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Weak bonding in transition metal complexes: The interactions of methyl thiocyanate and methyl isothiocyanate
J. inorg,nucl.Chem.,1968,Vol.30, pp. 583 to 589. PergamonPressLtd.Printedin Great Britain
W E A K B O N D I N G IN T R A N S I T I O N M E T A L COMPLEXES: THE INTERACTIONS OF METHYL THIOCYANATE AND METHYL ISOTHIOCYANATE A. O. S. M A C Z E K School of Chemistry, The University, Leeds 2, England (First received 20 June 1967; in revised forrn 2 ,4 ugust 1967) A b s t r a c t - H e a t s of interaction of MeSCN and MeNCS at the axial positions of Ni(ll) and Pd(ll) have been estimated using a gas chromatographic technique. The results indicate weak axial bonding (AH < 1 kcal/mole) except in the case of MeNCS-Ni(II) where there is no apparent interaction. The M e N C S - P d ( l l ) interaction is appreciably greater than that between MeSCN and either Ni(II) or Pd(II).
THE preference shown by class a metals [ 1] for the nitrogen end of the thiocyanate ion and of class b metals for the sulphur end, is well established. Some exceptions to this generalisation have been reported, but these exceptions are mostly cases of class b metals bonding with both the sulphur and nitrogen atoms of a bridging thiocyanate group. Some chelated Cu(II) ions[2,3], however, have been considered to show a preference for C u - N co-ordination of thiocyanate. But in the case of Cu(tren)(NCS)(SCN) [2], it has recently been shown [4] that the apparently S-bonded thiocyanate ion is not bonded to Cu at all; and in Cu(en)2(SCN)2 [3], the Cu-S bond is both longer and more acute than usual, and it seems probable[5] that the axial bonding strength of the copper ion is weak enough to cause the disposition of the thiocyanate ions to be primarily determined by packing effects rather than by a real tendency for a preference for Cu-S over C u - N co-ordination. If one of the ends of the thiocyanate ion is methylated, it should be possible to investigate the donor properties of the other, however modified these may be by the presence of a methyl group. The eutectic[6] at -61.2°C in the solution of cobaltous thiocyanate in methyl thiocyanate, which is taken as evidence of adduct or complex formation, indicates that CH3SCN can show donor properties towards a transition metal ion, though these appear to be weak. Such weak complexing lends itself to an investigation by gas chromatography, and the interactions of CH3SCN and CH3NCS with the nickel(II) and palladium(II) complex column liquids described by Cartoni et al.[ 7] are reported below. I. 2. 3. 4. 5. 6. 7.
S. Ahrland, J. Chatt and N. R. Davies, Q. Rev. chem. Soc. 12, 265 (1958). K.N. Raymond and F. Basolo, J. org. Chem. 5, 1632 (1966). B.W. Brown and E. C. Lingafelter, Acta crystallogr. 7,254 (1964). P. C. Jain and E. C. Lingafelter, J. A m chem. Soc. 89, 724 (1967). E. C. Lingafelter, Private communication. J. Gillis and A. de Sweemer, Chem. Zentbl. 1,685 ( 1935). G. P. Cartoni, R. S. Lowrie, C. S. G. Phillips and L. M. Venanzi, Gas Chromatography 1960 (Edited by R. P. W. Scott), p. 273. Butterworths, London (1961). 583
584
A . O . S . MACZEK
EXPERIMENTAL SECTION Reagents CH3SCN and CH3NCS were obtained in reagent grade from British Drug Houses Ltd. and were used without further purification. The column liquids, bis-(methyl-n-octyl glyoxime) nickel(ll) (m.p. 105°C), bis-(methyl-n-octylglyoxime)-palladium(ll)(m.p. 112°C), and n-heptadecyl-n-hexadecyl monoxime, sodium salt (m.p. 78°C) were prepared by Dr. G. P. Cartoni[7] and were kindly made available for this study. Apparatus Gas chromatographic measurements were made in all glass apparatus thermostatted with refluxing vapours at 78"5°C, 100°C, 110.6"C, 132°C, and 156"5°C. A flame ionisation detector was used. The column liquids were used in - 10 per cent (w/w) amounts on celite. Procedure Samples (0" 1 p.l) of 1 per cent solutions of benzene and either CH3SCN or CH3NCS were injected onto the columns, and the retentions of methyl thiocyanate and methyl isothiocyanate relative to benzene were measured. The relative retentions were averaged over six to ten determinations, and the specific retention volume (Vg) of benzene was measured on all three substrates with six different gas flow rates at each of the five temperatures. Accuracy in these measurements was ensured by determining retention-times with a stop watch and by metering the gas flow under carefully controlled conditions of temperature and pressure. Measurements of retention time were rejected if the gas flow-rates before and after an experiment varied by more than one part in three hundred; the probable errors in flow rate measurements are unlikely to be much larger than this. The precision of the retention time measurements was several times greater. The variation of V~ over six measurements indicates a probable error in the mean of between 1 and 2 per cent-though in one case (discussed below) this becomes almost 5 per cent.
THEORY I n g a s c h r o m a t o g r a p h y , t h e p a r t i t i o n coefficient H is d e f i n e d b y t h e e q u a t i o n [8] H=
c o n c n o f s a m p l e in l i q u i d p h a s e w t . / m l c o n c n o f s a m p l e in c a r r i e r g a s ' w t . / m l
a n d is r e l a t e d to t h e s p e c i f i c r e t e n t i o n v o l u m e V~ b y t h e e x p r e s s i o n H - VsTcpt 273 T h e r a t i o o f t w o s u c h p a r t i t i o n c o e f f i c i e n t s f o r a g i v e n s a m p l e in t w o different c o l u m n l i q u i d s at t h e s a m e t e m p e r a t u r e , is t h e e q u i l i b r i u m c o n s t a n t f o r t h e p a r t i t i o n o f t h e s a m p l e b e t w e e n t h e t w o l i q u i d s , t r e a t e d a s if t h e s e w e r e t o t a l l y immiscible. Thus
H1 H2
Kt~a~sfer
Vslpll V~pl2
Vg is c o n v e n t i o n a l l y e x p r e s s e d in ml. g-1 at N . T . P . , a n d Ktransfer d e f i n e s t h e e q u i l i b r a t i o n o f s a m p l e b e t w e e n e q u a l v o l u m e s o f l i q u i d 1 a n d liquid 2. T h e s t a n d a r d m o l a r f r e e e n e r g y o f t r a n s f e r (AG~r) o f s a m p l e f r o m a g i v e n v o l u m e o f s o l v e n t 1 to t h e s a m e v o l u m e o f s o l v e n t 2 is g i v e n b y A G E = - R T c In Ktr = - R T e In Vslpll V r~pl2 8. H.W. Johnson and F. H. Stross, Analyt. Chem. 30, 1586 0958).
Weak bonding in transition metal complexes
585
The use of the ratio HJH2 avoids the problems of standard state definitions, provided that Vgl and Vg2 are expressed in the same units. A more useful quantity, the molar free energy of transfer (AG~'r~m)) from one mole of solvent l to one mole of solvent 2 is given by: --AG~rtm) = --AG~'r + RTc In
M1/pt~= RTc'm Vg~M~ Vg----~z"
Mdpt2
If the molar specific retention volume (Vc = Vg.M) is used, this becomes: W6~ AG~'r(m) = - - R T c In Vo----~" RESULTS
Table 1 shows mean Vo values (in 1. mole -1) for methyl thiocyanate and methyl isothiocyanate on the three stationary phases. Also shown are the standard deviations (or) from six observations. A1OgloVG transfer [------logao(VcJVG2)] from reference to complex phase is plotted against reciprocal absolute temperature in Fig. 1. The indicated error ranges are the S.E. of the mean VG values, and are derived from the standard deviations inTable 1. Table 2 shows the molar enthalpies of transfer of methyl thiocyanate or methyl isothiocyanate from one mole of reference phase to one mole of complex phase. These enthalpies are calculated either from the slopes of the lines in Fig. 1 or (where only two points are available) by use of the van't Hoff isochore in its integrated form. Temperoture *C 1565
132'0
110"6 100"0
o U o 0.20
78 5
..." :"
E o
k
L 0"10
:::
""""
o
23
24
2~5
26
2J7
2~0
104/T
Fig. 1. The free energy of transfer for methyl thiocyanate and methyl isothiocyanate from reference to complex phase. --
I
MoSCN -MoNCS ± ±
Pd_bis(CH_dioxime) { CH3SCN CHaNCS
Ni-bis(C~-dioxime) { CH~SCN CHaNCS
CHaSCN Na-C34monoxime [ C H 3 N C S
Temperature °C ~r 1.21 1.44 1.73 2.01 2.32 2.74
Vc 37.15 44.06 27.54 33.19 20.65 24-32
78-5
13.93 15.14
17.70 20.61
23"33 25-70
VQ
0.48 0.51
0-61 0.71
0-64 0-71
o-
100-0
11.94 12.39
17-02 16.75
19-50 20.46
VG
0.41 0.44
0.59 0.58
0.56 0.59
o-
110.6
9.20 !1.69
10-33 11.17
12-88 12.94
VG
or
0-35 0-48
0.41 0-44
0"43 0"43
132.0
7.13 6.55
6.35 7.31
8"47 8"10
VQ
0-31 0.28
0.28 0.33
0.30 0"28
~r
156.5
Table !. Mean VG values (in 1. mole -1 at standard pressure and column temperatures) and their standard deviations o-
N
©
.>
Weakbondingin transitionmetalcomplexes
587
Table 2. A H trander (referenceto complex),in kcal/mole CH3SCN Referenceto n i c k e l Referenceto palladium
CH3NCS
--0.48__.0.14 +0.31__-0-07 --0.40 --4-0.28 -0.72 _+0.29
DISCUSSION The extended working range of the complexes warrants special mention. Cartoni et al.[7] quote a useful working range between the respective melting points and 180-190°C. It has been found that the nickel and palladium complexes act as "liquid" substrates down to temperatures some 30°C below their stated melting points. As the temperature is lowered, these column "liquids" show a gradual decrease in column efficiency, to the extent that peaks obtained with palladium complex at 78.5°C are too broad and trailing to allow Vg measurements to be quoted with great confidence, as may be seen from the large standard deviation in this case. The retention measurements made below the melting points of these complexes are of interest, for at low temperatures both nickel and palladium retain methyl isothiocyanate more than methyl thiocyanate; so too does the sodium reference compound. This is the expected "boiling-point" emergence order. However, immediately above its melting point, the nickel column shows a marked specific retardation of methyl thiocyanate, and the palladium column shows a specific effect with methyl thiocyanate and an even stronger one with methyl isothiocyanate. On the nickel column, methyl isothiocyanate is not specifically retarded, and Vc values in this case show a regular decrease with increasing temperature over the whole temperature range. In this, methyl isothiocyanate on the nickel column behaves qualitatively in the same way as benzene on both the nickel and the palladium column; there is no abrupt change in the behaviour of benzene on the two transition metal columns at their stated melting points. The transfer processes, whose molar Gibbs free energies are related to the points in Fig. 1, involve the removal of one mole of ligand from one mole of the environment in which it finds itself in n-heptadecyl-n-hexadecyl monoxime, sodium salt, and placing it in an environment consisting of bis-(methyl-n-octyl glyoxime)M(ll). The sodium salt acts as a reference phase, and it is assumed that, in the absence of complexing, the forces acting on a molecule dissolved in the complex phase are very closely similar to those acting on the same molecule dissolved in the sodium salt. Any differences in free energies or enthalpies of solution between individual compounds in the reference and complex phases may therefore be assigned to the effects of co-ordination at the vacant axial positions of the transition metal ion in the complex phase. This can, of course, at best be only an approximate assignment, for the reference and complex phases are sufficiently dissimilar for there to be a residual enthalpy and free energy of transfer even for molecules which do not co-ordinate to the transition metal ions. Moreover, the formation of a co-ordinate axial bond with a ligand molecule must involve the breaking of intermolecular metal-metal bonds which are known [9] to 9. F. A. Cotton and G. Wilkinson,Advanced Inorganic Chemistry 2nd edn, pp. 655, 1024. Wiley, New York (1966).
588
A.O.S. MACZEK
exist in these glyoxime complexes. Such factors too can have a marked effect on the relationship between the enthalpy of transfer and the enthalpy of formation of an axial co-ordinate link, and will be discussed later. However, though the absolute magnitudes of the enthalpies of formation of co-ordinate axial bonds at the transition metal ion may be in considerable error, arguments based on their relative sizes have extensive validity. The linearity of the Alog Vc transfer plot (reference ~ Ni(II)) over the whole temperature range in the case of CH3NCS indicates that the metal-ligand interactions which characterise the higher temperature transfer process in the other three cases are absent in this instance. As was pointed out earlier, methyl isothiocyanate in this case behaves exactly like a molecule (e.g. benzene) which has no tendency to co-ordinate at the vacant axial positions of the transition metal ion. Both methyl thiocyanate and methyl isothiocyanate show this reluctance to coordinate below the melting points of the transition metal phases, as may be inferred from the similarity of the sign and magnitude of the low temperature parts of the slopes of the A log Vc t~ansferplots in Fig. 1. It is probable that, in this low temperature region, solution occurs only in the outer bulky organic part of the complex molecules, and that residual crystalline structure in the stationary phase prevents molecules from coming close enough to the metal atoms to allow interactions to occur. Once the stationary phase is truly molten, metal-ligand interactions can take place quite freely. Three main conclusions relevant to the process of metal-thiocyanate interactions can validly be drawn from the enthalpies of transfer of Table 2. Firstly, and obviously, that the methylation of the thiocyanate group greatly decreases its tendency to co-ordinate to transition metal ions. The enthalpies of transfer are very low (< 1 kcal/mole) so the resultant metal-ligand bonds must be correspondingly very weak. Secondly, that the sulphur-free molecule shows no tendency to form bonds with class a nickel(II) whereas both the nitrogen-free and and the sulphur-free molecules interact with class b palladium(II). Moreover, the CH3NCS-Pd(II) bond is some 300 cal/mole stronger than the CH3SCN-Pd(II) bond. If the positive enthalpy of transfer of methyl isothiocyanate to the nickel phase is assumed to be a consequence of unavoidable imperfection in the choice of a reference compound (the ideal reference compound would have Alog V~transfer and hence AH~ both zero for all compounds which merely dissolved normally without interacting with the central metal atom) then it is possible to set upper and lower limits to the change in enthalpy when a bond from a transition metal atom to one of its own kind is replaced by a bond to methyl thiocyanate or methyl isothiocyanate. These estimates are:
and
CHaNCS-Ni(II) 0 CH3SCN-Ni(II) 0.79___0-21 CH3SCN-Pd(II) 0.71 ± 0-35 CH3NCS-Pd(II) 1.03 ___0.36
kcal/mole kcal/mole kcal/mole kcal/mole.
And thirdly, that the obsolute values of the enthalpies of formation of axial bonds by Ni(II) and Pd(lI) with methyl thiocyanate and by Pd(II) with methyl isothiocyanate are likely to be somewhat larger than the values shown above. It is not
Weak bonding in transition metal complexes
589
possible to make precise estimates of the strengths of the metal 8 bonds which the ligand bonds replace, but they are probably of the order of 1 kcal/mole [9]. The palladium 8 bond is likely to be the stronger of the two (cf. the dissolution in alkali of bis-(gloximato) Pd(II) but not of the Ni(II) analogue [9]) and it may therefore be concluded that the CHaSCN interacts more or less equally strongly at axial positions in Ni(lI) and Pd(II) and that the bond between CH3NCS and Pd(II) is some 15-30 per cent stronger. Acknowledgements-The author is indebted to Dr. G. P. Cartoni and Dr. C. S. G. Phillips for the gift of column materials; to Professor H. M. N. H. Irving for suggesting this work, and to Professor E. C. Lingafelter for his interest in it.
W E A K B O N D I N G IN T R A N S I T I O N M E T A L COMPLEXES: THE INTERACTIONS OF METHYL THIOCYANATE AND METHYL ISOTHIOCYANATE A. O. S. M A C Z E K School of Chemistry, The University, Leeds 2, England (First received 20 June 1967; in revised forrn 2 ,4 ugust 1967) A b s t r a c t - H e a t s of interaction of MeSCN and MeNCS at the axial positions of Ni(ll) and Pd(ll) have been estimated using a gas chromatographic technique. The results indicate weak axial bonding (AH < 1 kcal/mole) except in the case of MeNCS-Ni(II) where there is no apparent interaction. The M e N C S - P d ( l l ) interaction is appreciably greater than that between MeSCN and either Ni(II) or Pd(II).
THE preference shown by class a metals [ 1] for the nitrogen end of the thiocyanate ion and of class b metals for the sulphur end, is well established. Some exceptions to this generalisation have been reported, but these exceptions are mostly cases of class b metals bonding with both the sulphur and nitrogen atoms of a bridging thiocyanate group. Some chelated Cu(II) ions[2,3], however, have been considered to show a preference for C u - N co-ordination of thiocyanate. But in the case of Cu(tren)(NCS)(SCN) [2], it has recently been shown [4] that the apparently S-bonded thiocyanate ion is not bonded to Cu at all; and in Cu(en)2(SCN)2 [3], the Cu-S bond is both longer and more acute than usual, and it seems probable[5] that the axial bonding strength of the copper ion is weak enough to cause the disposition of the thiocyanate ions to be primarily determined by packing effects rather than by a real tendency for a preference for Cu-S over C u - N co-ordination. If one of the ends of the thiocyanate ion is methylated, it should be possible to investigate the donor properties of the other, however modified these may be by the presence of a methyl group. The eutectic[6] at -61.2°C in the solution of cobaltous thiocyanate in methyl thiocyanate, which is taken as evidence of adduct or complex formation, indicates that CH3SCN can show donor properties towards a transition metal ion, though these appear to be weak. Such weak complexing lends itself to an investigation by gas chromatography, and the interactions of CH3SCN and CH3NCS with the nickel(II) and palladium(II) complex column liquids described by Cartoni et al.[ 7] are reported below. I. 2. 3. 4. 5. 6. 7.
S. Ahrland, J. Chatt and N. R. Davies, Q. Rev. chem. Soc. 12, 265 (1958). K.N. Raymond and F. Basolo, J. org. Chem. 5, 1632 (1966). B.W. Brown and E. C. Lingafelter, Acta crystallogr. 7,254 (1964). P. C. Jain and E. C. Lingafelter, J. A m chem. Soc. 89, 724 (1967). E. C. Lingafelter, Private communication. J. Gillis and A. de Sweemer, Chem. Zentbl. 1,685 ( 1935). G. P. Cartoni, R. S. Lowrie, C. S. G. Phillips and L. M. Venanzi, Gas Chromatography 1960 (Edited by R. P. W. Scott), p. 273. Butterworths, London (1961). 583
584
A . O . S . MACZEK
EXPERIMENTAL SECTION Reagents CH3SCN and CH3NCS were obtained in reagent grade from British Drug Houses Ltd. and were used without further purification. The column liquids, bis-(methyl-n-octyl glyoxime) nickel(ll) (m.p. 105°C), bis-(methyl-n-octylglyoxime)-palladium(ll)(m.p. 112°C), and n-heptadecyl-n-hexadecyl monoxime, sodium salt (m.p. 78°C) were prepared by Dr. G. P. Cartoni[7] and were kindly made available for this study. Apparatus Gas chromatographic measurements were made in all glass apparatus thermostatted with refluxing vapours at 78"5°C, 100°C, 110.6"C, 132°C, and 156"5°C. A flame ionisation detector was used. The column liquids were used in - 10 per cent (w/w) amounts on celite. Procedure Samples (0" 1 p.l) of 1 per cent solutions of benzene and either CH3SCN or CH3NCS were injected onto the columns, and the retentions of methyl thiocyanate and methyl isothiocyanate relative to benzene were measured. The relative retentions were averaged over six to ten determinations, and the specific retention volume (Vg) of benzene was measured on all three substrates with six different gas flow rates at each of the five temperatures. Accuracy in these measurements was ensured by determining retention-times with a stop watch and by metering the gas flow under carefully controlled conditions of temperature and pressure. Measurements of retention time were rejected if the gas flow-rates before and after an experiment varied by more than one part in three hundred; the probable errors in flow rate measurements are unlikely to be much larger than this. The precision of the retention time measurements was several times greater. The variation of V~ over six measurements indicates a probable error in the mean of between 1 and 2 per cent-though in one case (discussed below) this becomes almost 5 per cent.
THEORY I n g a s c h r o m a t o g r a p h y , t h e p a r t i t i o n coefficient H is d e f i n e d b y t h e e q u a t i o n [8] H=
c o n c n o f s a m p l e in l i q u i d p h a s e w t . / m l c o n c n o f s a m p l e in c a r r i e r g a s ' w t . / m l
a n d is r e l a t e d to t h e s p e c i f i c r e t e n t i o n v o l u m e V~ b y t h e e x p r e s s i o n H - VsTcpt 273 T h e r a t i o o f t w o s u c h p a r t i t i o n c o e f f i c i e n t s f o r a g i v e n s a m p l e in t w o different c o l u m n l i q u i d s at t h e s a m e t e m p e r a t u r e , is t h e e q u i l i b r i u m c o n s t a n t f o r t h e p a r t i t i o n o f t h e s a m p l e b e t w e e n t h e t w o l i q u i d s , t r e a t e d a s if t h e s e w e r e t o t a l l y immiscible. Thus
H1 H2
Kt~a~sfer
Vslpll V~pl2
Vg is c o n v e n t i o n a l l y e x p r e s s e d in ml. g-1 at N . T . P . , a n d Ktransfer d e f i n e s t h e e q u i l i b r a t i o n o f s a m p l e b e t w e e n e q u a l v o l u m e s o f l i q u i d 1 a n d liquid 2. T h e s t a n d a r d m o l a r f r e e e n e r g y o f t r a n s f e r (AG~r) o f s a m p l e f r o m a g i v e n v o l u m e o f s o l v e n t 1 to t h e s a m e v o l u m e o f s o l v e n t 2 is g i v e n b y A G E = - R T c In Ktr = - R T e In Vslpll V r~pl2 8. H.W. Johnson and F. H. Stross, Analyt. Chem. 30, 1586 0958).
Weak bonding in transition metal complexes
585
The use of the ratio HJH2 avoids the problems of standard state definitions, provided that Vgl and Vg2 are expressed in the same units. A more useful quantity, the molar free energy of transfer (AG~'r~m)) from one mole of solvent l to one mole of solvent 2 is given by: --AG~rtm) = --AG~'r + RTc In
M1/pt~= RTc'm Vg~M~ Vg----~z"
Mdpt2
If the molar specific retention volume (Vc = Vg.M) is used, this becomes: W6~ AG~'r(m) = - - R T c In Vo----~" RESULTS
Table 1 shows mean Vo values (in 1. mole -1) for methyl thiocyanate and methyl isothiocyanate on the three stationary phases. Also shown are the standard deviations (or) from six observations. A1OgloVG transfer [------logao(VcJVG2)] from reference to complex phase is plotted against reciprocal absolute temperature in Fig. 1. The indicated error ranges are the S.E. of the mean VG values, and are derived from the standard deviations inTable 1. Table 2 shows the molar enthalpies of transfer of methyl thiocyanate or methyl isothiocyanate from one mole of reference phase to one mole of complex phase. These enthalpies are calculated either from the slopes of the lines in Fig. 1 or (where only two points are available) by use of the van't Hoff isochore in its integrated form. Temperoture *C 1565
132'0
110"6 100"0
o U o 0.20
78 5
..." :"
E o
k
L 0"10
:::
""""
o
23
24
2~5
26
2J7
2~0
104/T
Fig. 1. The free energy of transfer for methyl thiocyanate and methyl isothiocyanate from reference to complex phase. --
I
MoSCN -MoNCS ± ±
Pd_bis(CH_dioxime) { CH3SCN CHaNCS
Ni-bis(C~-dioxime) { CH~SCN CHaNCS
CHaSCN Na-C34monoxime [ C H 3 N C S
Temperature °C ~r 1.21 1.44 1.73 2.01 2.32 2.74
Vc 37.15 44.06 27.54 33.19 20.65 24-32
78-5
13.93 15.14
17.70 20.61
23"33 25-70
VQ
0.48 0.51
0-61 0.71
0-64 0-71
o-
100-0
11.94 12.39
17-02 16.75
19-50 20.46
VG
0.41 0.44
0.59 0.58
0.56 0.59
o-
110.6
9.20 !1.69
10-33 11.17
12-88 12.94
VG
or
0-35 0-48
0.41 0-44
0"43 0"43
132.0
7.13 6.55
6.35 7.31
8"47 8"10
VQ
0-31 0.28
0.28 0.33
0.30 0"28
~r
156.5
Table !. Mean VG values (in 1. mole -1 at standard pressure and column temperatures) and their standard deviations o-
N
©
.>
Weakbondingin transitionmetalcomplexes
587
Table 2. A H trander (referenceto complex),in kcal/mole CH3SCN Referenceto n i c k e l Referenceto palladium
CH3NCS
--0.48__.0.14 +0.31__-0-07 --0.40 --4-0.28 -0.72 _+0.29
DISCUSSION The extended working range of the complexes warrants special mention. Cartoni et al.[7] quote a useful working range between the respective melting points and 180-190°C. It has been found that the nickel and palladium complexes act as "liquid" substrates down to temperatures some 30°C below their stated melting points. As the temperature is lowered, these column "liquids" show a gradual decrease in column efficiency, to the extent that peaks obtained with palladium complex at 78.5°C are too broad and trailing to allow Vg measurements to be quoted with great confidence, as may be seen from the large standard deviation in this case. The retention measurements made below the melting points of these complexes are of interest, for at low temperatures both nickel and palladium retain methyl isothiocyanate more than methyl thiocyanate; so too does the sodium reference compound. This is the expected "boiling-point" emergence order. However, immediately above its melting point, the nickel column shows a marked specific retardation of methyl thiocyanate, and the palladium column shows a specific effect with methyl thiocyanate and an even stronger one with methyl isothiocyanate. On the nickel column, methyl isothiocyanate is not specifically retarded, and Vc values in this case show a regular decrease with increasing temperature over the whole temperature range. In this, methyl isothiocyanate on the nickel column behaves qualitatively in the same way as benzene on both the nickel and the palladium column; there is no abrupt change in the behaviour of benzene on the two transition metal columns at their stated melting points. The transfer processes, whose molar Gibbs free energies are related to the points in Fig. 1, involve the removal of one mole of ligand from one mole of the environment in which it finds itself in n-heptadecyl-n-hexadecyl monoxime, sodium salt, and placing it in an environment consisting of bis-(methyl-n-octyl glyoxime)M(ll). The sodium salt acts as a reference phase, and it is assumed that, in the absence of complexing, the forces acting on a molecule dissolved in the complex phase are very closely similar to those acting on the same molecule dissolved in the sodium salt. Any differences in free energies or enthalpies of solution between individual compounds in the reference and complex phases may therefore be assigned to the effects of co-ordination at the vacant axial positions of the transition metal ion in the complex phase. This can, of course, at best be only an approximate assignment, for the reference and complex phases are sufficiently dissimilar for there to be a residual enthalpy and free energy of transfer even for molecules which do not co-ordinate to the transition metal ions. Moreover, the formation of a co-ordinate axial bond with a ligand molecule must involve the breaking of intermolecular metal-metal bonds which are known [9] to 9. F. A. Cotton and G. Wilkinson,Advanced Inorganic Chemistry 2nd edn, pp. 655, 1024. Wiley, New York (1966).
588
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exist in these glyoxime complexes. Such factors too can have a marked effect on the relationship between the enthalpy of transfer and the enthalpy of formation of an axial co-ordinate link, and will be discussed later. However, though the absolute magnitudes of the enthalpies of formation of co-ordinate axial bonds at the transition metal ion may be in considerable error, arguments based on their relative sizes have extensive validity. The linearity of the Alog Vc transfer plot (reference ~ Ni(II)) over the whole temperature range in the case of CH3NCS indicates that the metal-ligand interactions which characterise the higher temperature transfer process in the other three cases are absent in this instance. As was pointed out earlier, methyl isothiocyanate in this case behaves exactly like a molecule (e.g. benzene) which has no tendency to co-ordinate at the vacant axial positions of the transition metal ion. Both methyl thiocyanate and methyl isothiocyanate show this reluctance to coordinate below the melting points of the transition metal phases, as may be inferred from the similarity of the sign and magnitude of the low temperature parts of the slopes of the A log Vc t~ansferplots in Fig. 1. It is probable that, in this low temperature region, solution occurs only in the outer bulky organic part of the complex molecules, and that residual crystalline structure in the stationary phase prevents molecules from coming close enough to the metal atoms to allow interactions to occur. Once the stationary phase is truly molten, metal-ligand interactions can take place quite freely. Three main conclusions relevant to the process of metal-thiocyanate interactions can validly be drawn from the enthalpies of transfer of Table 2. Firstly, and obviously, that the methylation of the thiocyanate group greatly decreases its tendency to co-ordinate to transition metal ions. The enthalpies of transfer are very low (< 1 kcal/mole) so the resultant metal-ligand bonds must be correspondingly very weak. Secondly, that the sulphur-free molecule shows no tendency to form bonds with class a nickel(II) whereas both the nitrogen-free and and the sulphur-free molecules interact with class b palladium(II). Moreover, the CH3NCS-Pd(II) bond is some 300 cal/mole stronger than the CH3SCN-Pd(II) bond. If the positive enthalpy of transfer of methyl isothiocyanate to the nickel phase is assumed to be a consequence of unavoidable imperfection in the choice of a reference compound (the ideal reference compound would have Alog V~transfer and hence AH~ both zero for all compounds which merely dissolved normally without interacting with the central metal atom) then it is possible to set upper and lower limits to the change in enthalpy when a bond from a transition metal atom to one of its own kind is replaced by a bond to methyl thiocyanate or methyl isothiocyanate. These estimates are:
and
CHaNCS-Ni(II) 0 CH3SCN-Ni(II) 0.79___0-21 CH3SCN-Pd(II) 0.71 ± 0-35 CH3NCS-Pd(II) 1.03 ___0.36
kcal/mole kcal/mole kcal/mole kcal/mole.
And thirdly, that the obsolute values of the enthalpies of formation of axial bonds by Ni(II) and Pd(lI) with methyl thiocyanate and by Pd(II) with methyl isothiocyanate are likely to be somewhat larger than the values shown above. It is not
Weak bonding in transition metal complexes
589
possible to make precise estimates of the strengths of the metal 8 bonds which the ligand bonds replace, but they are probably of the order of 1 kcal/mole [9]. The palladium 8 bond is likely to be the stronger of the two (cf. the dissolution in alkali of bis-(gloximato) Pd(II) but not of the Ni(II) analogue [9]) and it may therefore be concluded that the CHaSCN interacts more or less equally strongly at axial positions in Ni(lI) and Pd(II) and that the bond between CH3NCS and Pd(II) is some 15-30 per cent stronger. Acknowledgements-The author is indebted to Dr. G. P. Cartoni and Dr. C. S. G. Phillips for the gift of column materials; to Professor H. M. N. H. Irving for suggesting this work, and to Professor E. C. Lingafelter for his interest in it.