Dinitriles as ligands—I studies on complexes of cobalt(II) with adiponitrile

L inorg, nucl. Chem., 1973, Vo!. 35, pp. 1471-1480.

Pergamon Press.

Printed in Great Britain

DINITRILES AS LIGANDS-I S T U D I E S ON C O M P L E X E S OF COBALT(II) WITH ADIPONITRILE* D. L. G R E E N E r and P. G. SEARS* Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506

(Received 21 July 1972) Abstract-Several cobalt(lI)-adiponitrile complexes have been prepared with perchlorate and halide anions. Infrared data provide evidence that the bonding is through the lone pair of the nitrogen in the nitrile group in every case. This excludes chelation and indicates bridging adiponitrile moieties. Thermal decomposition temperatures are given. The spectral and magnetic parameters Dq, B, ~ and/z are given for both tetrahedral and octahedral species, indicating that adiponitrile is high in the spectrochemical series toward cobalt(ll), and that there is considerable covalence in the bonding to the metal ion. INTRODUCTION

A PREVIOUS paper has shown adiponitrile ( A D N ) , or 1,4-dicyanobutane, to be a non-aqueous electrolytic solvent with promising characteristics[l]. At that time solvate formation with a few salts was noted. Though there have been scattered reports in the literature concerning a few metal complexes of A D N , to our knowledge there has been no systematic investigation of this compound's ligand characteristics. In view of the multiple modes of bonding to a metal ion that are possible for nitriles in general, and the close resemblance of these compounds to molecular nitrogen in complexes[2], this was thought to be a particularly interesting area of investigation. To this end, several cobalt(II) complexes involving A D N as a ligand were prepared, and their spectral, magnetic and thermal characteristics are presented in this paper. EXPERIMENTAL

Materials All chemicals involved were of reagent grade with the exception of technical grade 2,2-dimethoxypropane (Dow Chemical Co.). A D N (Eastman Organic Chemicals) used for synthesis of complex compounds was purified by fractional distillation over P4010 at ~< 1 mm. A D N used in the study of conductances was further purified by fractional freezing.

Preparation of compounds [Co(ADNh](CIO4h. Hydrated cobalt(ll) perchlorate, Co(CIO4)2.6H20, (0.01 mole) was dehydrated by stirring magnetically for about 18 hr between 25* and 30"C with 100% excess of 2,2-dimethoxypropane. To this system was added a five-fold excess of ADN. Addition of 50 ml anhydrous ether *Presented in part at the Southeast-Southwest Regional Meeting of the American Chemical Society, New Orleans, December, 1970. tPresent address: Physical Sciences Department, Rhode Island College, Providence, Rhode Island 02908. ~tTo whom inquiries should be addressed. 1. P. G. Sears, J. A. Caruso and A. I. Popov, J. phys. Chem. 71, 905 (1967). 2. A. Misono, Y. Uchida, M. Hidai and T. Kuse, Chem. Comm. 208 (1969). 1471

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yielded a hygroscopic, light-orange, finely-divided precipitate which was recovered by filtration over sintered glass in a dry box. (Calc. for ClsH24N6CI2OaCo: C, 37"13; H, 4"16; Co, 10"12; N, 14"44; CIO4, 34' 16%. Found: C, 36.16; H, 4.06; Co, 10-08; N, 14-48; CIO4, 34.13%.) [(CoCI~. ADN)2], [(CoBr2. ADN)~] and [Co(ADN)3] [Co14]. To 0.01 mole of the appropriate anhydrous salt (Alfa lnorganics) was added a five-fold excess of ADN. Each system was magnetically stirred with heat (80°C) overnight. Upon cooling, 50 ml of anhydrous ether was added to each system yielding deep blue oils for the chloride and bromide, and a yellow-green oil for the iodide. Repeated washings with anhydrous ether yielded extremely hygroscopic powders of the same respective colors. These were vacuum dried at 60°C for several hours. (Calc. for CsHsN2CoCI2: Co, 24.76; CI, 29.80%. Found: Co, 24.39; CI, 29.47. Calc. for C~HsN2CoBr2: Co, 18.03; Br, 48.89%. Found: Co, 17.77; Br, 48.60. Calc. for CgH12N3CoI2: Co, 12.41 ; 1, 53.44%. Found: Co, 12.33; 1, 52.80%.)* [(COC12. A D N . H20)2]. Hydrated cobalt(lI) chloride, CoCI~. 6H~O, (0.01 mole) was stirred 18 hr with 100% excess 2,2-dimethoxypropane. To this system was added a six-fold excess of A D N and the solution was stirred 1 hr. A blue oil separated on addition of an equal volume of anhydrous ether. This oil was washed several more times in similar fashion and then cooled on ice. The resulting lightblue precipitate was washed with ether, filtered in a dry box, and dried under vacuum at room temperature about 10 rain to remove residual ether. The product was hygroscopic, yielding a pink product on exposure to moist air. (Calc. for C6H10N~OCoCI~: C, 28-15; H, 3.94; Co, 23.02; CI, 27.70%. Found: C, 28.13; H, 3.90; N, 10.91; Co, 23.23; CI, 27.81%.) [(COCI2. A D N . 2H20)2]. Finely ground COC12.6H~O (0.025 mole) was added to 50 ml of ADN. Endothermic dissolution with deep blue coloration occurred almost immediately. In about 1 min the blue color faded and a pink solid precipitated. This was filtered and dried under vacuum at room temperature for about 1 hr. The product then was washed with ether several times and subsequently dried briefly under vacuum to remove residual ether. The product did not appear hygroscopic. (Calc. for CnHI~N202CoCI2: C, 26.30; H, 4.42; N, 10.22; Co, 21.51; C1, 25.88%. Found: C, 26.00; H, 4.46; N, 10-42; Co, 21.42; CI, 25.95%.) CoBr~. A D N . 4H~O. Hydrated cobalt(ll) bromide, CoBr2.6H~O, (0.015 mole) was added to 25 ml of ADN. There was immediate dissolution to yield a deep blue color, then rapid deposition of what appeared to be a green solid. This system was allowed to stir about 1 hr, then the precipitate was recovered by filtration and washed several times with anhydrous ether. Residual ether was removed by drying under vacuum for 10 min at room temperature. The resulting light-orange powder was not hygroscopic. (Calc. for C6HI6N~O4CoBr2: Co, 14.77; Br, 40.06%. Found: Co, 14.63; Br, 40.28%.)*

Analytical procedures Cobalt was determined by EDTA complexometric titration. The halides were determined by volumetric and gravimetric procedures outlined in many standard analytical textbooks. Perchlorate was determined gravimetrically as the tetrapentylammonium salt [3]. Carbon, hydrogen and nitrogen were determined on a F. & M. Carbon, Hydrogen, Nitrogen Analyzer Model 185.

Spectra I.R. spectra were run on a Perkin-Elmer 621 recording spectrophotometer using Nujol and highboiling perfluorokerosene mulls between NaCI or Csl plates in the appropriate regions. Visible and near-i.r, spectra were obtained with a Cary Model 14 recording spectrophotometer, using Nujol mulls for solids [4] and optically matched cells for solutions.

Thermogravimetric analysis T G curves were obtained using a Perkin-Elmer TGS-I Thermobalance in conjunction with a Texas Instruments Serva Riter 11 recorder. The heating rate was 20°]min, and a dynamic helium *C, H, N analyses were attempted but the extremely hygroscopic nature of the compounds led to results which were not meaningful. tC. H. N. analyses were attempted, but rapid water loss in the analyzer entry port led to results which were not meaningful.

3. R. G. Dosch, Analyt. Chem. 40, 829 (1968). 4. J. R. Wasson, Chemist-Analyst 56, 36 (1967).

Cobalt(II)-adiponitrile complexes

1473

atmosphere was used over open aluminum pans. Sample sizes generally ranged from l to 2 mg. The instrument was calibrated using magnetic standards.

Conductance determinations Conductances were determined at 25°C using a Dike-Jones bridge assembly in conjunction with Jones-Bollinger cells which had been previously calibrated with aqueous KC1 solutions. Magnetic measurements Magnetic susceptibilities were determined by the Faraday method. Diamagnetic corrections for the ligands were made using Pascal's constants.

RESULTS AND DISCUSSION A D N has several possibilities for mode of bonding in its complexes with metals. These include chelation, bonding through the nitrile ~r system, or acting as a bridging ligand between metal ions. Dinitriles have been known to exhibit all three types of bonding[5-7], though chelation has been observed only when the bonding has been through the 7r system. This is due to the very stringent steric requirements involved in bending around the carbon chain with the linear arrangement of three terminal atoms, rendering the nitrogen or pair inaccessible at normal bond angles. This fact is readily verifiable for A D N with the use of good spacefilling models. 1.R. spectra in the region of the nitrile stretching frequency (ca. 2300 cm -1) are quite useful in determining the type of bonding to a metal. It has been demonstrated that nitriles act similarly to acetylenes, which show a decrease in the C - C stretching frequency of 50-250 cm -1 upon coordination through their ~r systems[8]. For example, a recent study has shown that the C--N stretching frequency in succinonitrile decreases by 185-230 cm -1 in a series of Mn and Re complexes where the bonding is through the 1r systems[5]. On the other hand, when nitriles coordinate normally through the nitrogen, then an increase in the C - N frequency is to be expected relative to the parent nitrile compound [9]. This has been shown in a wide variety of compounds to be the case, with the shift genera!!y having a magnitude of 30-110 cm -1. Two excepttions to this general rule should be noted, where bonding through the nitrogen lone pair is postulated though a decrease in the C--N frequency of ca. 45 cm -1 is observed[2, 10]. In both cases, this is attributed to an unusual degree of ~r-backbonding from the metal to the ligand. In any event, the negative shift is much smaller than that observed for the ~r-bonded systems. The cobalt(II)-ADN complexes, their colors, C-=N stretching frequencies, and the shifts in this frequency due to coordination are listed in Table 1. In every case there is a positive shift, ranging from 37-57 cm -~. This evidence indicates that these complexes must be bonded throdgh the tr lone pair of the nitrogen. This rules out the possibility of chelation of the ligand. Also, in no case is there any absorption around 2246 cm -1, which would indicate free or uncomplexed ligand. Thus, the dinitrile must be acting as a bridging group between cobalt ions. Similar bridging for A D N has been shown for the copper perchlorate and 5. M. F. Farona and K. F. Kraus, lnorg. Chem. 9, 1700 (1970). 6. D. M. Barnhart, C. N. Caughland and Mazhar-ul-Haque, lnorg. Chem. 8, 2768 (1969).

D. L. G R E E N E and P. G. SEARS

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Table I. Colors and infrared C=-N frequencies

Compounds

ADN [Co(ADN)3] (CIO4)2 [(COCI2. ADN)2] [(COC12. A D N . H~O)~] [(COCI2. A D N . 2H20)2] [(CoBr~. ADN)2] CoBr2. A D N . 4H20 [Co(ADN)a] [Col4]

Color

light orange deep blue blue pink blue light orange yellow-green

C ~ N frequency (cm -l)

Shift due to coordination (cm -1)

2246 2283 2291 2291 2286 2285 2303 2296

+ 37 + 45 +45 +40 + 39 + 57 + 50

nitrate adducts[7, 11], and a recent crystal structure determination verified it for the silver perchlorate complex[6/. The latter is described as being a twodimensional linear polymer, and it does not seem unreasonable to assume a similar structure in three dimensions for the cobalt(II) perchlorate-ADN adduct discussed here, polymerization in the third dimension arising from the increased coordination number from 4 to 6. A similar cation is postulated for the iodocomplex. The thermal behavior of the A D N complexes was studied from ambient temperature to 500°C. Stepwise decomposition temperatures are given in Table 2. It is worth noting that the decomposition temperatures corresponding to loss of the ligands are very nearly the same, within the limits of the method, for the corresponding hydrated and anhydrous halide complexes, suggesting that identical steps occur in the decomposition process. The possibility of preparation of the identical anhydrous complexes by this method is thus noted. For every compound included in this thermogravimetric study, the breaks in the TG curves were sharp enough and the plateau fiat enough for quantitative analysis. Thus the curves were useful in assigning compositions. For example, the two distinct and equal weight losses in the anhydrous cobalt(II) chloride complex suggests a correct formulation of (CoCIz. ADN)2, and the theoretical weight loss for each ligand corresponds within a few tenths of a per cent to that observed graphically. In every case, the end product of the decomposition was the anhydrous cobalt(II) halide, as verifiable by the expected quantitative weight loss and visual examination of the product. Duplicate molecular weight determinations were run on the chloride compound described above by vapor pressure osmometry in nitromethane solution. The experimental values of 476 and 461 g/mole compare quite favorably with the value of 476 g/mole calculated for (CoCIz. ADN)z. The bromide analogue was not attempted due to its more rapid solvolysis in nitromethane. Both of these 7. M. Kubota and D. L. Johnston, J. inorg, nucl. Chem. 29, 769 (1967). 8. G. E. Coates and F. Glockling, In Organometallic Chemistry (Edited by H. Zeiss), p. 461. Reinhold, New York (1960). 9. R.A. Walton, Q. Rev. chem. Soc. 19, 126 (1965). 10. R. E. Clark and P. C. Ford, lnorg. Chem. 9, 227 (1970). 11. M. Kubota, D. L. Johnston and I. Matsubara, Inorg. Chem. 5, 386 (1966).

Cobalt(I l)-adiponitrile complexes Table

2.

1475

Procedural decomposition temperatures of the complexes from their T G A curves*.t

Compound

Water loss (°C)

A D N loss (°C)

[COC12. A D N . H2Oh]

64

166,230

[(CoCh. ADN. 2H2Oh]

80, 109

170,228

[(CoCIz. A D N h ] CoBr~. A D N . 4H20 [(CoBr2. ADN)2] [Co(ADN)3] [COL]

46, 69

156,225 207, (237 sh)~ 207,244 146,213§

*Two temperatures indicate two distinct breaks. Decomposition temperatures obtained by taking the intersection point of extrapolation of the linear portions of the curve before and after the break. These are generally reproducible to -+ 5°C. tThe perchlorate compound was not run due to its potentially explosive nature. *sh indicates a shoulder in the curve. §Final weight loss not well resolved from the second at 213°C.

complexes are essentially insoluble in benzene and carbon tetrachloride which are among the more common solvents for such studies. Conductance data were obtained on those complexes where practicable. The complex postulated as (CoCl2. A D N . HzO)z was insoluble in nitromethane; in A D N it apparently converted to the anhydrous complex by solvent displacement of H20 as evidenced by spectral data. The other hydrated chloride and bromide complexes were insoluble in A D N but were soluble and readily solvolyzed to a deep blue color in a variety of other polar solvents including nitromethane. Solvolysis was, in fact, important in all of the cases involving nitromethane solutions. This is supported by the conductance data in Table 3, as well as the specTable 3. Molar conductances at 25°C

Compound [(COCI2. A D N h ] [(CoBr2. ADN)z] [Co(ADN)a] [Col4] [Co(ADN)a] (CIO4)2

Molarity (× 104)

Solvent

A (initial)* (f~-x cm z mole-l)

A (final)t (f/-I cm 2 mole-l)

6 11 5 7 9 6 6

NM~: ADN NM ADN NM ADN ADN

15.1 4.6 12-1 5-9 22-6 14.6 20-9

101.6 99.4 45.0

*These A values were based on resistance data taken shortly after preparation of the solutions, and should be recognized as being only approximate for the nitromethane solutions, since the resistance was decreasing rapidly with time. tValues taken about one month after the preparation of the nitromethane solutions. Equilibrium apparently had been achieved. Nitromethane.

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D.L. GREENE and P. G. SEARS

tral data in Table 4. The conductances for all of the anhydrous halide complexes in nitromethane increase fairly rapidly with time. This is consistent with nitromethane displacing the halogens from the coordination sphere, e.g. the formation of species such as [CoCl(ADN)(solvent)]Cl. Final conductance values for the chloride and bromide species are in the range expected for a I:1 electrolyte, while initial values are well below those expected for electrolytes in nitromethane [12], and thus these complexes prior to dissolution and solvolysis appear to be properly formulated as nonelectrolytes. The behavior of the iodide complex in Table 4. Electronic spectral data

Compound [(CoCh. ADNh]

[(CoB~ .ADNh]

[Co(ADNh][Col,]

Band maximum (absorbancy index) State (cm-~) (l/molecm) Nujol 17390 15970 15020 NM 17270 16310 15770 ADN 17510 17010 19920 14660 Nujol 16780 16500 15200 NM 16750 15800 15380 ADN 16810 16160 15800 14580 Nujol 21140 20100 18320 15870 14330 13810 12630 NM 15870 15240 14470 ADN 15700 15020 14350 13890 13570

(686) (949) (885) (539) (501) (502) (830)

(735) (1130) (1197) (348) (697) (734) (1030)

(859) (1059) (980) (382) (695) (941) (1029) (1052)

Band maximum (absorbancy index) (cm-1) (l/molecm) 8772 7246 5797 9191 6601

(87) (107)

7143 5882

(89) (75)

Band maximum (absorbancy index) (cm-~) (l/molecm)

8745 6920 5682 8772 (98) 6849 (126) 5747 (85) 8621 (38) 6897 (96) 5714 (48) 4545 (62) 5051 4619 Absorption broad, continues beyond4300

8264 6729 5435 7194 6579

(136) (156) (85) (117) (148)

12. A. K. R. Unni, L. Elias and H. I. Schiff,J. phys. Chem. 67, 1216 (1963).

Cobalt(l l)-adiponitrile complexes

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Table 4. (Contd.) P3

Compound [(COC12. ADN. H~O)2]

Band maximum (absorbancy index) State (cm-1) (l/mole cm)

19340 17390 16030 15670 15020 ADN 17450 17010 16290 14640 [Co(ADN)3] (CIO4)2 Nujol 21370 20490 19610 ADN 21140 20450 19610 [(COCI2. ADN. 2H20)2] Nujol 21010 20O80

CoB~ . A D N . 4 H 2 0

Nujol

Nujol 22470 21010

/'J2

Vl

Band maximum (absorbancy index) (cm-~) (I/mole era)

Band maximum (absorbancy index) (cm-~) (l/mole era)

8850 7143 5821

(540) (502) (518) (805)

8621 7123 4854

(41) (81) (70) 9259

(13"9) (14-3) (12.7)

9276 8333

(4-2) (3"2)

8347 6676 5780 8403 6711 5797

nitromethane is quite different; solvolysis of this compound is apparently immediate. Both the initial and final values of A for the compound in nitromethane are low for a dissociated electrolyte, but spectral evidence as well as the conductance value in A D N indicate a reasonable formulation as [Co(ADN)3] [Co14]. The solutions of the chloride and bromide complexes change from blue to green on standing, the iodide from green to brown. Resistance measurements on solutions of the anhydrous complexes in A D N (systems in which A D N functions as both solvent and ligand) were of more value, as these solutions were apparently stable and gave meaningful data in light of previous results for electrolytes in this solvent[l]. The perchlorate compound is thus seen to be properly formulated as [Co(ADN)3](CIO4)2. From the earlier study cited[l], the single ion limiting conductance for the perchlorate ion is 7.8 f~-i cm 2 mole-l, thus yield ing a value of 5-3 f~-i cm 2 mole-1 for the conductance of [Co(ADN)3 z+] in dilute A D N solution at 25°C. By comparison, the conductances for Na ÷ and Bu4N ÷ are 5.4 and 4-2 ~-1 cm 2 mole -~. The low value for the doubly charged cobalt(II)-ADN complex ion is consistent with the postulated polymeric structure. Using the value of 5.3 1~-1 cm 2 mole -1 for [Co(ADN)32÷], there results a value of 9.3 f~-i cm z mole-~ for [Co142-] in dilute A D N solution. In comparison, the limiting conductances for SCN-, CIO4- and BPh4- are 9-8, 7.8 and 3.8 f~-i cm z mole-~, respectively. The low conductance values for the chloride and bromide complexes confirm their nature as predominantly covalent compounds or weak electrolytes.

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D . L . GREENE and P. G. SEARS

The visible and near-i.r, spectral data for the complexes are given in Table 4. From these data and the magnetic data in Table 5 it is apparent that the following complexes have octahedral symmetry: [Co(ADN)z](CIO4)2, [ ( C o C I ~ . A D N . 2H20)2] and CoBr2. A D N . 4H20. The bands observed are those corresponding to the transitions 4TIo --~ 4T2a and 4Tla "-~ 4Tla(P), generally designated as Vl and v3, respectively[13]. The low energy band is difficult to place exactly in the halides, as there are considerable overtones from the IR in this region, probably from the H20 in the molecules. For this reason these complexes are not included in the calculations for Dq, B and/3 in Table 6. In all cases the values of/zeff clearly indicate that the cobalt ions are in a spin free configuration. The spectrum of cobalt(ll) in a tetrahedral environment has been well elucidated by Cotton et ai. [14]. Comparison of their data for the tetrahalocobaltate(II) species with our present work leads to several conclusions. First, the anhydrous halide compounds, as well as [(COC12. A D N . H20)2], exhibit the type of spectrum associated with tetrahedral cobalt. Second, neither the anhydrous bromide and chloride nor the above-mentioned hydrate exhibit the bands associated with the tetrahalocobalt(ll) ions. This would seem to rule out a formulation for the latter as [Co. A D N . 2H20] [COC14], o r the former two as [Co(ADN)2] [COX4]. Third, the Nujol mull spectrum of the anhydrous iodide complex does match that of the [Co14 2-] ion, thus leading to the formulation as [Co(ADN)3] [Co14]. Fourth, solvolysis occurs to a greater or lesser extent in all these complexes, and so the Nujol mull spectra shall be regarded as being the true spectra of the compounds. This solvolysis is most rapid and complete in nitromethane in the case of the iodide complex; however, solvolysis is much slower for [(COC12. ADN)2] and [(CoBr2. A D N h ] in nitromethane and almost true spectra can be obtained if these are run shortly after solution preparation. Within 24 hr, solutions of these latter two compounds change in color from blue to green. All spectra in A D N show slight changes, most noticeable being the change in the spectrum for [(COC12 : A D N . H20)2] to a spectrum resembling that of the anhydrous chloride in ADN. Table 5. Magnetic data Compound

T(°K)

XM¢°'r x 106*

/xat(B.M.)t

[Co(ADNh] (CIO4h [Co(ADNh] [Co14] [(COCI2. ADNh] [(COC12. ADN. H2Oh] [(CoBr2. ADNh] [(CoCI~. A D N . 2H20)~] CoBrz. ADN. 4HzO

293 293 293 293 295 295 295

9707 9960 8570 9453 7753 9078 9997

4.79 4.85 4"50 4-72 4.29 4.64 4.87

*Corrected for diamagnetism according to Pascal's constants. tAverage per cobalt(ll) ion. 13. T. M. Dunn, In Modern Co-ordination Chemistry (Edited by J. Lewis and R. G. Wilkins), p. 290. Interscience, New York (1960). 14. F. A. Cotton, D. M. L. Goodgame and M. Goodgame, J. Am. chem. Soc. 83, 4690 (1961), and preceding articles.

Cobalt(I l)-adiponitrile complexes

1479

The transitions associated with tetrahedral cobalt(ll) are 4A2(F) ~ 4T2(F ), 4Az(F) ~ 4TI(F) and 4A2(F ) ~ 4TI(P); their frequencies are generally designated as Vl, v2 and Va respectively[15]. The difficulty of assigning exact band positions in these spectra has been well documented. The procedure developed by Cotton et al. [14] was used and the resulting data are listed in Table 6. Values of Dq, B and/3 were obtained using the equations of Tanabe and Sugano [ 16] which are derived using a Hamiltonian which does not include spin-orbit coupling. As the iodide spectrum has been previously reported[14], it is not included. The rule of average environment is used to assign Dq values for [(COC12. ADN)2] and [(CoBr2. ADN)2], enabling us to place A D N in the spectrochemical series.

Table 6. Summary of spectral assignments and derived electronic structure parameters

Compound [(CoCI2. ADN)2] [(CoBr2. ADN)2]

[Co(ADN)a] [Co14]

v3

J'~

Vl

Dq

State

(cm -1)

(cm -1)

(cm-')

(cm -~)

Nujol mull ADN Nujol mull ADN

15750 15500 15500 14800

6743 6897 6849 6897

Nujol mull ADN

20700 20240

9259 9050

B

fl

467 487 509 519

565 519 475 408

0-50 0.46 0-42 0.36

1210 1i 80

791 772

0.71 0.69

As seen in Table 6, A D N ranks quite high in the spectrochemical series toward cobalt(II). Comparison of these Dq values with those of various ligands shows that A D N lies close to benzimidazole with a Dq of 510 cm -1 in the complexes [Co(CrHsN2)z] and [Co(CrHrN2)4](CIO4)2[17]. The calculated nephelauxetic parameters would also seem to indicate that there is a very large degree of covalent character in the bonding to the metal. In view of the data cited, it is logical to propose a dimeric structure for the anhydrous chloride and bromide complexes of the following type: X. NC(CH2),CN X ~'Cof ~Co ~ X "~'NC(CH2)4CN~ ~X

X=CI,Br.

This structure has the obvious advantage that one is able to account readily for the formation of the octahedral aquated species by addition of water molecules to cobalt ions in the available positions. The process is reversible with heating, and on the basis of the comparison of the TG curves of the hydrated and anhydrous cobalt complexes, there is little room for doubt that this mechanism does indeed occur. As this work neared completion, the binuclear nature of the anhydrous 15. F. A. Cotton and M. Goodgame, J. A m . chem. Soc. 83, 1777 (1961). 16. Y. Tanabe and S. Sugano, J. phys. Soc. J a p a n 9, 753 (1954). 17. M. Goodgame and F. A. Cotton, J. A m . chem. Soc. 84, 1543 (1962).

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D . L . G R E E N E and P. G. SEARS

chloride complex was postulated independently by Burmeister and AI-Janabi[18] solely on the basis of the color change from pink to blue on dehydration of [(COC12. A D N . 2H20)2]. They did not present any supporting analytical, spectral, or molecular weight data. It is interesting to note that their conclusion was nevertheless essentially correct. Acknowledgement- Prof. G. A. Melson of Michigan State University is gratefully acknowledged for measuring the magnetic susceptibilities. 18. J. L. Burmeister and M. Y. AI-Janabi, lnorg. Chim. Acta 4, 581 (1970).