- Email: [email protected]
Contact shifts and mechanism of spin delocalisation in hexa coordianted nickel complexes
volume 10, number
1
cxFM.IcAL PHY!ms
1 July 1971:
LEXTERS
CONTACI’ SHIFTS AND MECHANIS~I OF SPIN DELOCALISATION IN HEXA COORDINATED NKKEL COMPLEXES MM DHINGRA, G. GOVIL 3nd CR. ICANEKAR Tara IMi!ute of Fundaktental Resemch. Homi Bhabha Road, C~hzba,Bombay-S, India Received
11 May 1971
The contactshifts observed for protons of -y-picz&z in complexes of the type NiXi (r-picoline)s (where X = diethyldiUCophos@ate, ethybranthate and acetylacetonete) hwe been studied to understand the spin transfer mecbsnism.The revolts indicate that the spin transfer involvesnot only the highest bonding @and orbital of Qsymmetry but also to some extent, the lowest antibonding and the highest bonding orbitals of n-symmetry.
The large shifts observed for paramagnetic transiwe have relied on the SCF MO calculations in the . _ tion metal complexes have been attributed to the INDO framework as the method has proved very sucFermi contact and the dipole-dipole interactions becessf31 in predicting the spin densities and hypertie tween the nuclear and electronic spins [ 1] . For octacoupling constants (a) in a variety of organic syshedral complexes of nickel@) the ground state is an tems [4] (the program used is QCPE No. 142). In orbital singlet and the clipolsr contribution is absent. this note we report some interesting observations on The observed Isotropic shifts in such nickel comcomplexes where Y = -y-picoline. plexes therefore originate from the Fermi contact Table 1 gives the calculated hype&e interaction mechanism. In hexa coordinated Ni(II) complexes, / constants for the protons of ypicoline assuming a the unpaired electrons are i? the eg orbitals which kgle w+n in the orbit& u, Ir* and gb. It is-imporhave o-symmetry with respect to the ligand. It is tant to point out that the INDO calculation for the therefore expected that with ligands like pyridine and methyl substituted pyrldines the ligand to metal spin transfer should occur via the highest bonding tilled orbiTable 1 tal of the ligand. However, Cramer and Drago [2] showHvperfine coupling constants (a) for a singleCW@Iin daered that the experimental results for such systems indicate eat ortitals of r-picoline that to a small extent there is also a transfer of @pin INDO Observed [S] Eli-r I21 to the ligand Ir-orbitals (i.e., the lowest antibonding Ir* and/or the highest bonding orbital of a-symmetry, (a) ombital nb). Unfortunately, their analysis tias based on the +19.55 +?.23 OfthO extended Wckel theory (EHT) - a method which is +2.85 + 5.71 Meta +0.11 - 1.28 ParaCHs highly unreliable in the calcdation of spin de&ties. In recent years, we have been interested in the stucb) n*-orbital OrthO +2.92 - 3.09 (*).‘3.80 dy of contact shifts in complexes of the type Ni(W2Y2
where X = acetylacetonate (am), diethyldithiophos(exan), etc., and Y = the secondary and the tertiaryamines such at pyridine, piperidine, morpholine, etc. t. To’ explain these shifts f A short account of some experimental aspects of this phate (dtp), ethylxanthate
wosk hw @en reported previously [ 31.
86
Meta
- 0.26
+I.42
p-3
e13.49
-4.31
OX&O
+31.39
Meta
+X23
+1.75 +233
PafaX!Hs
+40.59
-6.80
(c) 7rb-orbitaJ
(*I! 0x0 (*)11.38
Volume10, numixr1
cIIF.bIIcAL PHYSICSLErrERs
Ir*
I July 1971
bitals can be calculated from the set of equations of the type:
(lowest mtkoildhg) orbital is in good agreement with the observed values, for vpicoline anion [S] . No such agreement is found for the values calculated by EXT. We have not been able to find any experimental data for the cation and tripositive ion of r_ picoline, corresponding to the o and VI’ orbitals respectively. However, in view of the wide-spread success in spin density calculations achieved by the INDO method 161, one can rely on the theoretical predictions for these orbitals. It has become customary in this kind of work to report contact shifts in terms of their ratios at the various proton sites. The ratios do not depend on the degree of dissociation for weak complexes. If the unpaired electron is only in the o-orbital of the &and then the ratios AOiA, and A,/+ (o = orrho, m = meta and p = para-CH3) are expected to be about 3.42 and -4.46 respectively, from INDO caldations. The experimental data on 7.picoline, for a series of complexes are given in table 2. In the com-
q&3.=-qJn*)v+cr,(+% % = % um(u)x-am(7r*)v*mf.srb)z
’
where x,y and z (= I -x-y) denote the fraction of spin densities transferred to the o, IF* and nb orbitals respectively. The significance of these numbers may become clear if one considers the possiile mechanisms by which the electronic spin centered on the metal ion can delocalise to the figand orbit& As already stated the direct spin transfer from the metal eg orbitals due to a partially comlcnt metal ligand bond will involve the o-orbitals, of the ligand Ieading to a a-spin in these orbitals. However, unpaired electrons in the eg orbitals can interact with the filled metal tzg orbitals (having n-symmetry) via spin exchange causing a residual spin in such orbit&. The mechankm of spin transfer to the ITSand x* orbit& of the
Table 2 Ratios of the contact shifts, values of spin dendty
fractions and &and field parametersin Ni(X)a (y-pic)z complexes
Ratio of contact shifts
Spin density fractions
%+m
“rnf%
Adap
x
Y
z
NWac)h-pi&
3.60 a)
-3.09 a)
Wr+)$(rpich Wdtpk h-+)2 Ni(exanh (rpic)2
3.05 c) 2.56 a) 1.85 d)
-3.56 C) -3.06 d) -3.41 di
-11.11 -10.83 - 7.74 - 6.31
1.00 0.805 0.557 0.200
0.00 0.157 0.352 0.631
0.00 0.037 0.090 0.171
a) C) a) a)
LODq (cm-9
y;: :; 8980 d) 9240 d)
a) Ref. [7];b) ref. (81 (for p@iine); c) ref. [Z] ;d) present work
plexes Ni(aca& (y-pit), the observed results are consistent with a spin transfer to only the o-orbitals of r_picoline. However, for other complexes, some contribution appears to arise from q* and nb orbitals. Attempts to fit the observed contact shift ratios and the calculated hyperfine interaction constants revealed that agreement can be reached only if it is assumed that a certain fraction of a-spin is present in the filled 0 and nb orbitals while &spin is present in R* orbital. Other [email protected], suck as u + n* and (I + nb for spin transfer did not give satisfactory agreement. The fraction of spin density transferred to the various or-
ligand is indicative of the certain amount of xcharacter in the metal ligand bond. The coefficients x md
y and z are a measure of relative (3and JTbonding respectively in the various compounds. The (I and II covalency in the various complexes wiU depend on the separation of the ep and ttg ocbitals. it is interesting to note that some correlation exists between the 10 &I parameters for these complexes and the relative spin delocalisation via the Q and r mechan&rns (table 2). A detailed paper on the other aspectsof this work wiiI be published elsetiere.
87
&CAL
voluuie 10. nu!Ilber 1 .
Refgrocea
.,r-
1 July 1971
_1
,
.- il] G.A. Webb, in: Agut
PliYSICS LErTERs
lcports oa~iMR spectroscopy,
VoL 3, ‘iid.E.F. Mmmey (Academic press, New York,
1970) p. 211. [2] RJL Crama and R.S. Draga. J-Am. C&n. Sot. 92 (1970)-66. [3]:MM. Dbingca,G.GaviSandC.R. Kanekar,Proc. 13th
[41 J.A: Pople, D-L. &vaidge and PA Dobosh, J. c&m. I’hys. 47 (1967) 2026. [5] CL. Talgott and R.J. MyerqbfoL Phys. 12 (1967) 549: [6] J+. Pople and D.L. Beveri~,App&imate molecular
.orbital theory @hGmw-Hill, New York, 1970). [7] JJL Happe and R.L. Ward, J. Chea Phys 39 (1963) 1211, (81 G. bfaki, I.ChemJbys
29 (1958) 162.
l.C.CC.. Cmcow, Pohnd (1970) I, 96.
ERRATUM
C. Lancelot and C. HWte, Spin-orbit coupling in sulphurcontaining purine derivatives. A comparis-on of caffeine and &thiocaffeine, Chem. Phys. Letters 9 (1971) 327. Since the publication of the above manuscript, we have been able to observe a shoulder on the long-wavelength side of the first nn* transition of bthio-
caffeine ($3~ = 100). Excitation in the wavelength rmp 380410 nm yields the same phosphorescence spectrum as excitation in any of the rrrr* transitions. ‘Thisweak absorption could thus be ascribed to an nn* transition of 6-thiocaffeine (and not to an impurity). If the triplet state is also an nn* state, then
strong coupling with lmr* states and in-plane polarization of phosphorescence are expected. However, the vibronic structure of the phosphorescence spectrum is quite simiiar to that of the fmt transition in absorp tion. This result and the absence of any solvent effect suggest that the lowest triplet state could be a 3~rr* state perturbed through spin-orbit coupling by lr~+ states as discussed in the above manuscript. Under excitation in either the first Im* transition or the nn* transition the same phosphorescence polarization ratio is obtained. This would indicate that, outside the O-O transition, the rur* transition borrows most of its intensity from the in-plane nn* transition through vibronic coupling.
1
cxFM.IcAL PHY!ms
1 July 1971:
LEXTERS
CONTACI’ SHIFTS AND MECHANIS~I OF SPIN DELOCALISATION IN HEXA COORDINATED NKKEL COMPLEXES MM DHINGRA, G. GOVIL 3nd CR. ICANEKAR Tara IMi!ute of Fundaktental Resemch. Homi Bhabha Road, C~hzba,Bombay-S, India Received
11 May 1971
The contactshifts observed for protons of -y-picz&z in complexes of the type NiXi (r-picoline)s (where X = diethyldiUCophos@ate, ethybranthate and acetylacetonete) hwe been studied to understand the spin transfer mecbsnism.The revolts indicate that the spin transfer involvesnot only the highest bonding @and orbital of Qsymmetry but also to some extent, the lowest antibonding and the highest bonding orbitals of n-symmetry.
The large shifts observed for paramagnetic transiwe have relied on the SCF MO calculations in the . _ tion metal complexes have been attributed to the INDO framework as the method has proved very sucFermi contact and the dipole-dipole interactions becessf31 in predicting the spin densities and hypertie tween the nuclear and electronic spins [ 1] . For octacoupling constants (a) in a variety of organic syshedral complexes of nickel@) the ground state is an tems [4] (the program used is QCPE No. 142). In orbital singlet and the clipolsr contribution is absent. this note we report some interesting observations on The observed Isotropic shifts in such nickel comcomplexes where Y = -y-picoline. plexes therefore originate from the Fermi contact Table 1 gives the calculated hype&e interaction mechanism. In hexa coordinated Ni(II) complexes, / constants for the protons of ypicoline assuming a the unpaired electrons are i? the eg orbitals which kgle w+n in the orbit& u, Ir* and gb. It is-imporhave o-symmetry with respect to the ligand. It is tant to point out that the INDO calculation for the therefore expected that with ligands like pyridine and methyl substituted pyrldines the ligand to metal spin transfer should occur via the highest bonding tilled orbiTable 1 tal of the ligand. However, Cramer and Drago [2] showHvperfine coupling constants (a) for a singleCW@Iin daered that the experimental results for such systems indicate eat ortitals of r-picoline that to a small extent there is also a transfer of @pin INDO Observed [S] Eli-r I21 to the ligand Ir-orbitals (i.e., the lowest antibonding Ir* and/or the highest bonding orbital of a-symmetry, (a) ombital nb). Unfortunately, their analysis tias based on the +19.55 +?.23 OfthO extended Wckel theory (EHT) - a method which is +2.85 + 5.71 Meta +0.11 - 1.28 ParaCHs highly unreliable in the calcdation of spin de&ties. In recent years, we have been interested in the stucb) n*-orbital OrthO +2.92 - 3.09 (*).‘3.80 dy of contact shifts in complexes of the type Ni(W2Y2
where X = acetylacetonate (am), diethyldithiophos(exan), etc., and Y = the secondary and the tertiaryamines such at pyridine, piperidine, morpholine, etc. t. To’ explain these shifts f A short account of some experimental aspects of this phate (dtp), ethylxanthate
wosk hw @en reported previously [ 31.
86
Meta
- 0.26
+I.42
p-3
e13.49
-4.31
OX&O
+31.39
Meta
+X23
+1.75 +233
PafaX!Hs
+40.59
-6.80
(c) 7rb-orbitaJ
(*I! 0x0 (*)11.38
Volume10, numixr1
cIIF.bIIcAL PHYSICSLErrERs
Ir*
I July 1971
bitals can be calculated from the set of equations of the type:
(lowest mtkoildhg) orbital is in good agreement with the observed values, for vpicoline anion [S] . No such agreement is found for the values calculated by EXT. We have not been able to find any experimental data for the cation and tripositive ion of r_ picoline, corresponding to the o and VI’ orbitals respectively. However, in view of the wide-spread success in spin density calculations achieved by the INDO method 161, one can rely on the theoretical predictions for these orbitals. It has become customary in this kind of work to report contact shifts in terms of their ratios at the various proton sites. The ratios do not depend on the degree of dissociation for weak complexes. If the unpaired electron is only in the o-orbital of the &and then the ratios AOiA, and A,/+ (o = orrho, m = meta and p = para-CH3) are expected to be about 3.42 and -4.46 respectively, from INDO caldations. The experimental data on 7.picoline, for a series of complexes are given in table 2. In the com-
q&3.=-qJn*)v+cr,(+% % = % um(u)x-am(7r*)v*mf.srb)z
’
where x,y and z (= I -x-y) denote the fraction of spin densities transferred to the o, IF* and nb orbitals respectively. The significance of these numbers may become clear if one considers the possiile mechanisms by which the electronic spin centered on the metal ion can delocalise to the figand orbit& As already stated the direct spin transfer from the metal eg orbitals due to a partially comlcnt metal ligand bond will involve the o-orbitals, of the ligand Ieading to a a-spin in these orbitals. However, unpaired electrons in the eg orbitals can interact with the filled metal tzg orbitals (having n-symmetry) via spin exchange causing a residual spin in such orbit&. The mechankm of spin transfer to the ITSand x* orbit& of the
Table 2 Ratios of the contact shifts, values of spin dendty
fractions and &and field parametersin Ni(X)a (y-pic)z complexes
Ratio of contact shifts
Spin density fractions
%+m
“rnf%
Adap
x
Y
z
NWac)h-pi&
3.60 a)
-3.09 a)
Wr+)$(rpich Wdtpk h-+)2 Ni(exanh (rpic)2
3.05 c) 2.56 a) 1.85 d)
-3.56 C) -3.06 d) -3.41 di
-11.11 -10.83 - 7.74 - 6.31
1.00 0.805 0.557 0.200
0.00 0.157 0.352 0.631
0.00 0.037 0.090 0.171
a) C) a) a)
LODq (cm-9
y;: :; 8980 d) 9240 d)
a) Ref. [7];b) ref. (81 (for p@iine); c) ref. [Z] ;d) present work
plexes Ni(aca& (y-pit), the observed results are consistent with a spin transfer to only the o-orbitals of r_picoline. However, for other complexes, some contribution appears to arise from q* and nb orbitals. Attempts to fit the observed contact shift ratios and the calculated hyperfine interaction constants revealed that agreement can be reached only if it is assumed that a certain fraction of a-spin is present in the filled 0 and nb orbitals while &spin is present in R* orbital. Other [email protected], suck as u + n* and (I + nb for spin transfer did not give satisfactory agreement. The fraction of spin density transferred to the various or-
ligand is indicative of the certain amount of xcharacter in the metal ligand bond. The coefficients x md
y and z are a measure of relative (3and JTbonding respectively in the various compounds. The (I and II covalency in the various complexes wiU depend on the separation of the ep and ttg ocbitals. it is interesting to note that some correlation exists between the 10 &I parameters for these complexes and the relative spin delocalisation via the Q and r mechan&rns (table 2). A detailed paper on the other aspectsof this work wiiI be published elsetiere.
87
&CAL
voluuie 10. nu!Ilber 1 .
Refgrocea
.,r-
1 July 1971
_1
,
.- il] G.A. Webb, in: Agut
PliYSICS LErTERs
lcports oa~iMR spectroscopy,
VoL 3, ‘iid.E.F. Mmmey (Academic press, New York,
1970) p. 211. [2] RJL Crama and R.S. Draga. J-Am. C&n. Sot. 92 (1970)-66. [3]:MM. Dbingca,G.GaviSandC.R. Kanekar,Proc. 13th
[41 J.A: Pople, D-L. &vaidge and PA Dobosh, J. c&m. I’hys. 47 (1967) 2026. [5] CL. Talgott and R.J. MyerqbfoL Phys. 12 (1967) 549: [6] J+. Pople and D.L. Beveri~,App&imate molecular
.orbital theory @hGmw-Hill, New York, 1970). [7] JJL Happe and R.L. Ward, J. Chea Phys 39 (1963) 1211, (81 G. bfaki, I.ChemJbys
29 (1958) 162.
l.C.CC.. Cmcow, Pohnd (1970) I, 96.
ERRATUM
C. Lancelot and C. HWte, Spin-orbit coupling in sulphurcontaining purine derivatives. A comparis-on of caffeine and &thiocaffeine, Chem. Phys. Letters 9 (1971) 327. Since the publication of the above manuscript, we have been able to observe a shoulder on the long-wavelength side of the first nn* transition of bthio-
caffeine ($3~ = 100). Excitation in the wavelength rmp 380410 nm yields the same phosphorescence spectrum as excitation in any of the rrrr* transitions. ‘Thisweak absorption could thus be ascribed to an nn* transition of 6-thiocaffeine (and not to an impurity). If the triplet state is also an nn* state, then
strong coupling with lmr* states and in-plane polarization of phosphorescence are expected. However, the vibronic structure of the phosphorescence spectrum is quite simiiar to that of the fmt transition in absorp tion. This result and the absence of any solvent effect suggest that the lowest triplet state could be a 3~rr* state perturbed through spin-orbit coupling by lr~+ states as discussed in the above manuscript. Under excitation in either the first Im* transition or the nn* transition the same phosphorescence polarization ratio is obtained. This would indicate that, outside the O-O transition, the rur* transition borrows most of its intensity from the in-plane nn* transition through vibronic coupling.