter are of an unexpected kind in the com- plexes considered here.

Experimental Materials Chloropentaamminerhodium(III), aquopentaammine-

rhodium(III), and aquopentaamminecobalt(1II) perchlo- rates were made as described previously (15, 16). Chloro- pentakis(methylamine)wbalt(III) chloride was made from dichlorotetrapyridinecobalt(II1) chloride (17) by the method of Mitzner et al. (18), with minor variations in reagent quantities and reaction conditions between suc- cessive preparations.

Preparation of Chloropentakis(methylamine)rhodium (111) Perchlorate

Liquid methylamine (30 cm3) was obtained by adding the 40% aqueous solution dropwise to NaOH pellets with warming, drying the liberated gas over sodalime, and condensing it under reduced pressure at -78'C. To this liquid, 0.5 g RhCl3.3H20 (freshly prepared from Rh residues) was added, a little at a time, allowing the solid to dissolve after each addition. The red solution was allowed to evaporate, the yellow residue was dissolved in the minimum amount of hot dilute HCl, and the solution was filtered. On cooling, yellow crystals of [Rh(NHz- CH3)5CI]Clz (0.4 g) separated, and were converted to the perchlorate salt by dissolving them in 10 cm3 water at 65"C, adding about 8 cm3 60% HC104, and cooling the solution to 0°C. The resulting pale yellow needles were filtered, washed with 2-3 cm3 ice-wld water, and dried in a vacuum desiccator over silica gel. Anal. calcd. for

[Rh(NHzCH3)sCll(ClO,), : C 12.2, H 5.1, N 14.2, C121.6, Rh 20.9; found: C 12.3, H 5.2, N 14.1, C1 21.5, Rh 20.9.

Preparation of Aquopentakis(methylamine)rhodium(III) Perchlorate

[Rh(NHzCH3)5Cl](C10,), (0.702 g) in 150 cm3 HC1O4 (0.01 M ; M = mol dm-9 was heated with AgC104 (0.305 g, a 3% excess) for 24 h at 95°C. The pale yellow solution was filtered, evaporated on the steam bath to about 15 cm3, and again filtered. The solution was treated with 5 cm3 60% HC104, evaporated to 10 cm3, filtered, and kept overnight at 0°C. The resulting pale yellow crystals were filtered, washed quickly three times with ice-cold water, and dried under vacuum over fresh NaOH pellets. Yield, 0.36 g. Anal. calcd. for [Rh(NH,- CH3)50HZ](C104)3: C 10.5, H 4.7, N 12.2, C1 18.5 (formula weight 575); found: C 10.6, H 4.5, N 12.3, C1 18.7 (formula weight by p H titration (see below) 580 k 6).

Spectra All spectrophotometric measurements were made using

a freshly serviced Cary 15 spectrophotometer. The spectra of the pure compounds in acidic aqueous solution are listed in Table 1.

Kinetics The kinetics of the aquation reactions were followed

by thermostatting (+0.0l0C) 10 cm3 aliquots of a solu- tion of the rhodium(II1) complex in HC1O4 in Pyrex ampoules in a darkened oil bath, quenching these to room temperature at appropriate times t, and measuring the optical density A, of the solution at 200 nm. At this wavelength, the molar absorptivities E of Rh(NHZCH3),- C12+, Rh(NHzCH3)50H23+, Rh(NH3)5ClZ+, and Rh- (NH3)50Hz3+ in aqueous HC1O4 are 35 400, 18 000, 11 200, and 300 M-' cm-I respectively, while E for C1- is negligible, and the final absorbance values A, of the reaction mixtures after 10 half-lives were compared to these to show that the reactions being followed were indeed chloride aquations (reaction 1) and went to com- pletion under the experimental conditions.

For the base hydrolysis reactions (reactions 2 and 3), accurately measured aliquots were pipetted from the thermostatted bulk reaction mixtures at appropriate in- tervals and quenched in a measured excess of 0.5 M HC104 at room temperature. The absorbances of the resulting solutions were measured at 200 nm, and again the final values A, verified that the products were indeed (RNH2)5RhOHZ+ (identified in acid as the aquo com- plex).

All temperature measurements were made with calibra- tions traceable to NBS.

Acid Dissociation Constants of Aquo Complexes For Rh(NHzCH3)50Hz3 +, Rh(NH3)50HZ3 +, and

C O ( N H ~ ) ~ O H ~ ~ + , aliquots of 0.0100 M solutions made from the solid perchlorates were thermostatted in a jacketted beaker and titrated with standardized 0.1 M NaOH solution from a microburette, while monitoring the p H with a Beckman 39502 combination electrode in conjunction with an Orion 801A potentiometer. This pro- cedure gave both the acid equivalent weights of the com- plexes (which agreed with the theoretical values within the experimental uncertainty in all cases) and the pK.

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CAN. J. CHEM. VOL. 55, 1977

TABLE 1. Spectra of (RNH2)5MX'3-n)+ in dilute perchloric acid"

M X" - R hl A2 h3 Reference

This work This work This work This work

19 19

16,20 This work, 20

21 2 1

OWavelengths h. of maxima in nm are followed by molar

values at the ionic strength I of the midpoint of the titration curve.

This procedure was inapplicable to CO(NH,CH~)~- 0Hz3+, which was not obtained as a solid salt and which decomposed slowly with methylamine release in the course of a p H titration. Instead, solutions of [Co(NH2- CH3),CI]C12 in dilute standard HC1O4 with the stoichio- metric amount of AgCIO, were kept at 22°C in the dark for 44 to 68 h, i.e., for times corresponding to over 99% chloride aquation even in the absence of the anticipated Ag+ catalysis (3, 5). The solutions were then chilled to 0°C to ensure maximum precipitation of AgCl, filtered, and made to 100 cm3 at 22°C such that [HC1O4] = [(CO(NH~CH~),OH~)(CIO,)~] = 0.0100 M. These freshly prepared solutions were used to obtain the spectrum of C O ( N H ~ C H ~ ) ~ O H ~ ~ + , given in Table 1, which agrees well with that reported by Mitzner and Blankenburg (20). Aliquots of these solutions were placed in the darkened, thermostatted beaker as above but at a low temperature (2"C), and enough 0.1 M NaOH was added under thorough stirring to neutralize all the HCIO, and pre- cisely one-half of the aquo complex. The p H of the mix- tures (=pK, of the aquo complex) was then measured as above, arranging for the momentary pH meter reading immediately before immersion into the test solution to be within 1 pH unit of the anticipated pK, value so as to expedite the attainment of a stable final reading without decomposition of the complex.

Results Aguation of Rh(NH2R) ,c12+

Plots of In (A, - A,) vs. t were linear over at least 87% reaction (correlation coefficients r 2 > 0.9994), and the corresponding pseudo- first-order rate coefficients k,, are collected in Table 2. For R = CH,, standard errors in k,, were typically +0.8%, and reproducibility was + 1.8%. Precision was better than this for R = H because the final absorbances A, were much smaller than for R = CH,. The rate coefficients were the same in 0.01 M as in 0.10 M HClO,, and over a twofold range of initial complex concentrations: within these limits of uncertainty.

The Eyring plot for R = CH, is linear (r2 =

absorptivities E in M-1 cm-'.

TABLE 2. Pseudo-first-order rate coefficients k,, for the aquation of Rh(NH2R),C12+ in 0.1 M HCIO,

Complex T ("c) loSk,, (s-l)

Rh(NH2CH3)sClzt a 113.59 32.6 108.00 20.5 100.25 10.5 92.36 4.90 84.90 2.40

Rh(NH3)sC12 + b 100.21 20.5 84.90 4.80

0.9998) and leads to AHa,* = 101.9 + 0.9 kJ mol-', withAS,,* = -50.2 + 2.3 J K - ' mol-'. The data for R = H indicate AH,,* - 102 kJ mol-' and AS,, * - - 44 J K-' mol-' ; the k,, values agree well with those interpolated from the work of Poe e t al. (22) and Lalor and Bushnell (23), and all these data taken together give AH,, * = 101.5 f 1.2 kJ mol- ' and AS,, * = -45.6 + 3.4 J K-' mol-' with r 2 = 0.9992 (the activation parameters calculated by Lalor and Bushnell are in error).

Base Hydrolysis of Rh(NH2R) ,CI2+ Plots of In (A, - A,) vs. t were linear to at

least 86% reaction, with r 2 always exceeding 0.995 for R = CH, and 0.999 for R = H. The corresponding pseudo-first-order rate coefficients kobs were seen to be directly proportional to [OH-] at constant I, and accordingly Table 3 lists koH = kobs/[OH-1, together with the stan- dard errors. The temperature dependence of koH gives AHoH* = 108.6 f 1.8 kJ mol-' and ASoH* = 74.1 f 6.1 J K-' mol-' with r2 = 0.9995 for Rh(NH2CH3),C12+, and AHoH* = 114.8 f 0.7 kJ mol-' and ASoH* = 66.4 f 2.1 J K-' mol-' with r 2 = 0.99997 for Rh(NH,),-

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SWADDLE

TABLE 3. Specific rate coefficients koH for the base hydrolyses of (RNHZ),- RhC12+ at ionic strength 0.100 M (NaOH/NaC104)

Complex T("C) [OH-] (M) 103kon (M-' S-')

Rh(NHzCH3)sCIZ+a 35.00 0.0251 18.6 k 0 . 4 30.00 0.0251 9 . 0 k O . l

0.0497 9 . 0 k O . l 0.0989 9 . 3 k 0 . 3

20.01 0.0989 2 . 1 4 k 0 . 0 5 13.43 0.0989 0 . 6 9 k 0 . 0 1

Rh(NH3)sCIZ+b 50.42 0.0497 5 . 7 7 k 0 . 0 2 44.98 0.0497 2 . 6 6 k 0 . 0 1 45.00 0.0989 2 . 8 7 k 0 . 0 3 34.99 0.0989 0 . 6 5 k 0 . 0 1 34.99 0.0873' 0 . 6 5 k 0 . 0 1

O[Rh] = 3.2 X 10-5 M. b[Rh] = 1.2 X M. =Ionic strength 0.088 M.

TABLE 4 . Acid dissociation constants for the ions (RNH2)sMOH23+ in aqueous solutiona

Complex T ("C) pKab AH0 (kJ mol-') AS0 (J K-' mol-l)

10.4 6.27 Rh(NH3)s0Hz3 + 35.0 6.24' 25 + 4' -38k12'

22 .0 6 . 5 3 k 0 . 0 2 9 . 4 6.63'

Co(NH2CH3)s0HZ3 + 1.9 5 .73k0 .01 5 . 6gd

Co(NH3)50HZ3+ 30.0 5.97" 33 + 2 - 5 k 6 22 .0 6 .08

1 .9 6 . 5 3 k 0 . 0 2

"Counterions Na+, C10,-; ionic strength 0.047 M except where indicated. bTriplicate measurements where error ranges cited; otherwise, single determinations. <Reference 22; ionic strength 0.2 M. *Using solution kept 24 h at 22'C after filtration. =Extrapolated from higher ionic strengths (ref. 25).

C12+, at I = 0.100 M. The latter parameters give k,, = 1.39 x M-' s-' for Rh(NH3),- C12+ at 40.16"C, in excellent agreement with the value reported by Bushnell et al. (24) for this temperature and similar ionic strength.

Acid Dissociation Constants Ka of the Aquo Complexes

Values of pKa (Table 4) obtained by the pH titration and the direct half-neutralization methods were equally reliable, according to experiments using CO(NH,),OH,~+, were in- dependent of variations in the mode of prepara- tion of the complexes, and agreed well with litera- ture values (22, 25, 26), allowing for ionic strength differences. The p H values of solutions of the pure solid salts were in good agreement with those calculated from the measured pKa data.

Discussion The retardation ( x 0.50 at 85°C) of the aqua-

tion of (RNH2),RhC12+ on N-methylation, though modest, originates in ASaq* rather than AHaq * and so persists over the entire temperature range of interest. It is opposite in direction to the acceleration ( x 2 2 at 25°C) observed for the cobalt(II1) analogues (3, 5, 6), and the simplest explanation is that the mechanism is I, for M = Rh and I, for M = Co, as previously contended (1, 2, 15). Steric effects can account for the con- sequences of the presumed mechanistic differ- ence, and indeed the retardation is less striking for Rh than for Cr (3, 5) , as expected on steric grounds. Steric factors in aliphatic substitution produce much larger effects than these, but organic S,2 reactions involve attack remote from the replaced ligand with stereochemical inversion of the entire molecule, whereas octahedral sub-

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3 170 CAN. J. CHEM. VOL. 55, 1977

stitution by an I, process evidently involves flanking ('cis') attack with little disturbance of the ligands other than the one being replaced (1). Similarly, the large kinetic effects of steric decompression associated with organic S,1 re- actions, in which the geometry goes from tetra- hedral to trigonal planar, may be matched in octahedral substitutions of the D type, in which the five-coordinate intermediate survives long enough to undergo major rearrangement (27), but will be less striking in I, processes in which the configuration of the intermediate is less likely to change prior to resumption of six-coordina- tion.

In the base hydrolysis of Rh(NH2R),C12+, we observe an acceleration (x29 at 35°C) on N- methylation. If a conjugate base mechanism (reactions 2 and 3) is operating, part of this acceleration could be due to an increased acidity of the N protons (reaction 2), but Rh(NH,CH,),- OH2,+ is only 2.7 times more acidic than Rh(NH3),OHZ3+ (Table 4), so that an accelera- tion of about 10-fold still needs to be accounted for, if N-proton and 0-proton acidity trends are at all similar. This acceleration contrasts with the retardation displayed in the aquation re- actions of the same compounds, but is in ac- cordance with the assignment of a D,, mechan- ism to base hydrolysis reactions in general, on the basis of steric acceleration of reaction 3. This argument applies much more forcefully when M = Co (6); Co(NH2CH3),OHZ3+ is 6.3 times more acidic then Co(NH3),OHZ3+ (Table 4) but this still leaves an acceleration factor of about 2 x lo3 to be attributed to N-methylation in reaction 3. The overall acceleration factors as- sociated with N-methylation in base hydrolysis of M(NH2R),C12+ are 1.5 x lo4, 225, and 29 for M = Co, Cr, and Rh respectively, as expected for a common D,, mechanism, for which the steric acceleration of reaction 3 will decline with the decrease in the number and severity of the nonbonded interactions as the ionic radii of M3+ increase (9, 10).

While the above analysis explains the pheno- mena on the basis of steric effects alone, consi- deration must be given to the contributions of the rather unusual electronic effects revealed by Tables 1 and 4, and to solvational factors, even though these contributions will be similar for all the three M considered and hence do not seriously affect the foregoing conclusions. Table 1 shows that N-methylation in (RNH2),MC12+ decreases

the energy of the lowest spin-allowed ligand field band in every case, i.e., it decreases the ligand field strength of the nonreacting ligands, which in (RNH2),MC12+ is a measure of o electron release from N to M. This is surprising, since the methyl group is invariably electron releasing in organic molecules, yet Table 4 shows that the (CH,NH2),MOHz3+ are more acidic than the corresponding (NH3),MOH23+ even though CH3NH2 itself is 25 times more basic than NH, (28). Parris and Feiner (29) noted a similar spec- troscopic effect in M(NH,R),~+, and attributed the anomaly to steric constraints on six-coordina- tion when R = CH,. The work of Foxman (9) suggests that the distortion of metal-to-ligand bond angles from 90" may account for the re- duced effectiveness of N + M o-electron dona- tion in the methylamine complexes.

It is difficult to gauge the effect of reduced electron release from RNH, on the aquation rates of (RNH,),MC~~+. A lowered rate would be expected for an I, process, on the basis of the lowered electron density at M, yet marked acceleration is observed when M = Co, where the I, mechanism is almost certainly operative (1, 14); the steric effects discussed above are evidently much more important than these elec- tronic factors. Furthermore, the replacement of NH, by H 2 0 in (NH3),CrC12+ results in a de- crease of over 3000 cm-' in the wavenumber 5, of the first ligand field band and a 31-fold de- crease in the chloride aquation rate (30), whereas replacement of NH, by CH3NH2 decreases V, by only 410 cm-' yet k,, is reduced 33-fold. These observations also run counter to the expec- tation that increasing the ligand field should de- crease the reaction rate of d6 (spin-paired) or d3 complexes (ref. 14, pp. 145-158). For M = Rh and Co, Vl decreases on N-methylation of (NH3),MC12+ by amounts that are sufficiently small and similar (810 and 640 cm-', or 2.8 and 3.4z7 respectively) to indicate that electronic effects cannot account for the differences in the kinetic consequences of N-methylation between M = Co, Rh, and Cr (compare ref. 14, p. 161).

Qualitatively, however, the spectroscopic data can be taken as evidence for a general electronic destabilization of (RNH,),MC~~+ through geo- metrical strain, leading to an expectation of labilization of the complexes in the order Co >> Cr > Rh, on N-meth.ylation. Viewed in this way, the deactivation of the Cr and Rh complexes to- wards aquation becomes all the more significant.

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SWADDLE 3171

Finally, solvational factors cannot account satisfactorily for the different kinetic conse- quences of N-methylation for M = Co, Cr, and Rh, since they will be rather similar in each case. Progress from R = CH, to R = C2H,, n-C3H,, etc. (the chief consequence of which will be desolvation) leads to modest stepwise accelera- tions of the aquations of (RNH2),MC12+ for both Co and Cr (4-6), indicating that the re- tardations of aquation observed in N-methylation of (NH3),RhC12+ and (NH3),CrCIZf would be more striking if solvational factors could be al- lowed for. Again, this effect can be viewed as one of general labilization through destabilization of the complexes.

In summary, then, the kinetic phenomena observed in reactions 1-3 can be rationalized in terms of steric effects if aquation proceeds by an I, mechanism for M = Rh or Cr but I, for M = Co, and base hydrolysis by a D,, process in all these cases.

Acknowledgements I thank Dr. S. J. Cartwright for preparing

chloropentakis(methylamine)rhodium(III) chlo- ride, Dr. B. M. Foxman for permission to refer to his results prior to publication, and the Na- tional Research Council of Canada for financial assistance.

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