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Nacht im Eisschrank wurde abfiltriert. Das isolierte Xan-thinsulfat enthielt nach Aussage des UV-Spektrums 81% Xanthin-base, reines Xanthin-sulfat enthält 84,6% Xan-thin-base. Wie das in Streifen zerschnittene und unter dem Zählrohr vermessene Papierchromatogramm zeigte, war keine andere radioaktive Verbindung nachweisbar. Da das eingesetzte Guanin ein Rohprodukt war, beträgt die Ausbeute über 60%.

H y p o x a n t h i n - [ 8 - 1 4C]: Obwohl die Synthese von Hypoxanthin-[8-14C] durch Desaminierung von Adenin-[8-14C] möglich ist, wurde doch versucht, aus 4.5-Diamino-6-oxy-pyrimidin durch Ringschluß mit Ameisensäure-14C das Hypoxanthin zu gewinnen. Zunächst scheiterten die Versuche unter den von W e y g a n d und G r o ß k i n s k y 4 für die Synthese von Guanin-[8-14C] angegebenen Reaktionsbedin-gungen (Ringschluß in Glykol). Die Vornahme des Ringschlusses in Formamid oder Formyl-morpholin verbietet sich, da hierbei isotopische Verdünnung eintritt. Schließlich wurden doch noch Bedingungen

gefunden, die den Ringschluß in Glykol ermöglich-ten. Die HCl-Menge mußte etwas erhöht werden.

70 mg 4.5-Diamino-6-oxy-pyrimidin-hydrochlorid (0,43 mMol) und 27,5 mg (0,40 mMol) Natriumformiat-14C der Aktivität 1,25 mC/mMol wurden in 0,8 ccm gut getrock-netem Glykol und 2 Tropfen gesättigter ätherischer Salz-säure langsam im Reinststickstoff-Strom auf 75° in einem Röhrchen mit Normalschliff unter Rühren mit einem Glas-stäbchen erhitzt. Nach 30 min bei 75° wurde die Tempe-ratur innerhalb 10 min auf 100° erhöht, wobei alles in Lösung ging. Nach weiteren 35 min bei 100° schied sich langsam wieder ein Niederschlag ab. Anschließend wurde 60 min lang auf 185—190° erhitzt. Nach dem Abkühlen wurde das Glykol i. Vak. abdestilliert. Der Rückstand wurde in 0,6 ccm 2-n. Salzsäure aufgenommen und fil-triert. Nach Zugabe von Ammonik und Verkochen des überschüssigen Ammoniaks fiel das Hypoxanthin aus. Nach UV-spektroskopischen Messungen war das Präparat reiner als 95%. Ausb. 35 mg (65%).

Der D e u t s c h e n F o r s c h u n g s g e m e i n s c h a f t und dem V e r b and d e r C h e m i s c h e n I n d u s t r i e — F o n d s d e r C h e m i e — danken wir bestens für Unterstützung.

On the question of octahedral binding in bis-cyclopentadienyl compounds B y F . A . C O T T O N 1 a n d G . W I L K I N S O N 1

(Z. Naturforschg. 9 b, 453—456 [1954]; eingegangen am 8. März 1954)

A comparison is made of the various theories of the electronic structure of bis-cyclopenta-dienyl compounds, and the deficiencies of the octahedral bonding theory are discussed.

Numerous bis-cyclopentadienyl2 derivatives of the transition elements have been made in this

laboratory and also by E. O. F i s c h e r and his coworkers3. W e have also shown the existence of the violet cation [C 1 0H 1 0V(III)]+obtained by the re-duction of an aqueous solution of C1 0H1 0V(IV)C12 4 '5 . W e have further prepared the orange - red C 1 0H 1 0Nb(V)Br 3 , the dark red C1 0H1 0Ta(V)Br3 and the dark green C 1 0H 1 0Ti(II) by the reaction of the anhydrous metal halides with cyclopentadienylsodium in tetrahydrofuran or 1.2-dimethoxyethane solution4. In a notice 6, we have also described bis-cyclopenta-dienyl derivatives of molybdenum and tungsten, and

1 Present Address: Chemistry Laboratory A, Technical University, Copenhagen, Denmark

2 We use the prefix "bis" to distinguish C10H10M compounds having the "sandwich" or "Doppelkegel" structure from compounds of the same formula which have an alkyl type structure.

3 a E. O. F i s c h e r , D. S e u s and R. J i r a , Z. Na-turforschg. 8 b, 692 [1953].

sb G. W i l k i n s o n , J. Amer. chem. Soc. 76, 209 [1954],

sc F. A. C o t t o n , P. L. P a u s o n and G. W i 1 -k i n s o n , ibid. 76, 1970 [1954],

separately7 the brown compound of formula C1 0H1 0Mn which is undoubtedly of alkyl type. A white C1 0H1 0Mn has been described8 by F i s c h e r . In addition to the above compounds which have been shown to have the "sandwich" or "Doppel-kegel" structure either by X-ray or infrared ab-sorption measurements, we have prepared the red-violet, diamagnetic compounds C5H5Mo(CO)5MoC5H5

and C 5 H 5 W(CO) 6 WC 5 H 5 3b. The analogous diama-gnetic green-blue C5H5Cr(CO)5CrC5Hg and the almost black C5H5Fe(CO)5FeC5H5 have also been characterized.

4 G . W i l k i n s o n and J . M . B i r m i n g h a m , J. Amer. chem. Soc., in press.

5 G . W i l k i n s o n , P. L. P a u s o n , J. M. B i r m i n g -ham and F . A . C o t t o n , J. Amer. chem. Soc. 75, 1011 [1953].

6 F. A. C o t t o n and G. W i l k i n s o n , Z. Natur-forschg. 9 b, 417 [1954].

7 G. W i l k i n s o n and F. A. C o t t o n , Chem. and Ind. 307 [1954].

8 E . O . F i s c h e r and W. H a f n e r , Z. Naturforschg. 8 b, 445 [1953].

N o t e : The brown CioHioMn has also been prepared by E. O. F i s c h e r , private communication, May 1954.

F i s c h e r and his colleagues have consistently maintained in their several papers9 that in bis-cyclopentadienyl compounds the metal atom is octahedrally coordinated. They also maintain that the attainment of the configuration of the next inert gas by the metal atom, where all the -T electrons from the C 5H 5 rings are involved in d2sp3 bonding, is of some significance. The repeated exposition of this viewpoint, and the recent interpretation of the magnetic properties of the compounds on similar lines 10 prompts us to point out the inadequacy of these ideas, which are inconsistent with facts already known. The original concept 9a of two C5H5" anions, each with three electron pairs disposed around it in the form of an approximately equilateral triangle, and giving a total of six dative bonds with the d-sp3 hybrid orbitals of the metal atom, was useful only in that it lead to the recognition of a kind of sandwich geometry. The main defects of the octa-hedral theory are as follows.

a) The precise symmetry conditions required by the octahedral theory are not provided by the actual molecule, C 1 0 H 1 0 Fe 11, but only an approxima-tion to them.

b) The evidence for aromatic character in the C5H5 rings is not explained. This evidence is as follows:

1. C 1 0 H 1 0 Fe has been shown to undergo acylation in the F r i e d e l - C r a f t s reaction12 .

2. The infrared absorption spectrum of C1 0H1 0Fe, and indeed of every bis-cyclopentadienyl compound, whether neutral or an ion, and also of the cyclo-pentadienyl-carbonyls, shows a single C — H stretch-ing frequency at ~ 3.27 p 3 b ' c ' 4 ' 5 ' 1 3 . This frequency is in the region characteristic of aromatic C—H bonds, whereas the frequency which would be expected on the octahedral model would be in the aliphatic region, 3.3—5.6 p 14.

3. The infrared absorption band at about 7.1 p which is found in C 1 0 H 1 0 Fe and all other bis-cyclo-

9 e. g., a) E. O. F i s c h e r and W. P f a b , Z. Na-turforsdig. 7b, 377 [1952]; b) E. R u c h and E. O. F i -s c h e r , ibid. 7b, 676 [1952]; c) W. P f a b and E. O. F i s c h e r , Z. anorg. Chem. 274, 316 [1953],

10 F. E n g e l m a n , Z. Naturforsdig. 8 b, 775 [1953]. 11 J. D. D u n i t z and L. E. O r g e l , Nature [Lon-

don] 171, 121 [1953], 12 R. B. W o o d w a r d , M. R o s e n b l u m and M.

C. W h i t i n g , J. Amer. chem. Soc. 74, 3458 [1952], 13 E. R. L i p p i n c o t t and R. B. N e l s o n , J. chem.

Physics 21, 1307 [1953]. 14 N. B. C o 1 t h r u p , J. optical Soc. America 40, 397

[1950],

pentadienyl compounds, has been attributed 13 to the stretching of "equivalent multiple bonds resulting from derealization of the ^ electrons of the cyclo-pentadienyl rings". Since the stretching frequencies for carbon-carbon double bonds1 4 are generally found in the range 5.9 — 6.6 p and for carbon-carbon single bonds in the range 8.5 — 1 2 . 5 p, it is apparant that the order of the C — C bonds in bis-cyclopentadienyl compounds is of an intermediate value. That they are perhaps of slightly lower order than in benzene (which absorbs at 6.75 p 15) would lead to an estimate of the bond order of approxim-ately 1.5 in agreement with the value from X-ray data. This clearly indicates aromatic character of the rings.

4. Although the measurements of the C — C distances in C 1 0 Hi 0 Fe are not of high accuracy, the agreement between the independent measure-ments n< 1 6 indicates that the value of 1.4 Ä is reliable. From the known correlations of bond distance and bond order17 , the C — C bond in C1 0H1 0Fe is approximately 1.6, whidi is of the same order as the C—C bond in benzene.

5. In the mass spectrometric studies of Ci 0H 1 0Ru 1 8 , the nature and yields of the various molecular species are similar to those observed in the mass spectra of fused ring systems such as naphthalene and anthracene.

It has been claimed1 9 that the evidence for the aromatic character of C 1 0H 1 0Fe is inconclusive, because the [C10H10Co] + ion cannot be nitrated or sulfonated, and also because F r i e d e l - C r a f t s reactions have been performed with cycloparaffins. Both of the arguments are purely negative. On the basis of present theories of aromatic substitution20, the failure of a positively charged species, sudi as [C 1 0 H 1 0 Co] + to be nitrated or sulfonated is not to be expected. Whilst substitution reactions have been reported only for C 1 0H 1 0Fe (C1 0H1 0Ru should behave

15 G. H e r z b e r g , Infrared and Raman Spectra of Polyatomic Molecules, D. Van Nostrand, New York 1945, p. 362.

16 P. F. E i 1 a n d and R. P e p i n s k y , J. Amer. chem. Soc. 74, 4971 [1952].

17 C. A. C o u 1 s o n , Valence, Oxford University Press 1952, p. 253.

18 L. F r i e d m a n and A. P. I r s a , J. Amer. chem. Soc. 75, 5741 [1953].

»s» E. O. F i s c h e r and R. J i r a , Z. Naturforschg. 8 b, 1 [1953].

-'o cf. P. D. B a r 11 e 11, Organic Chemistry', H. G i l -m a n , Editor, John Wiley, New York, Vol. I, 2nd Ed. 1944, p. 205—13.

similarly) the failure of other neutral bis-cyclo-pentadienyl compounds, e.g., C10H10Ni, to react, is due to the fact that they are not chemically stable enough to survive the reaction conditions3C. The oxidation or destruction of the neutral compounds with nitric or sulfuric acid results from the accessi-bility of the metal atom to chemical attack, and this point therefore cannot be used as evidence against the aromatic character of the C 5 H 5 - rings. The aromatic character of the rings not only in C1 0H1 0Fe but also in the other bis-cyclopentadienyl derivatives must therefore be accepted, this shows that most of the n electron density is retained in the C5H5-rings and is not withdrawn into bonding with the metal as is required by the octahedral theory.

c) The dative bonding required by the octahedral theory would result in the accumulation of negative charge on the central metal atom. Such charge separation is not in keeping with P a u l i n g ' s electroneutrality principle, and it is impossible to visualize how the resulting negative charge on the central metal atom could be dissipated. Charge dissipation by multiple bonding as in the metal carbonyls, isocyanides, etc., can take place only from filled d Orbitals of the metal; in the case of the bis-cyclopentadienyl compounds, this leads to the wholly incongruous picture of back-donation to the C 5H 5 rings of the same electrons that were withdrawn from the rings into bonding with the metal. In the case of the diamagnetic [ C 1 0 H 1 0 T i ] + +

compounds, if one made the assumption of drsp3

bonding there would be no electrons in the remaining 3 d orbitals for such back donation; yet the iso-electronic derivatives of Ti(IV), Zr(IV), Nb(V) and Ta(V) are stable 4' 5. Thus the inescapable conclusion of the octahedral theory is the development of charge separation. Proof that such charge separation does not exist in C 1 0 H 1 0 Fe was elegantly given by W o o d w a r d et a l 1 2 ; thus the similarity between both the acid dissociation constants for the dicarb-oxylic acid (C 5H 4COOH) 2Fe and that of benzoic acid indicates that " the ring carbon atoms of C 1 0H 1 0Fe, and thence, necessarily the central iron atom as well, are substantially electrically neutral".

d) The known magnetic data on bis-cyclopenta-dienyl compounds is not satisfactorily accomodated

21 L. P a u l i n g , Victor Henri Memorial Volume, Liege 1948.

22 F. H. B u r s t a l l and R. S. N y h o l m , J. chem. Soc. 3574 [1952],

23 H. H. J a f f e , J. chem. Physics 21, 156 [1953].

by the octahedral theory. F i s c h e r and E n g e 1 m a n have explained the observed triplet state of C1 0Hi0Ni by assigning the two unpaired electron to the 4 d level. There is, however, no reason to suppose that the 4 d orbitals are of lower energy than the 5 s, and in fact the accepted principles of atomic structure lead to the opposite conclusion. The two electrons in question might hence be expected to enter the 5 s orbital leading to a diamagnetic C1 0H1 0Ni. Such pairing of electrons in the 5 s level is already known to occur for octahedral penetration complexes of nickel(II); thus the complex [Ni(II)R3](C104)2 , where R = C6H4[As(CH3)2]2 with the configuration 3d2

4s4p3 is diamagnetic22 .

e) The existence of the stable derivatives4 '5 of the transitional elements of groups IV and V of the periodic table shows that the attainment of the structure of the next inert gas by the metal atom is of little significance.

Molecular orbital treatments of the bonding in C 1 0H 1 0Fe have been given by D u n i t z and O r g e l 1 1

and by J a f f e 2 3 ; the work of the first authors has been recently amplified 24. J a f f e s treatment is unsatisfactory since it again postulates the attain-ment of the Krypton-structure by the iron-atom and the formation of four metal to ring-bonds; it also leads to an incorrect prediction for the magnetic moment of C1 0H1 0Cr 3c.

In contrast to the complicated bonding envisaged by both the octahedral theory and J a f f e s theory, the D u n i t z and O r g e l treatment provides a simple and sound basis for the electronic structures of bis-cyclopentadienyl compounds. It shows that the primary metal to ring-bond is formed using the 3 d orbital of the metal which has unit angular momentum (E t g) and the electron orbital of the ring which has the same symmetry properties ( E l g ) . That this theory, which was based on C1 0H1 0Fe, and also its extension, is not entirely adequate is evident from the fact that it does not correctly and unambiguously predict the magnetic moments of C 1 0 H 1 0 Fe and C1 0H1 0Ni3 C . The theory does, however, nicely account for the aromatic properties of C 1 0H 1 0Fe since each CgH5-ring is left with four paired n electrons.

24 D. P. C r a i g , A. M a c c o 11, R. S. N y h o 1 m , L. E. O r g e l and L. E. S u 11 o n , J. chem. Soc. 352 [1954],

25 W. M o f f i 11, J. Amer. chem. Soc., in the press. See also U.S. Atomic Energy Commission Unclassified Report, NYO 6132, August 1953.

It is only with M o f f i t t s 2 5 ingenious modification of the molecular orbital view that the magnetic properties of neutral bis-cyclopentadienyl compounds can be correctly predicted. M o f f i t t s innovation consists in recognizing that the electrostatic field of the 7i electrons of the C5H5-rings can produce a mixing and splitting of the 3 dz2 and 4 s orbitals both of which have A t g symmetry. The arrangement of the energy levels of the metal-atom is then as follows. There are four approximately degenerate d levels; two of these have E l g - s y m m e t r y and are responsible for bonding as in the D u n i t z and O r g e l treatment, and the other two have E 2 g -symmetry. There is then an sd hybrid orbital (A l g

symmetry and designated as hag) lying below these four levels, and the other sd hybrid (ka g ) lying above, probably at about the level of the metal 4 p orbitals. In the neutral bis-cyclopentadienyl-molecules, these orbitals are then filled as follows. In C1 0H1 0Cr, the four non-bonding electrons are disposed two paired in the hag orbital and two unpaired in the doubly degenerate E 2 g orbitals. The additional two elec-trons in C 1 0 H 1 0 Fe enter the remaining E 2 g orbitals leading to diamagnetism; had the d orbital of A l g -symmetry remained degenerate with the orbital of

Eo g - symmetry, paramagnetic C 1 0 H 1 0 Fe would have been predicted as in the D u n i t z and O r g e l -theory. Since the kag orbital is in the region of the 4 p orbitals, for C10H10Ni, which has two electrons more than C1 0H1 0Fe, these electrons will go unpaired into two of these orbitals following H u n d s rule; the triplet state of C10H10Ni is thus explained. On this view, the bond between each C5H5-ring and the metal-atom is in essence quite simply a two-electron covalent bond, and with the assumption of 3 d — 4 s hybridization all the difficulties of the octahedral, J a f f e ' s and D u n i t z and O r g e 1' s treatments are resolved. It is inconsequential whether the bis-cyclopentadienyl-compounds are considered to be derived from the neutral metal plus the two C5H5-radical according to M o f f i 11, or from a metal ion plus the C5H5" anions, since in the molecules the result is the same. The chemical facts indicate however, that in the [C10H10Mn]Xn-2-compounds, it is more appropriate to consider the metal to be in oxidation state n. It may be further noted that the estimation of the metal to ring-bond-energies in C 1 0H 1 0Fe and C10H10Ni, 73.5 Kcal/Mol and 61.5 Kcal/Mol respectively 3c, are quite in line with those to be expected for strong single covalent bonds.

Über das Kultivieren und Konservieren verschiedener Pilze und Bakterien in Reinkultur

(Erfahrungen aus einer Mikroorganismen-Sammlung, die mit Unterstützung der Deutschen Forschungsgemeinschaft aufgebaut wurde)

V o n A . N I E T H A M M E R u n d H . B A E S S L E R *

(Z. Naturforschg. 9 b, 4 5 6 ^ 6 0 [1954]; eingegangen am 7. Januar 1954)

Auf Grund eigener, langjähriger Erfahrungen wird über Kulturmethoden und Lebens-dauer zahlreicher Stämme niederer und höherer Pilze sowie Bakterien berichtet. Das Material wird durch Angaben aus der Literatur ergänzt.

Oft benötigen Biologie, Medizin und Technik physiologische Leistungen verschiedener Mikro-

organismen und haben Interesse an entsprechenden Reinkulturen. Es wurden daher die besten Kultur-methoden für verschiedene Arten in Bezug auf Nähr-boden, Aufzucht und Lagertemperatur ermittelt und vor allem untersucht, wie lange ein bestimmter Mikroorganismus ohne Substratwechsel lebensfähig bleibt. Denn allzu häufiges Uberimpfen auf frische Nährsubstrate bedingt erhöhte Infektionsgefahr und

* Anschrift: Korntal (Württ.), Standtstr. 19.

den möglichen Verlust der physiologischen Lei-stungsfähigkeit.

Die. im Folgenden angegebenen günstigsten Lebens-bedingungen und die Daten der Lebensdauer sind in jahrelanger Arbeit an Stämmen geprüft, welche zumeist noch in der Korntaler Sammlung sind.

A l l g e m e i n e s ü b e r K u l t i v i e r e n u n d K o n s e r v i e r e n

Die Substratansprüche der einzelnen Arten, Gat-tungen und Familien sind unterschiedlich. Die üb-