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Electron spin resonance of some paramagnetic compounds of iridium(II)
Spectrochimico Acta, Vol. 38A. No. II, pp. 1177-l 179. 1982 Printed in Great Britain.
058&fIS39/82/1II 17743503.0010 @ 1982 Pergamon Press Ltd.
Electron spin resonance of some paramagnetic compounds of iridium(II) M. ANGOLETTA,T.BERINGHELLI and F.MORAZZONI* Istituto di Chimica Generale e Inorganica dell’Universiti di Milano, Via Venezian 21, 20133 Milano, Italy (Receioed
10 April 1982)
Abstract-This paper deals with some electronic problems suggested by Malatesta. Electron spin resonance with [IrBr,(NO)(PPh&] shows that the electronic configuration of the paramagnetic compound is described by that of the k-NO moiety. Specifically the three values of g tensor components demonstrate that the unpaired electron is located in a largely comprised 7r* (NO) orbital, so that the electronic state of NO is similar to that of gaseous trapped NO. The overlap between Ir and NO orbitals is through the d, (d,, or dyz) metal orbitals, with consequent bent coordination of NO ligand. In conclusion the stability of d’ configuration of Ir in this monomeric compound is due to the easy electron transfer between metal and ligand, in agreement with that described in other monomeric Ir(II) compounds.
INTRODUCTION Stable monomeric iridium(H) compounds are rare examples in the coordination chemistry of this metal [ l-41, so that it seemed important to examine the nature of the chemical bond which induces Ir(I1) paramagnetic compounds. A survey of the related literature shows that the d7 electronic configuration of this ion is generally found in dinuclear systems and that dimerization is metal-metal either by direct obtained interaction[5] or by metal-ligand-ligand-metal interaction[6]. If one considers that the metalmetal interaction requires location of the unpaired electron on Ir(II) centre, the metal-ligand-ligandmetal interaction requires the location on the ligand, one can suggest that a rigid localization of the unpaired electron either on the metal or on the ligand prevents stable Ir(II) paramagnetic monomeric compounds being obtained. This hypothesis explains well the stability of a series of Ir(II) monomeric compounds[3, 41, whose ESR investigation revealed that a free exchange of the unpaired electron occurs between metal and ligands, through the T overlap of the respective symmetry adapted orbit&. The aim of this paper is to extend the ESR investigation to [IrBr,(NO) (PPh& which was the first isolated paramagnetic Ir(II) compound [ I], in order to explain the stability of its electronic configuration. RIISULTSANDDISCUSSION ESR investigation was performed on a solid undiluted sample. [IrBr,(NO) polycrystalline (PPh&j is insoluble in all non-interacting solvents, but dissolves in hot dimethylformamide or dimethylsulphoxide with loss of nitrosyl ligand. The ESR spectrum at R.T. has broad unresolved resonance lines; only at 4.2K the rhombic aniso*To whom correspondence should be addressed.
tropy of the g magnetic tensor becomes evident and two components can be easily read, g1 = 2.0218, g, = 1.8128, while a third one is tentatively located at g, = 1.9515 (Fig. 1). One can hypothesize three possible electronic configurations of [IrBr,(NO)(PPh,)J, whose main differences lies in the electronic distribution within the Ir-NO moiety. Indeed the interaction between metal and nitrosyl can be formulated as (i) Ir(II)d’-NO’; (ii) Ir(IV)d5-NO-; (iii) Ir(III)d6-NO. The first hypothesis localizes the unpaired electron on the metal ion, with a d’ electronic configuration; the second one on the metal ion with a ds configuration; the third one localizes the unpaired electron on the NO molecule. Decision about the most probable electronic state has been drawn by the following arguments. If Ir(II)-NO’ formulation was the real one, the values of the g tensor components should be reproduced by the expressions available for a d' rhombic configuration in a crystal field symmetry[7]. In this sense, all possible assignments of g tensor components, for the two probable ground states configuration (~z)~(yz)~(x* yp2(zz) and (~z)‘(yz)~(x~ - yy(xy) were attempted. But the calculated energy dif[erences between the ground configuration and the excited ones are in both cases too high to be attributed to electronic d-d transitions. By a more qualitative argument, though an exhaustive one, the location of the unpaired electron in the dz2 or d,, orbital of iridium@) can be excluded by the g, value, much lower than 2. If the Ir(IV)-NO- formulation was the real one, one could expect lower frequency for the Y (N-O) frequency vibration than it was found 1177
1178
M. AWXEITA
29ca
et al.
3900
G Pig. 1. ESR spectrum of IrBr3(NO)(PPh&, recorded at 4.2 K in polycrystalline sample.
(1730 cm-‘) [ 11, as a consequence of the increased antibondiig population on NO moiety. Finally Ir(III)-NO formulation has been considered. By this hypothesis the unpaired electron is located in a 7~ orbital largely centred on NO ligand; and the g tensor components should be reproduced by the expressions available for NO molecule, where the interaction with Ir centre is described by the energy difIerence between the rr*(NO) orbitals. In this light a simple perturbation calculation, performed on the NO ground state conftguratton, ((T~$(?T~~~)z(~2pY)2(~llr, or &,). gives the following expressions g,, = 2++
l/2&
gxx=2-A2A grv = 2+E-
A = I&., - &I,
1/2;
l/2&
1/2$
h2 l/22+
2 112%
E
=
1~9pr
or 2py
(1)
-
apt
A = spin orbit coupling constant; L is assumed to be the internuclear N-O axis; with the reasonable hypothesis that the crystal field perturbation is much higher than the spin-orbit coupling perturbation. A more rigorous treatment has been given by MERGERWN and MARSHALL[~], but the essential features of our spectrum may be reproduced by the simplified expressions of g tensor components above given. Really the trend expected for the g tensor components is g,, < g, < g,,, with g, and gzz < 2, and it is in agreement with an assignment of the experimental g values of [IrBr,(NO)(PPh,)2] as
g1 to g,,,
gz to 8xX, g3 to g*z.
The trend of g values observed for [IrBr,(NO) (PPhp)J is diagnostic of an electronic state similar to that observed in trapped NO centres[9a] or in the surface adducts between NO and inorganic supports[9bl. However, the g expressions given above [equation (l)] usually employed in the interpretation of ESR spectra of these systems, cannot explain the large displacement from 2 observed for g,, and g, of [IrBrJ(NO)(PPh~)2]. It is our opinion that the overlap between the r*(NO) orbitals and the d,(B) orbitals plays an important role in the electronic structure of Ir-NO moiety and that the crystal field alone, employed in calculating the g expressions (l), is not able to well explain the experimental g values. Specifically the d,-W* overlap can increase the energy of the half occupied &,, 0r z,,(NO) orbital, while it does not affect the energy of empty pY0I 2,,(NO) orbital; this leads to a decrease of mTB the A value, impossible to be described by the crystal field model. Hence only the trend of g values can be obtained from the expressions (1). The formulation which describes the Ir-NO moiety in terms of Ir(III) d6-NO and locates the unpaired electron mainly on the r*(NO) orbital seems to be the real one. As a consequence the low percent of the unpaired electron located on the iridium centre lies in the d,, or d,, orbital, and the d,+r* overlap is the electron path of exchange between metal and NO ligand. The overlap between the d,(B) orbitals and the rr*(NO) orbitals is confirmed by the low extent of the energy difference between them observed from the electron ditIuse reflectance spectrum; really the absorption at 11300 cm-’ (Fig. 2) can be assigned to a d, + r*(NO) transition, in agreement with the position of 1r*(N0) orbitals proposed by MANOHARANand GRAY[ lo] for nitrosyl derivatives. A bent coordination of nitrosyl group is the other consequence of the electronic configuration
Electron spin resonance of some paramagnetic compounds of iridium(H)
700
I
I
900
llco
I 1300
I500
1179
I 2000
2500
nm
Fig. 2. Electronic diffuse reflectance spectrum of IrBr,(NO)(PPh&.
proposed for the Ir-NO moiety. This conclusion can be drawn also from the extent of rhombic anisotropy of ESR spectrum, which seems too high to be due only to the effect of rhombic ligand field around the iridium centre. CONCLUDING REMARKS
Three main features of the reported magnetic analysis can be emphasized: 1. The electronic properties of coordinated NO resemble those of nitric oxide, while in the most part of nitrosyl derivatives the electronic state of NO is strongly perturbed by the coordinative interaction. An Ir-NO bond strength anomalously low is expected from the described electronic configuration. 2. Paramagnetic nitrosyl complexes where the unpaired electron lies on NO moiety are rare examples in the coordination chemistry. Really the only known described case[9a] is [Fe(CN)5N0]3-, obtained by y-irradiation of [Fe(CN)sN0]2at 77 K. The examples of coordinated NO, where the unpaired electron rests on this molecule are confined to the surface adducts between NO and inorganic supports (MgO, ZnO, ZnS, NaY zeolite) [9b]. 3. The unpaired electron [IrBrs(NO)(PPh&j is freely exchanged between Ir and NO by overlap of the symmetry adapted respective orbitals. On these bases the stability of the paramagnetic Ir nitrosyl compound is mainly due to the possibility that the unpaired electron is distributed between metal and ligands, as was suggested in the Ir(I1) paramagnetic compounds previously considered [3,41.
ExpERlMENTAL
[IrBrr(NO)(PPh&] was prepared as described in literature[l]. ESR measurements were performed on a Varian E 109 spectrometer. eauipoed _ __ with a Varian temperature control in the range 298-77 K, and with an Oxford Instruments ESR 900-A continous tlow couulina system, operating in the range 4.2-3OOK, continuously adjustable. Electronic ditTuse reflectance spectra were recorded on a Beckman DK-2A spectrophotometer.
Acknowledgements-Authors
thank the Italian C.N.R. for supporting this research. This paper is dedicated to LAMBERTOMALATESTA on the occasion of his 70th birthday.
[l] L. MALATESTA, M. ANGOLEITA and G. CAGLIO,Ang. Chem. 2,739 (1%3). M. ANGOLETTA and G. CAGUO, Garretta Chim. It. 93, 1584 (1%3). [21 R. MASON, K. M. THOMAS,H. D. EMPSALL,S. R. FLETCHER,P. N. HEYS. E. M. HYDE. C. E. JONES and B. L. SHAW,J. Chem. Sot. Chem. Comm. 612 (1974). [3] A. ARANEO,F. MORAZZONI and T. NAPOLETANO, J. Chem. Sot. Dalton 2039 (1975). and F. MORAZZONI.J. Chem. Sot. Dal[41 G. MERCATI ton 569 (1979). PI L. MALATES~Aand F. CANZIANI,J. Inorg. Nucl. Chem. 19, 81 (1961). bl H: J. KELLERand H. WAWERSIK,J. Orgnnometal. Chem. 8, 185 (1%7). [71 B. R. MCGARVEY,Can. J. Chem. 53 2498 (1975). and S. A. MARSHALL, Phys: Rev: 127, 181 D. MERGERIAN 2015 (1%2). 191 (a) M. B. ‘D. BLOOM,J. B. RAYNORand M. C. R. SYMONS,J. Chem. Sot. A 3843 (1972). (b) J. H. LUNSFORD,J. Phys. Chem. 72,214l (1%8); ibid. 72. 4163 (1%X). and H. B. GRAY,Inorg. Chem. 5, I101 P. T. MANOHAIZAN 823 (1%6).
058&fIS39/82/1II 17743503.0010 @ 1982 Pergamon Press Ltd.
Electron spin resonance of some paramagnetic compounds of iridium(II) M. ANGOLETTA,T.BERINGHELLI and F.MORAZZONI* Istituto di Chimica Generale e Inorganica dell’Universiti di Milano, Via Venezian 21, 20133 Milano, Italy (Receioed
10 April 1982)
Abstract-This paper deals with some electronic problems suggested by Malatesta. Electron spin resonance with [IrBr,(NO)(PPh&] shows that the electronic configuration of the paramagnetic compound is described by that of the k-NO moiety. Specifically the three values of g tensor components demonstrate that the unpaired electron is located in a largely comprised 7r* (NO) orbital, so that the electronic state of NO is similar to that of gaseous trapped NO. The overlap between Ir and NO orbitals is through the d, (d,, or dyz) metal orbitals, with consequent bent coordination of NO ligand. In conclusion the stability of d’ configuration of Ir in this monomeric compound is due to the easy electron transfer between metal and ligand, in agreement with that described in other monomeric Ir(II) compounds.
INTRODUCTION Stable monomeric iridium(H) compounds are rare examples in the coordination chemistry of this metal [ l-41, so that it seemed important to examine the nature of the chemical bond which induces Ir(I1) paramagnetic compounds. A survey of the related literature shows that the d7 electronic configuration of this ion is generally found in dinuclear systems and that dimerization is metal-metal either by direct obtained interaction[5] or by metal-ligand-ligand-metal interaction[6]. If one considers that the metalmetal interaction requires location of the unpaired electron on Ir(II) centre, the metal-ligand-ligandmetal interaction requires the location on the ligand, one can suggest that a rigid localization of the unpaired electron either on the metal or on the ligand prevents stable Ir(II) paramagnetic monomeric compounds being obtained. This hypothesis explains well the stability of a series of Ir(II) monomeric compounds[3, 41, whose ESR investigation revealed that a free exchange of the unpaired electron occurs between metal and ligands, through the T overlap of the respective symmetry adapted orbit&. The aim of this paper is to extend the ESR investigation to [IrBr,(NO) (PPh& which was the first isolated paramagnetic Ir(II) compound [ I], in order to explain the stability of its electronic configuration. RIISULTSANDDISCUSSION ESR investigation was performed on a solid undiluted sample. [IrBr,(NO) polycrystalline (PPh&j is insoluble in all non-interacting solvents, but dissolves in hot dimethylformamide or dimethylsulphoxide with loss of nitrosyl ligand. The ESR spectrum at R.T. has broad unresolved resonance lines; only at 4.2K the rhombic aniso*To whom correspondence should be addressed.
tropy of the g magnetic tensor becomes evident and two components can be easily read, g1 = 2.0218, g, = 1.8128, while a third one is tentatively located at g, = 1.9515 (Fig. 1). One can hypothesize three possible electronic configurations of [IrBr,(NO)(PPh,)J, whose main differences lies in the electronic distribution within the Ir-NO moiety. Indeed the interaction between metal and nitrosyl can be formulated as (i) Ir(II)d’-NO’; (ii) Ir(IV)d5-NO-; (iii) Ir(III)d6-NO. The first hypothesis localizes the unpaired electron on the metal ion, with a d’ electronic configuration; the second one on the metal ion with a ds configuration; the third one localizes the unpaired electron on the NO molecule. Decision about the most probable electronic state has been drawn by the following arguments. If Ir(II)-NO’ formulation was the real one, the values of the g tensor components should be reproduced by the expressions available for a d' rhombic configuration in a crystal field symmetry[7]. In this sense, all possible assignments of g tensor components, for the two probable ground states configuration (~z)~(yz)~(x* yp2(zz) and (~z)‘(yz)~(x~ - yy(xy) were attempted. But the calculated energy dif[erences between the ground configuration and the excited ones are in both cases too high to be attributed to electronic d-d transitions. By a more qualitative argument, though an exhaustive one, the location of the unpaired electron in the dz2 or d,, orbital of iridium@) can be excluded by the g, value, much lower than 2. If the Ir(IV)-NO- formulation was the real one, one could expect lower frequency for the Y (N-O) frequency vibration than it was found 1177
1178
M. AWXEITA
29ca
et al.
3900
G Pig. 1. ESR spectrum of IrBr3(NO)(PPh&, recorded at 4.2 K in polycrystalline sample.
(1730 cm-‘) [ 11, as a consequence of the increased antibondiig population on NO moiety. Finally Ir(III)-NO formulation has been considered. By this hypothesis the unpaired electron is located in a 7~ orbital largely centred on NO ligand; and the g tensor components should be reproduced by the expressions available for NO molecule, where the interaction with Ir centre is described by the energy difIerence between the rr*(NO) orbitals. In this light a simple perturbation calculation, performed on the NO ground state conftguratton, ((T~$(?T~~~)z(~2pY)2(~llr, or &,). gives the following expressions g,, = 2++
l/2&
gxx=2-A2A grv = 2+E-
A = I&., - &I,
1/2;
l/2&
1/2$
h2 l/22+
2 112%
E
=
1~9pr
or 2py
(1)
-
apt
A = spin orbit coupling constant; L is assumed to be the internuclear N-O axis; with the reasonable hypothesis that the crystal field perturbation is much higher than the spin-orbit coupling perturbation. A more rigorous treatment has been given by MERGERWN and MARSHALL[~], but the essential features of our spectrum may be reproduced by the simplified expressions of g tensor components above given. Really the trend expected for the g tensor components is g,, < g, < g,,, with g, and gzz < 2, and it is in agreement with an assignment of the experimental g values of [IrBr,(NO)(PPh,)2] as
g1 to g,,,
gz to 8xX, g3 to g*z.
The trend of g values observed for [IrBr,(NO) (PPhp)J is diagnostic of an electronic state similar to that observed in trapped NO centres[9a] or in the surface adducts between NO and inorganic supports[9bl. However, the g expressions given above [equation (l)] usually employed in the interpretation of ESR spectra of these systems, cannot explain the large displacement from 2 observed for g,, and g, of [IrBrJ(NO)(PPh~)2]. It is our opinion that the overlap between the r*(NO) orbitals and the d,(B) orbitals plays an important role in the electronic structure of Ir-NO moiety and that the crystal field alone, employed in calculating the g expressions (l), is not able to well explain the experimental g values. Specifically the d,-W* overlap can increase the energy of the half occupied &,, 0r z,,(NO) orbital, while it does not affect the energy of empty pY0I 2,,(NO) orbital; this leads to a decrease of mTB the A value, impossible to be described by the crystal field model. Hence only the trend of g values can be obtained from the expressions (1). The formulation which describes the Ir-NO moiety in terms of Ir(III) d6-NO and locates the unpaired electron mainly on the r*(NO) orbital seems to be the real one. As a consequence the low percent of the unpaired electron located on the iridium centre lies in the d,, or d,, orbital, and the d,+r* overlap is the electron path of exchange between metal and NO ligand. The overlap between the d,(B) orbitals and the rr*(NO) orbitals is confirmed by the low extent of the energy difference between them observed from the electron ditIuse reflectance spectrum; really the absorption at 11300 cm-’ (Fig. 2) can be assigned to a d, + r*(NO) transition, in agreement with the position of 1r*(N0) orbitals proposed by MANOHARANand GRAY[ lo] for nitrosyl derivatives. A bent coordination of nitrosyl group is the other consequence of the electronic configuration
Electron spin resonance of some paramagnetic compounds of iridium(H)
700
I
I
900
llco
I 1300
I500
1179
I 2000
2500
nm
Fig. 2. Electronic diffuse reflectance spectrum of IrBr,(NO)(PPh&.
proposed for the Ir-NO moiety. This conclusion can be drawn also from the extent of rhombic anisotropy of ESR spectrum, which seems too high to be due only to the effect of rhombic ligand field around the iridium centre. CONCLUDING REMARKS
Three main features of the reported magnetic analysis can be emphasized: 1. The electronic properties of coordinated NO resemble those of nitric oxide, while in the most part of nitrosyl derivatives the electronic state of NO is strongly perturbed by the coordinative interaction. An Ir-NO bond strength anomalously low is expected from the described electronic configuration. 2. Paramagnetic nitrosyl complexes where the unpaired electron lies on NO moiety are rare examples in the coordination chemistry. Really the only known described case[9a] is [Fe(CN)5N0]3-, obtained by y-irradiation of [Fe(CN)sN0]2at 77 K. The examples of coordinated NO, where the unpaired electron rests on this molecule are confined to the surface adducts between NO and inorganic supports (MgO, ZnO, ZnS, NaY zeolite) [9b]. 3. The unpaired electron [IrBrs(NO)(PPh&j is freely exchanged between Ir and NO by overlap of the symmetry adapted respective orbitals. On these bases the stability of the paramagnetic Ir nitrosyl compound is mainly due to the possibility that the unpaired electron is distributed between metal and ligands, as was suggested in the Ir(I1) paramagnetic compounds previously considered [3,41.
ExpERlMENTAL
[IrBrr(NO)(PPh&] was prepared as described in literature[l]. ESR measurements were performed on a Varian E 109 spectrometer. eauipoed _ __ with a Varian temperature control in the range 298-77 K, and with an Oxford Instruments ESR 900-A continous tlow couulina system, operating in the range 4.2-3OOK, continuously adjustable. Electronic ditTuse reflectance spectra were recorded on a Beckman DK-2A spectrophotometer.
Acknowledgements-Authors
thank the Italian C.N.R. for supporting this research. This paper is dedicated to LAMBERTOMALATESTA on the occasion of his 70th birthday.
[l] L. MALATESTA, M. ANGOLEITA and G. CAGLIO,Ang. Chem. 2,739 (1%3). M. ANGOLETTA and G. CAGUO, Garretta Chim. It. 93, 1584 (1%3). [21 R. MASON, K. M. THOMAS,H. D. EMPSALL,S. R. FLETCHER,P. N. HEYS. E. M. HYDE. C. E. JONES and B. L. SHAW,J. Chem. Sot. Chem. Comm. 612 (1974). [3] A. ARANEO,F. MORAZZONI and T. NAPOLETANO, J. Chem. Sot. Dalton 2039 (1975). and F. MORAZZONI.J. Chem. Sot. Dal[41 G. MERCATI ton 569 (1979). PI L. MALATES~Aand F. CANZIANI,J. Inorg. Nucl. Chem. 19, 81 (1961). bl H: J. KELLERand H. WAWERSIK,J. Orgnnometal. Chem. 8, 185 (1%7). [71 B. R. MCGARVEY,Can. J. Chem. 53 2498 (1975). and S. A. MARSHALL, Phys: Rev: 127, 181 D. MERGERIAN 2015 (1%2). 191 (a) M. B. ‘D. BLOOM,J. B. RAYNORand M. C. R. SYMONS,J. Chem. Sot. A 3843 (1972). (b) J. H. LUNSFORD,J. Phys. Chem. 72,214l (1%8); ibid. 72. 4163 (1%X). and H. B. GRAY,Inorg. Chem. 5, I101 P. T. MANOHAIZAN 823 (1%6).