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Electronic structure of metallocenes—II
J. inorg,nucl.Chem., 1969,VoL31, pp. 3103to 3104. PergamonPress. Printedin Great Britain
ELECTRONIC
STRUCTURE
OF
METALLOCENES-II*
MOSSBAUER M E A S U R E M E N T S A N D M O L E C U L A R O R B I T A L D E S C R I P T I O N S OF F E R R O C E N E J. T. D E H N and L. N. M U L A Y t Materials Research Laboratory, Pennsylvania State University, University Park, Penna. 16802
(First received 13 July 1967; in revised form 30 May 1968) A b s t r a c t - M f s s b a u e r studies on single crystals of ferrocene confirm results obtained by magnetic perturbation techniques. These show that the molecular orbital theory in general give correct sign for the electric field gradient and is superior to a crystal field description.
COLLINS[I] has reported the use of a magnetic perturbation technique to determine the sign of the axially symmetric electric field gradient tensor for ferrocene. He concluded that molecular orbital (MO) theory gives the correct sign for the field gradient and so is superior to a crystal field description which gives the wrong sign. His conclusion was questioned by Simkin [2] who made Mrssbauer measurements on single crystals of ferrocene supplied by one of us (L. N. Mulay). By observing the variation in the intensity ratio of the lines in the quadrupole-split doublet with crystal orientation, Simkin determined that the _+3/2 level lies below the _+ 1/2 level. This seemed to contradict the energy level scheme given in Fig. 4 of Collins' paper, so Simkin concluded that the molecular orbital description of ferrocene predicted the wrong sign for field gradient. It is the purpose of the present paper to point out that Collins' conclusion is correct while Simkin's is wrong. From X-ray studies [3-4], it is known that each unit cell of ferrocene contains two molecules. The molecular axes (which pass through the iron atoms and are perpendicular to the cyclopentadiene rings) lie in the crystal a-b plane, are at right angles to each other and make 45 ° angles with the a and b axes (See Fig. 1). The ratio of the absorption intensities for an unpolarized source is I±~/2/I~-1/2= 3(1 + cos20)/(5 - 3 cos20), where 0 now is the angle between the y-ray beam and a molecular axis. A beam perpendicular to the a-b plane (001) with 0 = 90° gives a ratio of 3 : 5. This experiment was carried out and the _+3/2 line was found to lie at larger velocity so that the quadrupole splitting parameter is positive. Simkin was apparently misled by the external morphology of the ferrocene crystal he used (supplied by L. N. Mulay). This is an easy mistake to make and *Some phases of this work have been supported by grants from the Advanced Research Project Agency of the U.S. Government and from the Pennsylvania State University (SI R-Project 00667). TInquiries should be addressed to this author. 1. 2. 3. 4.
R. L. Collins,J. chem. Phys. 42, 1072 (1965). D.J. Simkin, Ph.D. Thesis, The Pennsylvania State University, 1966. W. Pfab and E. O. Fischer, Z. anorg, allg. Chem. 274, 316 (1953). J. D. Dunitz, L. E. Orgel and A. Rich,Acta Crystallogr. 9, 373 (1956). 3103
3104
J.T. D E H N and L. N. M U L A Y
4J 4; b--7.59A
? a:1o.511 Fe(CsHs)2 Fig. 1. Unit cell of ferrocene in a-b plane. (After W. Pfab and E. O. Fischer [3].)
was at first repeated by us. The (001) and (110) pinacoids can easily be confused and are clearly distinguished only by X-ray examination. For y-rays incident perpendicular to the (001) face, the ratio of intensities is 0.6, as mentioned above, while if they are incident perpendicular to the (110) face, the ratio is 1.71 = (0.585) -1 ~ (0.6) -t so that the ratios in the Mrssbauer experiments are almost exactly inverted and X-ray orientation is required to distinguish these experiments. I t would be desirable to compare MO descriptions of ruthenocene and o s m o c e n e (See Part I) with Mrssbauer measurements on these compounds. However, the ruthenocene spectrum [5] is not yet sufficiently resolved for this purpose, while osmocene has not been done. This work is now in progress. Acknowledgements-The authors wish to thank Dr. Spijkerman of the National Bureau of Standards for allowing them to use his equipment for some of the experiments described in this paper. They also express their gratitude to Dr. K. Vedam and Mr. J. Caslavsky of the Materials Research Laboratory for their assistance in orienting our samples and to Dr. R. L. Collins of the University of Texas for pointing out the necessity of X-ray orientation. 5. O. C. Kistner, Phys. Rev. 144, 1022 (1966).
ELECTRONIC
STRUCTURE
OF
METALLOCENES-II*
MOSSBAUER M E A S U R E M E N T S A N D M O L E C U L A R O R B I T A L D E S C R I P T I O N S OF F E R R O C E N E J. T. D E H N and L. N. M U L A Y t Materials Research Laboratory, Pennsylvania State University, University Park, Penna. 16802
(First received 13 July 1967; in revised form 30 May 1968) A b s t r a c t - M f s s b a u e r studies on single crystals of ferrocene confirm results obtained by magnetic perturbation techniques. These show that the molecular orbital theory in general give correct sign for the electric field gradient and is superior to a crystal field description.
COLLINS[I] has reported the use of a magnetic perturbation technique to determine the sign of the axially symmetric electric field gradient tensor for ferrocene. He concluded that molecular orbital (MO) theory gives the correct sign for the field gradient and so is superior to a crystal field description which gives the wrong sign. His conclusion was questioned by Simkin [2] who made Mrssbauer measurements on single crystals of ferrocene supplied by one of us (L. N. Mulay). By observing the variation in the intensity ratio of the lines in the quadrupole-split doublet with crystal orientation, Simkin determined that the _+3/2 level lies below the _+ 1/2 level. This seemed to contradict the energy level scheme given in Fig. 4 of Collins' paper, so Simkin concluded that the molecular orbital description of ferrocene predicted the wrong sign for field gradient. It is the purpose of the present paper to point out that Collins' conclusion is correct while Simkin's is wrong. From X-ray studies [3-4], it is known that each unit cell of ferrocene contains two molecules. The molecular axes (which pass through the iron atoms and are perpendicular to the cyclopentadiene rings) lie in the crystal a-b plane, are at right angles to each other and make 45 ° angles with the a and b axes (See Fig. 1). The ratio of the absorption intensities for an unpolarized source is I±~/2/I~-1/2= 3(1 + cos20)/(5 - 3 cos20), where 0 now is the angle between the y-ray beam and a molecular axis. A beam perpendicular to the a-b plane (001) with 0 = 90° gives a ratio of 3 : 5. This experiment was carried out and the _+3/2 line was found to lie at larger velocity so that the quadrupole splitting parameter is positive. Simkin was apparently misled by the external morphology of the ferrocene crystal he used (supplied by L. N. Mulay). This is an easy mistake to make and *Some phases of this work have been supported by grants from the Advanced Research Project Agency of the U.S. Government and from the Pennsylvania State University (SI R-Project 00667). TInquiries should be addressed to this author. 1. 2. 3. 4.
R. L. Collins,J. chem. Phys. 42, 1072 (1965). D.J. Simkin, Ph.D. Thesis, The Pennsylvania State University, 1966. W. Pfab and E. O. Fischer, Z. anorg, allg. Chem. 274, 316 (1953). J. D. Dunitz, L. E. Orgel and A. Rich,Acta Crystallogr. 9, 373 (1956). 3103
3104
J.T. D E H N and L. N. M U L A Y
4J 4; b--7.59A
? a:1o.511 Fe(CsHs)2 Fig. 1. Unit cell of ferrocene in a-b plane. (After W. Pfab and E. O. Fischer [3].)
was at first repeated by us. The (001) and (110) pinacoids can easily be confused and are clearly distinguished only by X-ray examination. For y-rays incident perpendicular to the (001) face, the ratio of intensities is 0.6, as mentioned above, while if they are incident perpendicular to the (110) face, the ratio is 1.71 = (0.585) -1 ~ (0.6) -t so that the ratios in the Mrssbauer experiments are almost exactly inverted and X-ray orientation is required to distinguish these experiments. I t would be desirable to compare MO descriptions of ruthenocene and o s m o c e n e (See Part I) with Mrssbauer measurements on these compounds. However, the ruthenocene spectrum [5] is not yet sufficiently resolved for this purpose, while osmocene has not been done. This work is now in progress. Acknowledgements-The authors wish to thank Dr. Spijkerman of the National Bureau of Standards for allowing them to use his equipment for some of the experiments described in this paper. They also express their gratitude to Dr. K. Vedam and Mr. J. Caslavsky of the Materials Research Laboratory for their assistance in orienting our samples and to Dr. R. L. Collins of the University of Texas for pointing out the necessity of X-ray orientation. 5. O. C. Kistner, Phys. Rev. 144, 1022 (1966).