Nuclear Magnetic Resonance Study

Nmr Study of Thiocarbonyl Derivatives of Ре and Mn were made with solutions of the tungsten and molybdenum species using the Cary 60 as modified Ьу the Cary Division of Varian Instru­ ments to accommodate а Varian superconducting magnet, employing а magnetic field of 44 kG. Samples and pure solvent were studied in 1- and 2-cm Supracil cells. Only spectral observations made for dynode voltages <400 V were considered as reliaЫe. Spectra are base line corrected for solvent and cell absorptions. (2 5) Note Added in Proof. The spectrum recentld reported as that of tetra-n-propylammonium octacyanotungstate(IV) 5 is very similar to the spectra reported earlier for other salts of the tungstate(V) ion. 8 In fact, the tetra-n-propylammonium octacyanotungstate(V) salt has been isolated in our laboratory [ С. J. D::>naht1e, unpuЬlished results] using the synthetic procedure 5d for the "I.V" salt. Apparent\y the octacyanotungstate(IV) ion is more susceptiЬle to oxidation than the corresponding molybdenum complex; e.g., the tungsten ion readily oxidizes in aqueous acid [А. Samotus and В. Kosowicz-Czajkowska, Rocz. Chem., 45, 1623 (1971)]. А Fourier Transform Carbon-13 Nuclear Magnetic Resonance Study of Thiocarbonyl and Other 1r-(C 5 H 5 )M(CO) 2 L Derivatives of Iron and Manganese "С nmr spectra have been oЬtained for а series of complexes of the types 1r-(C,H 5 )Cr(CO) 3 ·, 1r-(C,H,)Mn(CO) 2 L (L = CS, СО, P(OPh) 3 , Р(ОСН 3 )3, PPh 3 , PBu 3 , C,H 10 NH, С 8 Н 14 ), and тr-(C,H,)Fe(CO) 2 U (L = CS, СО, PPh 3 , NH,). The observ­ ed carbonyl chemical shift data are in agreement with а previously suggested hypothesis of increasingly deshielded carbonyl resonances with increasing transition metal ➔ carbonyl 1r back-donation and suggest that the CS ligand is а significantly bet­ ter тr acceptor than СО in тr-(C,H,)Mn(CO) 2 (CS). The thiocarbonyl in 1r-(C,H,)Mn(CO) 2 (CS) exhiЬits the most intensely deshielded carbon resonance yet reported. This would appear to indicate .the localization of а substantial positive charge on the thiocarbonyl carbon atom, in close similarity to that suggested previously for the carbene carbon in (CO),CrC(OCH,)­ CH,. Introduction The characterization of transition metal-carbonyl com­ plexes 1• 2 preceded the first reports of analogous thiocarbonyl complexes Ьу over 75 years. 3 While only а lirnited number of thiocarbonyl derivatives have been synthesized to date, their ublquitous nature has been dempnstrated Ьу the char­ acterization of complexes containing Rh,3-6 Ir , 7-9 Ru, 10, 11 Со 12 Fe 13•14 Mn 15• 1 6 Cr Мо and W 17 and Ni Pd Pt and os:18 ' ' ' ' ' ' ' ' (1) L. Mond, С. Langer, and F. Quinke, J. Chem, Soc., 57, 749 ( 1890). (2) L. Mond and С. Langer, J. Chem. Soc., 59, 1090 (1891). (З) М. С. Baird and G. Wilkinson, Chem. Соттип., 267 (1966). (4) J. L. De Boer, D. Rogers, А. С. Skapski, and Р. G. Н. Troughton, Chem, Соттип., 756 (1966). (5) W. Н. Baddley, J. Amer. Chem. Soc., 88, 4545 (1966). (6) М. С. Baird, G. Hartwell, Jr., and G. Wilkinson, J. Chem. Soc., А, 2037 (1967), (7) М. Р. Yagupsky and G. Wilkinson, J. Chem. Soc. А, 2813 (1968). (8) М, Kubota and С. R. Carey, J. Organometa/. Chem., 24,491 (1970). (9) М. J. Mays and F. Р. Stefanini, J. Chem. Soc. А, 2747 (1971), (10) J. D. Gilbert, М. С. Baird, and G. Wilkinson, J, Chem. Soc. А, 2198 (1968). (11) Т. А. Stephenson and Е. Switkes, Inorg, Nucl. Chem. Lett., 7, 805 (1971), (12) Е. Кlumpp, G. Bor, and L. Marko,J, Organometal, Chem., 11,207 (1968), (13) L. Busetto and R. J. Angelici, J. Amer. Chem. Soc., 90, 3282 (1968), (14) L. Busetto, U. Belluco, and R. J. Angelici, J, Organometal. Chem., 18,213 (1969), (15) I. S. Butler and А. Е. Fenster, Chem. Соттип., 933 (1970), (16) А. Е. Fenster and I. S. Butler, Сап. J, Chem., 50, 598 (1972). (17) В. D. Dombek and R. J. Angelici, J. Amer, Chem. Soc., 95, 7516 (1973). The relative scarcity of thiocarbonyl complexes does not arise from an inherent instabllity, but rather from synthetic complications. Thiocarbonyl complexes have been oЬtained via two synthetic schemes (eq 1 and 2) and thus depend either 1r-(C,H,)Fe(CO) 2 • + ClC(S)OR ➔ 1r-(C,H,)Fe(CO) 2 C(S)OR 1r-(C,H,)Fe(CO),C(S)OR +НС! ➔ 1r-(C,H,)Fe ( CO) 2 (CS)• (Ph 3 P) 3 RhC1 + CS 2 ➔ (Ph 3 P) 2 RhCl(тr-CSz) (Ph,P) 2 RhCl(1r-CS 2 ) + PPh, ➔ (Ph 3 P) 2 RhCl(CS) + S=PPh, (1) (2) upon the nucleophilicity of the starting ma terial toward а derivative of thiophosgene or the relative stabllity of а transi­ tion metal-тr-CS 2 intermediate. Substantial evidence suggests that the CS ligand is а better 1r acceptor toward transition metals than its СО analog. Baird and Wilkinson 3 have argued that the absence of oxida­ tive addition of НС! to (Ph3P)2RhCl(CS) and the instabllity of this complex in the 2resence of HgC12 suggest the impor­ tance of а (Ph 3P)2C1Rh6 +=C=S 6 - resonance structure. This formulation is in accord with the crystallographic data of De Boer, et al., 4 who found that the Rh-C bond length in (Ph3P)2RhCl(CS) is 0.1 А shorter than in (Ph3P)2RhCl(CO), while the C-S bond length is only 0.018 А shorter than the C=S bond length in CS2• Yagupsky and Wilkinson 7 and Mays and Stefanini9 have noted а reluctance of Ir-CS complexes to undergo oxidative addition with Н 2 under conditions where their Ir-CO analogs undergo rapid reaction, and they have invoked an increased 1r acidity ofthe CS ligand to explain (18) G. Wilkinson, U, S. Patent 3,452,068 (1969). ТаЬ!е 1. Infrared Stretching Frequencies (cm - 1 ) and Force Constants (mdyn/A) for the Carbonyl Mode i n 1r-(C 5 H 5 )Mn(CO) 2 L Derivatives v cs , v CHC1 3 v CHC1 3 va c s , k с0 с н с 1 , kc o CS 2 L . 5 a 1955 2007 cs 15.88 1 9 54 1 5.86 20 1 0 2024 1 9 39 со 1 9 20 1 5 .4 5 2025 1 5 .64 а P(OPh) 3 1 5 .08 а 1 9 00 1 96 3 1962 1 5 .0 2 1 894 1 8 9 3 14.97а 1 9 5 6 14.92 1 887 1955 с.н,. Р(ОСН 3 ) 3 1 8 84 1 94 9 14.82 1 4 . 84 1 8 80 1950 PPh 3 1 9 34 14.58 1 874 1 8 64 1 4 .6 5 а 1 9 34 PBu , 1 9 29 14 .s з а 1 86 3 1852 14.42 1 9 25 а 1 9 20 C , H 1 0 NH 1 4 .29 1 840 14.20 1 832 1917 а Data taken from W. Р . Anderson , Т . В . Brill, А . R . S choenberg, and С. W. Stanger , Jr. , J. Organometal. Chem. , 44 , 161 ( 1 9 7 2) . these result s . Cotton-Кraihanzel for ce con stant calculations on 1r- (C 5 H 5 )M(C0) 2 L complexes (L = CS , СО; М = F e + , Mn ° ) are also consistent with the formulation of the C S ligand as а good 1r acceptor 1 3 (see ТаЫе 1 ) . C onversely , Busett o , et а/. , 1 9 have observed preferential at ­ tack of nucleo phili c reagents such as СН 3 0 - , N 3 - , and RNH 2 at the C S ligand in 1r- (C 5 H 5 )Fe(CO)iCS) + which occurs at а rate substantially faster than for the analogous 7Т - (С 5 Н 5 )Fе­ (С О ) 3 + co mple x . In the ab sence o f an unknown stabllization of the t ra11sition state for the CS co mple x , this wo uld indicate а11 increased ele ctrophilic character for the C S carb on ato m , which is inconsistent \Vith the results discus se d ab ove . An explanation of t his behavior might arise fro m the comparison of S C F -MO calculations fo r CS 20 and С О 2 1 which sugge st а higher ene rgy for the 7 а donor orЬ ital o n C S t han fo r the S a orbltal o n С О and а lower ene rgy for the З 1r * orbltals o n CS than for the 21r* o rb ltals on СО, thereb y implying an incre ase in both the a-donor and 1r-acceptor abilities for the C S ligan d . Recent studies of а series of 1r- (C 6 H 5 X)Cr(CO) 3 derivative s have sugge sted that the 1 3 С nmr carb onyl che mi cal shifts are а linear measure of the extent of transition metal ➔ carbonyl 1Т back-do natio n in these complexes .22 We re port here а systemati c study of the 1 3 С nmr spe�tra o f а series of 1r- (C 5 H 5 )M(C0) 2 L derivatives (L = CS , С О , PPh 3 , PBu 3 , P(OPh) 3 , Р(ОСН 3 ) 3 , C 5 H 1 0 NH , NH 3 , С 8 Н 14 ; М = Cr- , Mn ° , Fe + ) , under­ taken to develo p further the pro posed relationshi p b etween 13 С nmr carb onyl che mical shifts and the carb onyl 1r acidity . The results o f this study are then applied t o а consideration of the transition metal-thio carb onyl b on d . Experimental Section Materials. Cr(CO), and [ 1r- (C , H , )Fe(CO) 2 ] 2 were purchased from the Pressure Chemical Со. А sample of тr- (C , H , )Mn(CO ), was generously provided Ьу t h e Ethyl Corp. Samples of [ 1r- (C , H ,)­ Fe(CO) 2 (CS) ]PF 6 and 1r- (C , H , )Mn(CO) 2 (CS) were oЬtained from Dr. R. J. Angelici and Dr. I. S. Butler. [ п- (С 5 Н 5 )Сr(СO) 3 ]Na was synthesized Ьу the method of Piper and Wilkinson 2 3 and characterized Ьу infrared spectroscopy. [ тr- (C , H , )Fe(CO) , L]PF 6 derivatives, L = N H 3 2 • and PPh , , 2 5 were synthesized via literature methods. [ тr - (C , H ,)Fe(CO) 3 ]PF 6 was synthesized independently Ьу the methods of Busetto and Angelici 2 6 and Reimann and Singleton. 27 1r- (C , H , ) Mn(CO) 2 L de­ rivatives, L = C 8 H 1 4 , 2 8 P(OPh) 3 , 2 9 PPh 3 , 2 9 PBu 3 , 3 0 Р(ОСН 3 ) 3 , ( 1 9 ) L. Busetto, М. Gra ziani, and U. Belluco , Inorg. Chem., 1 0 , 78 ( 1 9 7 1 ). ( 2 0 ) W. G. Richards, Trans. Faraday Soc. , 6 3 , 2 5 8 ( 1 9 6 7 ) . ( 2 1 ) R . К. Nesbet , J, Chem, Phys. , 4 0 , 3 6 1 9 ( 1 9 6 4 ) . ( 2 2 ) G. М . Bodner and L. J , T o d d , lnorg. Ch em., 1 3 , 1 3 3 5 ( 1 9 74). ( 2 3 ) Т. S. Pi per and G . Wilkinson , J. Inorg. Nuc l. Chem., Э , 1 04 ( 1 9 56). ( 2 4 ) Е . О . Fischer a n d Е . Moser, Inorg. Sуп . , 1 2 , 3 5 ( 1 9 7 0 ) . ( 2 5 ) А . Davison, М . L . Н . Green , an d G . Wil kinson , J. Chem. Soc., 3 1 7 2 ( 1 96 1 ) . ( 2 6 ) L. Busetto and R . J. Angelici, Jnorg. Chim. A c ta, 2 , 3 9 1 ( 1 9 6 8) . ( 2 7 ) R . Н. Reimann an d Е. Singleton, J. Organometa l, Chem,, 3 2 , С44 ( 1 9 7 1 ) . ( 2 8 ) Е . О . Fis cher and М . H erberhold, Experien tia, Suppl. , No. 9 , 2 5 9 ( 1 9 6 4) . and NHC, H , 0 , 3 1 were synthesized via the photoly sis of п- (С , Н , ) ­ Мn(СO) 3 with а high-pressure H g lamp in either b enzene or hexane solution in the presence of а slight excess of the re spective ligand. The С , Н , 4 , PPh , , P(O Ph) 3 , and NHC , H , 0 derivatives were recrystal­ lized fro m benzene-hexane and characterized Ьу infrared spectroscopy and microanalysis. The PBu , and Р(ОСН 3 ) 3 derivatives were oЬtain­ ed as viscous oils , purified Ьу suЫimation of unreacted п- (С , Н , )Мn­ (СО) 3 , and then characterized Ьу infrared spectro scopy and mass spectroscopy. Instrumentation. ' 'С nmr spectra were oЬtained in deuterio­ chloroform , deuterioacetone, or tetrahydrofшan solution on а Varian Associates XL- 1 0 0-FT spectrometer operating at 2 5 . 1 6 MHz. Deuteriobenzene was added to tetrahydrofuran so lutions to provide an internal deuterium lock. The 1 3 С nmr chemical shifts were meas­ ured relative to the internal solvent resonance and are reported in ppm downfield from TMS using the conversions Б тмs = Б c n ci3 - 7 6 .9 8 ppm Б тмs = Б (сD з >, со - 2 9 .6 6 ppm Б тмs = Б т н F {с<, , s ) } - 68 .05 ppm Infrared spectra were oЬtained in cy clohexane , carbon disulfide, and chloroform solution on а Beckman IR-1 2 spe�trometer calibrated below 2000 cm - 1 with amЬient water vapor. А 1 0-fold ordinate expansion was used with а 20 ст · ' /min scan speed. Microanalytic data were oЬtained in the microanalysis laboratory of the S chool of Chemical Sciences, University of Illinois. Results and Discussion Previous re ports of the 1 3 С nmr spectra of tran sition metal carb onyl comple xe s o f manganese have suggested that the carbonyl resonan ces cannot Ь е ob served in the ab sence of shiftless relaxation reagents such as tris(acetylacetonato )­ chromium(I II) d ue to co upling with the 5 /2 -spin 55 Mn nucleus undergoing rapid quadrupole relaxation . 32 • 3 3• Figure 1 shows the proton-coupled 1 3 С nmr spe ctrum of 1r-(C 5 H 5 )Mn(CO) 3 in deuteriochloroform. We note that allowing suffi cient time for re laxation of the carb onyl resonance between pulses yields high-r esolution spe ctra of derivative s of the type 1r­ (C 5 H 5 )Mn(C0) 2 L in the absence o f parama gnetic relaxation reagents . Darensb ourg and Brown 34• 3 5 have shown that changes in the magnitude of carbonyl mo de stret ching frequencies for transition metal carbonyl comple xes may Ь е bro ught about Ьу changes in either the a-donor or 1Т-acceptor chara cter of the carb onyl ligand . Rather than pursuing а dublous se para­ tion of the а and 7r contrib utions to the for ce constant , (29) ( 1 964). ( 3 0) G. Е. Schroll , U. S. Patents 3 , 0 5 4 ,740 ( 1 96 2 ) and 3, 1 30,2 1 5 W. Р. Anderson, Т . В . Brill, А. R . Sc hoen berg, an d С . W. Stanger, Jr., J. Organometal. Chem. , 44, 1 6 1 ( 1 9 7 2 ) . ( 3 1 ) W . Strohmeier and J . F . Guttenberger, Chem. Ber. , 9 7 , 1 2 5 6 ( 1 964). (32) О . А. Gansow , А. R . Burke and G. N . La Mar, J. Chem. Soc. , Chem. Сот тип., 4 5 6 ( 1 9 7 2 ) . ( 3 3 ) Р. С. 3 2 6 6 ( 1 96 5). ( 34) Т . L. ( 1 9 6 7). ( 3 5) D . J . (1 968). Lauterbur and R . В. Юng, J. A mer. Chem Soc. , 8 7 , Brown an d D. J . Darensbourg, Inorg. Chem,, 6, Darensbourg and Т . L . Brown , Inorg, Chem., 971 7, 9 59 ТаЬ!е 11. ''С Nmr Chemical Shifts (ppm) and 'J1 'сЗ! р Coupling Constants (Hz) in 1r-(C,H,)M(CO),L Derivatives 1r-(C,H,)Cr(CO),Na 1r-(C,H,)Mn(CO), (CS) 1r-(C,H,)Mn(CO),(CS) 1r-(C,H,)Mn(CO), 1r-(C,H,)Mn(CO) 2 [P(OPh) 3 ] 1r-(C 5 H 5 )Mn(CO), [Р(ОСН 3 )3] 1r-(C,H,)Mn(CO) 2 (PBu,) 1r-(C,H,)Mn(CO) 2 (PPh 3 ) 1r-(C 5 H 5 )Mn(CO) 2 (C,H 14 ) 1r-(C,H,)Mn(CO) 2 (NHC,H 10 ) 1r-(C,H,)Fe(CO),PF 6 1r-(C,H,)Fe(CO),(CS)PF 6 1r-(C,H,)Fe(CO) 2 (PPh 3 )PF6 1r-(C,H,)Fe(CO) 2(NH,)PF6 а Chemical shifts in ppm dиvnfield from TMS; 'J1 3 c 31 p CS СО С,Н, Complex a -246.75 -8l.87 -442.6 -224.0 -85.71 -442.9 -224.5 -86.0 -225.1 -82.69 36 -228.8 -81.11 34 -229.5 -80.94 26 -231.1 -79.Оо 23 -232.8 -82.40 -234.5 -83.90 -236.2 -81.94 -202.98 -90.81 -203.32 -92.16 -307.90 24 -210.53 -90.81 -211.76 -86.70 Mn-CO chemical shifts ±0.1 ppm; all other chemical shifts ±О.Об ppm. Solvent THF-c.o. CDC!, Acetone-d 6 CDCI, CDC!, CDCI, CDC!, CDC!, CDCl 3 CDC1 3 Acetone-d 6 Acetone-d 6 Acetone-d 6 Acetone-d 6 -236 -234 111 .,..,,.1 1 ,.,,... ..,, ... ..,.......�.�, 'v, 'i'.щ,. •� 1 Figure 1. Proton-coupled Fourier transform 13 С nmr spectrum of 1r-(C Н 5 )Mn(CO) 3 in deuteriochloroform, with 1000 scans, а 10sec p�!se de!ay between scans, and а flip angle of 45° . Resonance assignments and chemical shifts (in ppm downfield from TMS): СО, -225.1; С,Н,, -82.69. The three small peaks at high field arise from the CDC!, solvent. Darensbourg and Darensbourg 36 have argued that the magni­ tude of the stretching force constant is directly proportional to the positive character of the carbonyl carbon atom. One might therefore suggest that the order of increasing electron density on the carbonyl carbon for а series of 7Т-(C 5 H 5 )Mn­ (CO) 2 L derivatives (see ТаЫе I) is CS < СО < Р(ОРh)з < СвН 14 < Р(ОСН 3 ) 3 < PPh 3 < РВuз < C 5 H 1 oNH.37 This postulate has led several authors to investigate the nature of the correlation between 1 3 С nmr carbonyl chemical shifts and infrared stretching frequencies or force constants. While а gratifyingly high linear correlation has been observed for several classes of closely related derivatives,22• 38• 39 this correlation has been questioned when applied to а broad range of derivatives.40 The 13С nmr carbonyl chemical shift data for а series of 1r-(C 5 H 5 )M(CO) 2 L derivatives (М = Cr-, Mn° , Fe +) are presented in ТаЫе II. Figure 2 shows а plot of the carbonyl chemical shifts for 7Т-(C 5 H 5 )Mn(CO) 2 L com­ plexes vs. the infrared stretching force constants calculated via the Cotton-Кraihanzel approximations41 assuming ideal­ ized C2v symmetry. As noted previously22 the sign of this correlation is opposite to that expected on the basis of the Darensbourg-Brown model and а dependence of the carbonyl chemical shift upon electron density at the carbonyl carbon atom. We have argued22 that the observed correlation is con­ sistent with the hypothesis of increasingly deshielded car­ bonyl resonances with increasing transition metal ➔ carbonyl (36) D. J. Darensbourg and М. У. Darensbourg, lnorg. Chem., 9, 1691 (1970). (37) W. Р. Anderson, Т. В. Brlll, А. R. Schoenberg, and С. W. Stanger, Jr., J. Organometal. Chem., 44, 161 (1972). (38) О. А. Gansow, D. А. Schexnayder, and В. У, Кimura, J. Amer. Chem. Soc., 94, 3406 (1972). (39) G. М. Bodner, S. В. Kahl, К. Bork, В. N. Storhoff, J. Е. Wuller, and L. J. Todd, Inorg. Chem., 12, 1071 (1973). (40) в. Е. Mann, J. Chem. Soc., Dalton, Trans., 2012 (1973). (41) F. А. Cotton and С. S. Кraihanzel, J. Amer. Chem. Soc., 84, 4432 (1962). -232 13С NMR -230 -228 -226 - 22 4 /�о '----4 cs .__-'-......J.-...J_---'---'---l...--'---J 15,8 15.4 ·,5,0 14.6 14,2 Figure 2. P!ot of the 13 С nmr carbonyl chemical shifts in ppm down­ field from TMS vs. the infrared stretching force constants in mdyn/A for 1r-(C,H,)Mn(C0) 2 L derivatives. Relative error bars for k co of ±О.Об mdyn/A are included. The re!ative error in carbonyl chemical shift is approximated Ьу the size of the circ!e. 1Т back-donation. Thus the data presented in ТаЫе II for the isoelectronic 7Т-(С 5 Н 5 )М(СО) 3 complexes suggest an increase in transition metal ➔ carbonyl 1r back-donation with increas­ ing negative charge on the complex in accord with infrared data26• 42 and the results of SCF-MO calculations on the iso­ electronic М(СО) 6 complexes (М = v-, Cr 0 , Mn +). 43 Deviations from а linear correlation in Figure 2 may arise from several complications. First, while it has been suggest­ ed that comparisons of infrared and 13 С nmr data Ье restrict­ ed to а common solvent,40 the use of chloroform leads to broadened infrared absorption bands with а concomitant de­ crease in the accuracy of the stretching force constants. Sec­ ond, the force constants were calculated using an approximate theory with an assumed C2v symmetry instead of the more exact С, symmetry. Third, infrared stretching force constants have been shown to Ье proportional to changes in either the а basicity or 1Т acidity of the carbonyl ligand, whereas the 13С nmr carbonyl chemical shifts appear to Ье proportional to changes in the extent of transition meta,l ➔ carbonyl 7Т back-donation alone. 22 Transition metal ➔ carbonyl 1Т back-donation increases with increasing electron density on the transition metal. The (42) R. D. Fischer, Chem. Ber,, 93,165 (1962). (43) К. G. Caulton and R. F. Fenske, Inorg. Chem., 7, 1273 (1968). 1 3 С nmr carbonyl chemical shift should thus Ье proportional to the electron density on the transition metal and therefore should Ье а good measure of the relative а basicity/1r acidity of ligands in substituted metal carbonyl complexes. The data in ТаЫе II suggest that the order of increasing electron density at the transition metal in 1r-(C 5 H 5 )Mn(CO) 2 L deriva­ tives is CS <СО< P(OPh) 3 < Р(ОСН 3 ) 3 < PBu 3 < PPh 3 < С 8 Н 14 < C 5 H 10 NH. The relative order СО P(OPh) 3 > Р(ОС­ Н 3 ) 3 > PBu 3 > PPh 3 > C 5 H 10 NH. The close similarity between the carbonyl chernical shifts for the cyclooctene (С 8 Н 14 ) and piperidine (С 5 Н 10 NН) de­ rivatives is indicative of extensive donation of electron densi­ ty from the olefin to the transition metal. The shielding of the carbonyl resonance in 77-(С 5 H 5 )Mn(CO)iCS) relative to 7Т-(С 5 Н 5 )Мn(СО) 3 is indicative of а decrease in electron density at the transition metal in the CS complex which rnight arise from an increased 1Т acidity of the CS ligand. The reduced range of carbonyl chernical shifts for 1r-(C 5H 5 )Fe(CO) 2 L + relative to 1r-(C 5H 5 )Mn(CO) 2 L derivatives is in agreement with а decrease in transition metal ➔ carbon­ yl 1Т back-donation in the positively charged complex. The data in ТаЫе II suggest that PPh 3 cannot compete with t,vo strong 1r-acceptor СО ligands toward а weak 1r-donor F е + complex as well as it can to,vard the stronger 1r-donor Mn ° complex. Demarco, Doddrell, and Wenkert 49 have observed а de­ shielding of the С(2) carbon resonance iп thiocamphor of 54.3 ppm relative to the С(2) carbon resonance in camphor. They have argued that this deshielding is inconsistent with either the relative electronegativities or dipole moments for C=S and С=О but is iп agreemeпt with the known de­ crease in the energy of the n ➔ 1r* transition upon replace­ ment of С=О with C=S 50 •51 and with the expressioп for the 52 paramagnetic screeпing tensor derived Ьу Karplus and Pople. Кalinowski and Kessler 53 have noted а fairly constant de­ shielding of thiocarbonyls Ьу 25-30 ppm relative to their carbonyl analogs in R 2 C=X derivatives (Х = О, S), which they suggest is due to the change in ЛЕ. In 1r-(C 5 H 5 )Mn­ (CO) 2 (CS) the thiocarbonyl resonance is over 218 ppm downfield from the carbonyl resonance. While а portion of this chernical shift difference is to Ье expected from changes in дБ, the magnitude of this effect is reminiscent of the 135ppm difference betweeп the chernical shifts of the carbene and carbonyl resoпances iп (CO) 5 CrC(OCH 3 )CH 3.39 The observed thiocarbonyl chernical shift rnight therefore Ье best understood in terms of а -8+C=S 8 - resonance structure which is stabilized Ьу transition metal-thiocarbonyl 1Т back­ donation. This hypothesis is in agreemeпt with studies of the Raman spectra of thiocarbonyl complexes which suggest а significant dipole momeпt for the thiocarbonyl ligand. 54 Acknowledgment. We wish to thank Dr. I. S. Butler and Dr. R. J. Angelici for their gracious gifts of samples of 1Т· (C 5 H 5 )Mn(CO)z(CS) and 1r-(C 5 H 5 )Fe(CO) 2 (CS)PF 6 and the Ethyl Corp. for providing а generous sample of 11-(C 5 H 5 )Mn­ (CO) 3 . The Variaп Associates XL-100-FT spectrometer was oЬtaiпed in part with an instrument grant from the National Science Foundation, GP-28262. Registry No. п-(C,H 5 )Cr(CO) 3 Na, 12203-12-2; п-(C,H,)Mn­ (CO) 2 (CS), 51804-24-1; п-(С,Н,)Мn(СО),, 12079-65-1; п-(С,Н,)­ Мn(СO) 2 [P(OPl1) 3 ], 12278-56-7; п-(С ,H,)Mn(CO) 2 [Р(ОСН,) 3 ], 34922-82-2; п-(C,H,)Mn(CO) 2 (PBu,), 12277-85-9; п-(C,H 5 )Mn­ (CO) 2 (PPh,), 12100-41-3; п-(С,Н,)Мn(СО) 2 (С,Н 14 ), 12088-20-9; п-(С,Н 5 )Мn(СО) 2 (NHC,H 10 ), 38497-86-8; п-(C,H,)Fe(CO),PF 6, 34 738-62-0; п-(С,Н,)Fе(СО), (CS)PF 6, 34 738-61-9; п-(С,Н,)Fе­ (СО) 2 (PPh,)PF 6, 12100-39-9; п-(С ,H,)Fe(CO) 2 (NH,)PF" 5222544-2; "С, 14 762-74-4. (49) Р. V. Demarco, D. Doddrell, and Е. Wenkert, Chem. Сот­ (44) Е. О. Fischer, L. Knauss, R. L. Keiter, and J. G. Verkade, J. Organometal. Chem., 37, С7 (1972). (45) С. G. Swain and Е. С. Lupton, Jr., J. Amer. Chem. Soc., 90, 4328 (1968). (46) J. А. Рор\е and D. Р. Santry, Mol. Phys., 8, 1 (1964); С. J. Jameson and Н. S. Gutowsky, J. Chem. Phys., 51, 2790 (1969). (47) D. М. Grant and W. М. Litchman, J. Amer. Chem. Soc., 87, 3994 (1965). (48) G. М. Bodner and L. J. Todd, unpuЫished results. тип., 1418 (1969). (50) J. J. Worman, G. L. Роо\, and W. Р. Jensen, J. Chem. Educ., 47, 709 (1970). (51) R. N. Nurmukhametov, L. А. Mi\eshina, D. N. Shigorin, and G. Т. Khachaturova, Russ. J. Phys. Chem., 43, 24 (1969). (52) М. Karplus and J. А. Pople,J. Chem. Phys., 38, 2803 (1963). (53) Н.- О. Kalinowski and Н. Kess\er, Angew. Chem, lnt. Ed. Engl., 13, 84 (1974). (54) С. F. Shaw, Ш, and 1. S. Butler, personal communication of unpuЫished data.