Infrared and carbon-13 NMR study of phosphine monosubstituted group VI B metal carbonyls

Journal o f Molecular Structure, 53 (1979) 45--53 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

INFRARED AND CARBON-13 NMR STUDY OF PHOSPHINE MONOSUB-STITUTED GROUP VI B METAL CARBONYLS

M. F. GUNS*, E. G. CLAEYS and G. P. VAN DER KELEN

Laboratory for General and Inorganic Chemistry-B, University o f Ghent, Kr~]gslaan 271, B-9000 Ghent (Belgium) (Received 15 September 1978)

ABSTRACT IR and 13C NMR data are presented for a series of LM(CO)s compounds with L = (CH3)3P, (C2Hs)3P and (C6Hs)3P for M ffi Cr, Mo, W and L = PC13, C~HsPCI 2 and (C6Hs)2PC1 for M = Cr. The correlation between the Al(eq) v(CO) stretching frequency and the sum of Taft's o*X parameters for the phosphorus substituents is discussed in terms of a~ and bonding. Relative inductive and mesomeric parameters, derived according to both the treatment of Graham and of Dobson, are compared. A linear correlation has also been found between v(CO) and 813c(o) .for axial as well as equatorial CO groups. Finally 2JP--M--C(O) a x ' 2JP--M--C(O)eq and IJw_.c(o) e are discussed in view of the recent literature; a negative sign is proposed for some ~ - ~ O ) coupling constants in LCr(CO) 5 compounds in order to obtain logical trends for the carbonyls of the three transition metals Cr, Mo and W. INTRODUCTION

The substitution of a CO group in a metal carbonyl b y another ligand is thought to result mainly in a redistribution of the electron density in the system of the whole molecule. Generally this change is discussed in terms of different a~lonor and ~-acceptor capacities of the ligand, b u t the interaction of o- and ~- t y p e orbitals of the central metal can also be a determining factor [1, 2]. The first results of spectroscopic research on carbonyl c o m p o u n d s i.e., the CO stretching frequencies, were explained in terms of either inductive (o~lonor) [3, 4] or mesomeric (~-acceptor) [5, 6] effects of the ligands. Subsequent investigations have a t t e m p t e d to separate the o- and ~- bonding contributions from dipole m o m e n t measurements [7] or IR data [8, 9]. In this report we studied a series of phosphine monosubstituted group VI B metal carbonyl complexes by IR and 13C NMR spectroscopy of the CO groups, in order to distinguish the influence o f inductive (o) and mesomeric (n) effects. Literature data are combined with our o w n measurements on the Cr, Mo and W pentacarbonyl complexes LM(CO)s, L being (CH3)3P, (C2Hs)3P *Present address: Ministry of Agriculture, Institute for Chemical Research, Museumlaan 5, B-1980 Tervuren, Belgium.

46

and (C6Hs)3P and on the chromium pentacarbonyls with the ligandsPCI3, C6HsPCI2 and (C6Hs)2 PCI. EXPERIMENTAL

The synthesis of the carbonyl complexes has been described in a previous paper [21]. IR spectra were recorded on a Perkin-Elmer 225 spectrophotometer; the samples were studied in the carbonyl region (5 u ) as concentrated solutions in cyclohexane. The complementary Raman spectra were detected using a Coderg PH1 spectrometer with a continuous wave He--Ne laser (200 mW) as excitation source. The ~3C NMR spectra were obtained by Fourier transform NMR technique with a Bruker HF-X5 spectrometer at 22.63 MHz on nearly saturated solutions of the carbonyls in CDC13 in 10-mm NMR tubes at r o o m temperature. RESULTS AND DISCUSSION

IR v(CO) frequencies The octahedral complexes of the t y p e LM(CO)s theoretically show the following CO stretching vibrations: 2A~ + B~ + E; the assignment of these fundamentals and their activity and intensity in the IR spectra have been established earlier [ 1 0 ] . In a previous report from this laboratory [11] the v(CO) modes in LCr(CO)s and, in a first attempt, some aspects concerning o and ~ bonding have been treated. We wished to examine further the influence of inductive (o) and mesomeric (~) effects of the ligand L in the L--M--C--O chain. Therefore we collected an internally consistent set of data, all measured under the same conditions and shown in Table 1. The sum Y,d*x of the Taft substitution constants [22] is taken as a measure of the a
47 TABLE 1 v(CO) IR frequencies, CO stretching force constants and inductive and mesomeric parameters for LlVI(CO)s compounds L

A~(eq) B~

M = Cr (CH3)3P (C~Hs)3P (C6Hs)3P (C6Hs)2PC1 C,HsPCI 2 PC13 C6H1~NH2a

Al(ax) E

kax

keq

Graham

Dobson

Ao

Ad

A~

A~

2062 2058 2062 2070 2078 2085

1973 1972 1981 1992 2004 2008

1949 1942 1948 1963 1981 1999

1937 1934 1941 1950 1967 1981

15.46 15.55 15.58 15.88 16.12 16.29 14.63

15.71 15.76 15.81 15.94 16.17 16.35 15.79

--0.98 --0.99 --0.91 --0.95 -0.73 --0.54

0.95 0.91 0.93 1.10 1.11 1.10

--0.490 --0.495 --0.455 --0.475 --0.365 --0.270

0.460 0.415 0.475 0.625 0.745 0.830

M = Mo (CH3)3P 2069 (C2Hs)~P 2067 (C6Hs)3P 2072 C6HI1NH~

1978 1978 1986

1950 1946 1949

1942 1942 1949

15.48 15.55 15.50 14.70

15.84 15.85 15.95 15.85

--0.85 -0.80 -0.60

0.85 0.79 0.70

--0.425 --0.400 --0.300

0.425 0.390 0.400

M=W (CH3)3P 2067 (C2Hs)3P 2066 (C,Hs)3P 2071 C,HI~NH~

1972 1970 1979

1945 1941 1943

1936 1934 1940

15.40 15.46 15.46 14.69

15.74 15.77 15.84 15.74

-0.71 --0.71 --0.57

0.74 0.71 0.67

--0.355 --0.355 --0.285

0.385 0.355 0.385

aReference ligand (cannot act as ~ acceptor).

At(tq) cm'l 2O9Ot '' t C°I 10,

Z080

~

.~.7" /"

"

B 1

.'" 2070

..~"

"'"" 3* -2

2

/, ,""

2060

2050

I 0

I I

(C2HsllP

3 1C4Hgl3P lal 4 IC6H$)3P IC 6Hs)2PC[ (CHIOllP (bl 7 C6HsPC( 2 8 ICGHsO}3P Iol 9 PI 3 1¢1 }o PBr 3 (¢) 11 pet 3 I 2

I 3

[o'i, x

Fig. 1. Correlation (least-squares) between v(CO)A,(eq) for LCr(CO)s and ~a* x for ligand L; (a) ref. 24; (b) ref. 25; (c) ref. 26.

48

this could be interpreted as reflecting increased (M-~P)~ back bonding with respect to (C6Hs)aPCr(CO)s on successively replacing phenyl groups with chlorine atoms. From the position of PC13Cr(CO)s it can be shown by the same reasoning that the (M~P)~ back bonding increases with the number of chlorine atoms. Indeed, the strongly electronegative chlorine atoms decrease the (P-~M)o donation b u t cause a contraction of the P 3d orbitals and increase the (M-*P)~ back donation. Obviously this does not fit in with the view of a synergic P--M double bond (i.e. the weaker the (P-~M)o donation the smaller the (M~P)~ back bonding). Here the electronegativity of the P substituents influences b o t h o-donor and ~-acceptor capacities with an accumulating increasing effect on v(CO); hence a separation between b o t h effects on the basis of v(CO) frequencies alone cannot be achieved.

Relative inductive and mesomeric parameters from CO stretching force constants The CO stretching force constants can be calculated from the CO stretching frequencies b y the general FG matrix m e t h o d of Wilson. The secular equations for the LM(CO)s complexes were derived b y Cotton and Kraihanzel [ 1 3 ] , the solution of these equations being based upon a good approximation concerning the set o f stretching and interaction force constants [14]. Actually, the four different CO stretching frequencies should be known for this calculation; as most authors do n o t give the frequency o f the "activated" IR forbidden B1 mode, we used our consistent set of results only (see Table 1; only the stretching force constants kax and keq are given). Obviously the trend in the force constants parallels the conclusions drawn from CO stretching frequencies. First Graham [8] elaborated a m e t h o d to calculate ligand o and ~ parameters from kax and keq of the M(CO)s moiety. Next Brown and Dobson [9] proposed the "direct d o n a t i o n " mechanism as a model of inductive interaction b e t w e e n the ligand and the carbonyl group and obtained analogous d and ~ parameters. The results of b o t h methods are also summarized in Table I and allow a comparison, taking into account that a lower (more negative) A o (A.d) value represents a higher o-donor capacity of the ligand and a greater A~ value a better ~-acceptor capacity. As expected, the values o f Ao and Ad show a regular increase in o-donor capacity going from PC13 to (C6Hs)2PC1; both values for (C6Hs)3P are, however, anomalous. From b o t h inductive parameters for the three series of metal carbonyls it can further be seen that the trialkylphosphines are better o donors than triphenylphosphine. The Dobson A~ values, in contrast with those of Graham, are in good agreement with the idea of increasing ~-acceptor capacities in the series from (C6Hs)aP to PCla, due to the increasing contraction of the P 3d orbitals. Furthermore, Ad and A~ change in this series by a broadly equivalent amount, which should indicate that the inductive and mesomeric effects are equally important in determining v(CO). From b o t h A~ parameters it follows that (CH3)3P should be a better acceptor than (C2Hs)aP; the lower position of (C2Hs)3PCr(CO)s in the graph

49

of v(CO) vs. Y,o~.x (Fig. 1) is also an indication of the t r u t h of this assumption, the explanation of which, however, is not so obvious. A different ~-acceptor capacity for the trialkylphosphines vs. (C6Hs)3P is suggested by the Graham A~ values, but is n o t so obvious from those of Dobson. We can conclude t h a t the inductive and mesomeric parameters obtained are in most cases in good agreement with the current ideas about the ligands used, with a preference for Dobson's model; nevertheless we have to take into account t h a t these separations o f a and ~ effects are based on simplifying approximations.

~3C N M R o f the CO group Gansow et al. [17, 18] were the f i r s t t o publish ~3C NMR d a t a o n a s e r i e s o f group VI B carbonyls and concluded from the linear relation between 5,3c(o) and the CO stretching force constants that variations in 8,3c(o) were mainly due to changes in M - C - O ~ bonding. This assumption has been further examined by Braterman et al. [19]. They argued t h a t ~'3c(o) depends on AE, the average excitation energy, which is influenced by the filling of the CO ~* orbitals. During the elaboration of this work ~3C NMR data on a large series of group VI B metal carbonyl complexes were presented by Bodner [20]. The results of our carbonyl investigations are complementary to this work and contain new spectroscopic data for the complexes containing (C2Hs)3P, (CH3)3P, (C6Hs)2PCI, C6HsPCI2 and PC13 as ligand. Table 2 presents all measured ~3C N M R parameters. The chemical shift, in p.p.m., is calculated vs. tetramethylsflane, with the convention that a positive value stands for a downfield shift or a shift to higher frequency. 3C(0) chemical shift The main conclusion from Table 2 concerning the chemical shift of the CO carbon atoms is that a deshielding of the carbonyl resonance occurs when the n back donation from transition metal to carbonyl increases. In agreement with Braterman et al. [19] and Bodner [20] we think t h a t the observed downfield shift must be attributed to a decrease in AE, as a result o f increased interaction of metal d orbitals with CO~* orbitals. A comparison between homologous complexes of different transition metals is also o f interest: the neighbout anisotropy o f the transition metal acts equally on the carbon and on the phosphorus atom [21], as has been discussed earlier [19, 20]. From Table 2 it follows further t h a t the difference in chemical shift between the axial and equatorial CO groups decreases in the series Cr-*Mo-*W, which implies a decreasing !n~lonor strength of the metal. This is probably due to poorer contact between the metal d orbitals and the CO ~* orbitals as the former become more diffuse in going f r o m Cr to W. The plot o f 81~(o)e q vs. ZO*x (r 2 = 0.94) for different LCr(CO)s complexes (Fig. 2) is very similar to the graph of ~(CO) Al(eq) vs. Zo* x (Fig. 1); the deviations for most compounds are in the same direction and the explanation given in the preceding paragraph is still valid. Obviously the correlation between the A 1 stretching frequency of the equatorial CO groups and the

50 TABLE 2 13C NMR data of LM(CO)s compounds L

6C(O)eq

2JP'--C(O)eq

5C(O)ax

2Jp--c(O)ax

(p.p.m.)

(Hz)

(p.p.m.)

(Hz)

Jcis

IJw.-.c(o~

(Hz)

Jtrans

M = Cr (CH3)3P (C2Hs)3P (C6Hs)3P (C,Hs)2PC1 C,HsPCI 2 PC13 (C,H50)3P a (CH30)3P a

{Cr(CO),: 5C(O) = 211.9 p.p.m.} 217.4 --14.4 222.0 217.9 --13.7 221.8 217.3 --13.2 222.1 215.5 --14.2 220.8 213.7 --15.1 219.3 212.1 --16.8 217.6 (--)20 (--)21

--8.5 --8.1 --6.8 --4.9 --1.7 +4.6 0 (--)4

M = Mo (CH3)3P (C2H s)3P (C,Hs)3P PCI3a (C~HsO)3Pa (CH30),P a

{Mo(CO)~ :5 c(o) = 201.4 p.p.m. } 206.3 10.0 210.7 206.7 9. 5 210.4 206.3 9.0 210.8 11 13 14

20.1 20.8 22.7 66 46 40

M=W (CH3)3P (C2Hs)3P (C,Hs)3P (C6HsO)3P a

{W(CO)~:6c(o) = 191.5 p.p.m;Jw_c(O)eq 197.4 7.5 200.6 197.6 7.3 199.8 197.8 7.0 199.7 10

125.7} 18.6 19.0 21.5 44

124.4 124.6 125.7

aAbsolute values taken from Bodner [ 20 ]. c h e m i c a l s h i f t o f t h e e q u a t o r i a l CO c a r b o n is e x c e l l e n t (Fig. 3; r 2 = 0.99). T h e a n a l o g o u s c o r r e l a t i o n f o r t h e axial CO g r o u p ( n o t s h o w n ) is n o t as g o o d (r 2 = 0.93), p o s s i b l y d u e t o a g r e a t e r u n c e r t a i n t y o n t h e m e a s u r e d values in I R a n d N M R . I t is t h u s clear t h a t v(CO) a n d 5 c(o) are b o t h d e t e r m i n e d b y t h e charge o n t h e c e n t r a l m e t a l a t o m , w h i c h itself d e p e n d s o n t h e a ~ l o n o r a n d ~ - a c c e p t o r c a p a c i t i e s o f t h e ligands. F i n a l l y w e c o r r e l a t e d 8 ~3c(o)~ a n d 813c(o)eq w i t h t h e i n d u c t i v e a n d mesomeric parameters of Graham and Dobson; therefore, the data of the six c h r o m i u m c a r b o n y l s (see T a b l e 1) w e r e u s e d a n d t h e results are r e p r e s e n t e d in T a b l e 3. T h e degree o f l i n e a r i t y f o r all c o r r e l a t i o n s is s a t i s f a c t o r y , w i t h a preference for the parameters of Dobson.

Spin--spin couplings with the 13C(0) nuclei T h e o b s e r v e d c o u p l i n g c o n s t a n t s 2Jp_M__C(O)ax, 2Jp_M__C(O)eq a n d IJw_.c(o)eq ( s i m p l y d e n o t e d as Jrron,, J c s a n d J w - c ) are n o w discussed in t h e light o f r e c e n t l i t e r a t u r e .

51 813C(Oleq (ppm) ] 220[

1 2 3 4 5 6

/~,

(C2Hs)3P (CH3)3P (C4HgI3P (o) (C6Hs)3P (C6Hs)2PCI (CH30)3P (a)

215

9"

210

O

~

I

3,

~-o'i,

Fig. 2. C o r r e l a t i o n ( l e a s t - s q u a r e s ) b e t w e e n t h e c h e m i c a l s h i f t ~ 13C(O)eq f o r L C r ( C O ) s a n d ] c a * j ( f o r l i g a n d L; ( a ) r e f . 2 0 .

VICO) ¢m-I AIIPql

9

I 2 3 4 5 6 7 8

.

2080

6 ~'e

2070

,

(C2Hs)3P (CH3)3P IC4HgI3P IC6H,~I3P IC6H5)2PCI (CH3OI3P C6H5P¢I2 ICgHsO13P

°o

2060 i

210

220

215

6 13C,Oleq

Fig. 3. C o r r e l a t i o n ( l e a s t - s q u a r e s ) b e t w e e n t h e c h e m i c a l s h i f t 813C(O)eq a n d v ( C O ) A , ( e q ) f o r LCr(CO)s. TABLE 3 L i n e a r c o r r e l a t i o n d a t a o f 6,3C(O) i n L C r ( C O ) s c o m p o u n d s x

y = 6C(O)eq

y = 5 C(O)ax

Equation Ao an Ad an

(Graham) (Graham) (Dobson) (Dobson)

y y y y

= = = =

--12.2 --21.6 --24.4 --13.7

x x x x

+ + + +

205 237 205 224

r2

Equation

r2

0.88 0.78 0.88 0.99

y y y y

0.91 0.66 0.91 0.93

= = = =

---9.6 x ~+ 2 1 2 --15.4 x + 236 --19.2 x + 212 --10.3 x + 227

52

Braterrnan et al. [19], who studied Mo and W carbonyl complexes, drew the following conclusions: (1) Jtrans ~" Jcs' due to a larger mutual polarisability for ligands in trans positions. (2) 2Jp__c(o) depends on the electronegativity of the groups attached to phosphorus; it thus parallels ~JP-w. From~is more compIete study on Cr, Mo and Wcarbonyl complexes, Bodner [20] concluded that: (1) Jt~a,~ ~" Jci~ for Mo and W complexes, but this order is reversed for the chromium compounds. (2) Jc, decreases in t h e series Cr>Mo>W and Jt~ans decreases in the series Mo>W>>Cr. (3) The sign of both P--C(O) coupling constants in cis (PH3)2Cr(CO)4 must be equal in contrast to 2jp_p in c/s and trans L2M{CO)4 compounds with M = Mo or W. It is striking that the absolute values of the P--C(O) coupling constants (Table 2) do not follow a logical pattern. Verkade and co-workers [23] determined the P--M--P coupling constants in ( ( C H 3 ) 3 P } 2 C r ( C O ) 4 and found --36 Hz for the cis derivative and --28.5 Hz for the trans compound. In combination with assertion (3) of Bodner [20], we accept a negative sign for both c/s and trans coupling constants in all LCr(CO)s compounds, except for PC13Cr(CO)s, which yields the signs proposed in Table 2. Now the trends in the coupling constants become valid for the three group VI B metal carb0nyls: firstly, the assumption Jt~o,,, ~> Jci~ is also true for the LCr(CO)s derivatives; secondly, bothcoupling constants decrease in the same series: Mo ~> W >~ Cr; thirdly, in the series of chromium carbonyls a good correlation exists between Jt,~n, and earlier-obtained parameters, as summarized in Table 4. The dependence of Jt~,, on Z o*x shows that this coupling constant is in accordance with Bent's rule [16] as was concluded for ~JP-w in tungsten carbonyl compounds [ 15]. Both coupling constants seem to be dominated by rehybridization effects around the phosphorus atom under the influence of inductive mechanisms. It is noteworthy that the zero value for Jt,~,,8 in (C6HsO)3PCr(CO)s fits very well into the correlation of this coupling constant with ZO~x and supports the proposed sign inversion in the series (C6Hs)" C13 _. TABLE 4

Correlation data o f 2JF_M_.C(O)ax (Jtmm) w i t h several parameters y

X = Jfrarul

Equation

r2

o(CO) a 2;0*

Y = 3 . 3 2 x + 1999 Yffi0.27 x + 2 . 3

0.95 0.90

kax keq Ao (Graham)

Y =0.06 x +16.1 Y=O.05 x +16.2

0.90 0.96

Y =0.035 Y =0.015 Y =0.017 Y =0.032

0.95 0.59 0.95 0.90

~ (Graham) Ad (Dobson) A~ ( D o b s o n )

~c!--0.70 x + 1.08 x --0.35 x + 0.73

",,~"~"~ = 1/5 (A, (eq) + A, (ax) + S, + 2 E ] .

53

PCr(CO)s. The good agreement between Jt~an, and v(CO), k ~ and keq, allows us to suggest that even mesomeric (~) influences are of interest. The equations for A o and A ~ show that the coupling constant increases on increasing z-acceptot and decreasing o-donor capacity of the ligand. So Jtra,, depends on the electron withdrawing character of the ligands by inductive or mesomeric action. The values of Jc~ and J w - c , even in a more extended series of ligands, do not show great variations. As was noted for other coordination compounds, spin---spin coupling between t w o atoms in trans position is larger and more sensitive to substituent effects than the analogous c/s coupling. ACKNOWLEDGEMENT

The authors wish to thank Mr. T. Haemers and Mr. F. Persyn for recording the IR, Raman and NMR spectra. REFERENCES 1 H. B. Gray, E. Billig, A. Wojcicki and M. Farona, Can. J. Chem., 41 (1963) 1281. 2 L. F. Wuyts, G. P. Van Der Kelen and Z. Eeckhaut, J. Mol. Struct., 33 (1976) 107. 3 M. Bigorgne, J. Inorg. Nucl. Chem., 26 (1964) 107. 4 R. J. Angelici, J. Inorg. Nuel. Chem., 28 (1966) 2627. 5 W. D. Horroeks, Jr. and R. C. Taylor, Inorg. Chem., 2 (1963) 723. 6 E. W. Abel, M. A. Bennett and G. J. Wilkinson, J. Chem. Soc., (1959) 2323. 7 C. Barbeau and J. Turcotte, Can. J. Chem., 48 (1970) 3583. 8 W. A. G. Graham, Inorg. Chem., 7 (1968) 315. 9 R. A. Brown and G. R. Dobson, Inorg. Chim. Aeta, 6 (1972) 65. 10 D. M. Adams, Metal--Ligand and Related Vibrations, E. Arnold, London, 1967. 11 F. T. Delbeke and G. P. Van der Kelen, J. Organomet. Chem., 64 (1974) 239. 12 L. F. Wuyts and G. P. Van der Kelen, J. Organomet. Chem., 129 (1977) 163. 13 F. A. Cotton and C. S. Kraihanzel, J. Am. Chem. Soc., 84 (1962) 4432. 14 F. T. Delbeke and G. P. Van der Kelen, J.Organomet. Chem., 23 (1970) 497. 15 R. L. Keiter and J. G. Verkade, Inorg. Chem., 8 (1969) 2115. 16 H. A. Bent, Chem. Rev., 61 (1961) 275. 17 O. A. Gansow and B. Y. Kimura, Chem. Commun., (1970) 1621. 18 O. A. Gansow, B. Y. Kimura, G. R. Dobson and R. A. Brown, J. Am. Chem. Soc., 93 (1971) 5922. 19 P. S. Braterman, D. W. Milne, E. W. Randall and E. Rosenberg, J. Chem. Soe., Dalton Trans., (1973) 1027. 20 G. M. Bodner, Inorg. Chem., 14 (1975) 2694. 21 M. F. Guns, E. G. Claeys and G. P. Van der Kelen, J. Mol. Struct., in press. 22 C. G. Swain and E. C. Lupton, Jr., J. Am. Chem. Soc., 90 (1968) 4328. 2 3 R . D. Bertrand, F. B. Ogilvie and J. G: Verkade, J. Am. Chem. Soe., 92 (1970) 1908. 24 T. A. Magee, C. N. Matthews, T . S. Wang and J. H. Wotiz, J. Am. Chem. Soc., 83 (1961) 3200. 25 R. Mathieu, M. Lenzi and R. Poilblanc, Inorg. Chem., 9 (1970) 2030. 26 E. O. Fisher and L. Knauss, Chem. Ber., 102 (1969) 223.