Organometallics 1995, 14, 151-156
151
Ab Initio Molecular Orbital and Experimental Studies of Hydride Addition to Phosphine-SubstitutedManganese Carbonyl Complexes Dermot F. Brougham, David A. Brown,* Noel J. Fitzpatrick, and William K. Glass Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland Received June 29, 1994@ Ab initio molecular orbital calculations on [Mn(Co)61+,[Mn(CO),(PH,)]+, and the formyl and hydride complexes derived from these show that the increased stability of cis-Mn(CO)c(PRd(CH0) and similar formyl complexes is largely kinetic in origin. The transition state for the unsubstituted formyl complex, which forms the corresponding hydride by a simple dissociative mechanism, differs from the transition states for the substituted formyl complexes, which form the corresponding hydrides by concerted mechanisms. Experimental studies of hydride addition to diphenylphosphinoalkane-substitutedhexacarbonylmanganese cations are in accord with the theoretical conclusions.
Introduction For nearly two decades, there has been continued interest in the hydride reduction of metal carbonyl complexes, motivated a t least in part by interest in the Fischer-Tropsch reaction involving hydrogenation of carbon monoxide catalyzed by transition metal comp0unds.l In a number of cases, e.g., hydride addition t o [CpFe(C0)31+, low-temperature spectroscopy has provided clear evidence for the intermediacy of metal formyl complexes such as CpFe(C0)2(CH0).2 In the case of the prototype manganese formyl, Mn(C0)5(CHO),no spectroscopic evidence has yet been obtained for its existence although it is strongly implicated as an intermediate in both the reaction of NaMn(C0)s with acetic [13Clformicanhydride3 and the substitution reactions of Mn(C0)sH with labeled C0.4 In general, attempts to prepare stable metal formyls by CO insertion into metal hydrogen bonds (pathway a) have been
unsuccessful in contrast to the well-documented CO insertion into metal-alkyl bonds; this failure is attributed to the greater M-H bond strength compared t o the homologous M-C bond^.^^^ It is still not clear whether the reverse reaction (pathway b) of a metal formyl to form the corresponding metal hydride with loss of CO is determined by thermodynamic factors or is kinetic in origin. The balance between these will depend on the mechanism of CO loss. For example, third-row metals (e.g., Re) form stronger metal ligand bonds than first row (e.g., Mn), so if the loss of CO from @Abstractpublished in Advance ACS Abstracts, November 1,1994. (1)Pichler, H.; Schulz, H. (?hem.-Ing.-Tech. 1970,42,1162. (2)Brown, D.A.; Glass, W. K.; Ubeid, M. T. Inorg. Chim.Acta 1984, 89,L45. (3)Fiato, R.A.; Vidal, J. L.; F'ruett, R. L. J . Organomet. Chem. 1979, 172,C4. (4)Byers, B. H.; Brown, T. L. J. Organomet. Chem. 1977,127,181. ( 5 ) Berke, H.; Hoffmann, R. J. Am. Chem. SOC.1978,100,7224. (6)Ziegler, T.; Versluis, L.; Tschinke, V. J. Am. Chem. SOC.1988, 108,612.
the formyl proceeds by a dissociative mechanism, then third-row metal formyls should have greater kinetic stability as discussed by G l a d y ~ . ~ The nature of the L, ligands is also very important, and as noted above, if the ligands L, are replaced by a n-acid such as a Cp ring, the formyls can be observed spectroscopically and only lose CO to form the corresponding metal hydrides on raising the temperature. In the case of Mn(C0)5(CHO),replacement of some of the carbonyl groups by phosphines and phosphites leads to the formation of stable metal formyls. For example, the crystal structure of [Mn(C0)2(P(OPh)3)3(CHO)lhas been reported8 and a range of the trans complexes, [Mn(CO)5-,(PPh3),(CHO)l, n = 1,2, ~ r e p a r e d .Clearly, ~ replacement of CO by weaker n-acceptors such as phosphites and phosphines, which should lead to increased electron density on the metal, also leads to enhanced stability of the corresponding metal formyls. The object of the theoretical part of this paper is to examine in detail the various factors influencing the relative stabilities of a series of manganese formyls and hydrides of the above types and, in particular, to assess the relative importance of thermodynamic and kinetic factors. Ab initio calculations were completed on the cationic parent complexes, [Mn(C0)61+and [Mn(C0)5(PH3)1+,on the three daughter formyl complexes, Mn(C0)5(CHO), cis-Mn(C0)4(PH3)(CHO),and trans-Mn(C0)4(PH3)(CHO),and also on the corresponding hydride complexes, Mn(C0)5H, cis-Mn(CO)r(PH3)H,and transMn(C0)4(PH3)H. In addition, stationary points for the decomposition of each of the three formyl complexes to the corresponding hydride complexes were located.
Computational Details Ab initio molecular orbital calculations on the species I-XI were carried out using the Gaussian 92 program.1° The stationary points were initially located at the Hartree-Fock level using the 3-21G** basis set.'l (7)Gladysz, J. A. Adv. Organomet. Chem. 1982,20,1. ( 8 )Berke, H.; Huttner, G.; Scheidsteger, 0.; Weiler, G. Angew. Chem., Int. Ed. Engl. 1984,23,737. (9)Gibson, D.H.; Owens, K.; Mandal, S. K.; Sattich, W. E.; Franco, J. 0.Organometallics 1989,8,498.
0276-733319512314-0151$09.00/0 0 1995 American Chemical Society
Brougham et al.
152 Organometallics, Vol. 14, No. 1, 1995
energy process; thus, the stationary state is considered a minimum. A similar result was obtained for CpFeI II (C0)2(CH0).13 Mn(C0)5(CHO) cis-Mn(C0)4(pEQ)(CHO) ~ ~ ~ ~ s - M ~ ( C O ) ~ ( P H ~ ) ( C H O ) The complex cis-Mn(C0)4(PH3)(CHO)(IV)has all In IV V frequencies real and thus is a minimum. However trans-Mn(C0)4(PH3)(CHO)(V) has one imaginary freMn(C0)5H n's-Mn(CO)4(PHg)H trans-Mn(CO)$(PH3)H quency, corresponding to a movement of two mutually VI VI1 VI11 trans carbonyls towards the phosphine and the other T.S. T.S. T.S. carbonyls away from it. Thus the minimum of this Lx X XI species has a slightly distorted octahedral structure. The three hydride structures (VI-VIII) have imagiThese points were characterized by harmonic vibranary frequencies. However, Mn(C015H is a stable tional analyses at the HF/3-21G** level (designated by molecule with geometry14quite close to the calculated HF). The geometrical parameters were then refined HF geometry. Calculations of the frequencies at the using second-order Mgller-Plesset perturbation calcuMP2 level were computationally too expensive. lations for all the species considered, except the transiStationary points along the decomposition pathway tion states (E-XI), using the 3-21G basis set with d of Mn(C0)5(CHO)(111)to Mn(C0)5H (VI)and CO and polarization function on phosphorus only, MP2/3-21G for cis- and trans-Mn(C0)4(PH&CHO)(IVand V) to the (d p) (designated by MP2). The zero-point corrections corresponding hydrides VI1 and VI11 and CO were were estimated from HF harmonic vibrational wavelocated a t the HF level, using the constraint, as noted number calculations. above, of keeping the 90" angles at Mn. Due to this All the bond angles at Mn were assumed to be 90" constraint the "transition states" exhibited three imagithroughout. The formyl groups were kept planar. This nary frequencies. In the cis transition state X these assumption is justified by published X-ray data.8 All were at -934, -392, and -156 cm-l, essentially corthe carbonyl groups were assumed t o be linear. C-0 responding to in-plane H migration, out-of-plane H and M-C bond lengths of the carbonyl groups were migration, and formyl group rearrangement. In the considered equal if they possessed the same trans group, trans transition state XI, the corresponding frequencies and the geometry of the PH3 group was frozen throughwere at -914, -335, and -242 cm-l, with assignments out at the standard value.12 The other geometrical as in the cis case. parameters were allowed to optimize freely. Table 1 gives the total and relative energies for These choices were dictated by computational limitavarious species, at both HF and MP2 levels, with and tions. Similar choices have been adequate in discussing without ZPE corrections. From these values it is noted hydride attack on [CpFe(C0)31+.13 that, at both the HF and the MP2 levels, the cis isomers Initial calculations indicated that the energy surface have lower energies than the trans isomers for the for rotation of the formyl group is almost flat in formyl and hydride complexes. The extra stability is agreement with the conclusions for the CpFe(C0)spossibly due to weaker x accepting ligands trans to CO (CHO) system.13 At the HF level optimization of Mnenhancing the x-back-bonding t o the carbonyls. This (COk(CH0) (111)gave the eclipsed form, which is less mechanism to increase the thermodynamic stability of than 1 kcal mol-l lower in energy than the staggered the cis isomers is vindicated by the details of this study form. The cis-Mn(C0)4(PH3)(CHO)complex (IV)optidiscussed below. mized to the eclipsed form, with the formyl hydrogen Table 1 also gives the enthalpies for the formation syn to the PH3 group. The trans-Mn(C0)4(PH3)(CHO) (Mf) [Mn(C0)5(L)l+ H- Mn(C0)4(L)(CHO) reaction complex optimized t o the eclipsed form. Preliminary and for the dissociation reaction ( m d ) Mn(C0)4(L)calculations, at the HF level, showed that rotation of (CHO) Mn(C0)4(L)H CO, where L = CO, PH3. AHf the phosphine group is a very low energy process FO.01 and were calculated from the MP2 results, and a kcal mol-') in these systems. Thus, the phosphine zero-point correction calculated at the HF level, due t o group was rotationally frozen during the geometry computational limitations, was included. The results optimizations a t values which maximize the distances in Table 1 show that the formation of the formyl is from the phosphine hydrogens to the atoms of the other exothermic in all three cases, and the results in the ligands. three series differ little. The evidence for the existence of Mn(C0)5(CHO)is indirect, yet phosphine-substituted Results and Discussion formyls exist. Thus AHf, whose absolute value is Energies. At the HF level both the parent cations I greater in the unsubstituted case, does not reflect the and I1 are minima, having all real frequencies. Mninstability of Mn(C0)5(CHO). The theoretical enthal(CO)s(CHO) (111) has one imaginary frequency corpies of formation presented here compare favorably with responding to a rotation of the formyl group. As values in other studies. In CpFe(C0)2(CHO), a cormentioned above, rotation of the formyl group is a lowresponding value of -202 kcal mol-l was ca1c~lated.l~ Lane and Squires15obtained a value of -194 kcal mol-l (10)Frisch, J. M.; Head-Gordon, M.; Schlegel, H. B.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; De Frees, D. J.; Fox, D. J.; Whiteside, for the gas phase enthalpy of formation of [Fe(C0)4R. A.; Seeger, R.; Melius, C. F. Baker, J.; Kahn, L. R.; Stewart, J. J. (CH0)I- from the corresponding carbonyl complex and P.; Fluder, E. M.; Topiol, S.; Pople, J. A. GAUSSIAN 92;Gaussian H-. Similarly, the M d values do not explain the Inc.: Pittsburgh, PA, 1992. (11)Binkley, J.S.;Pople, J. A,; Hehre, W. J. J . A m . Chem. SOC.1980, increased stability of the formyls with monosubstitution 102.939. bfn(CO)6j+
[Mn(CO)5(PH3)]+
-
(12)Hehre, W.J.;Ditchfield,R.; Stewart, R. F.; Pople, J. A. J.Chem. Phys. 1970,52,2769. (13)Brown, D. A.; Fitzpatrick, N. J.; Groarke, P. J.; Koga, N.; Morokuma, K.Organometallics 1993,12,2521.
+
-
+
(14) La Placa, S. L.; Hamilton,W. C.; Ibers, J. A.; Davidson, A. Znorg. Chem. 1969,9,1928. (15)Lane, K. R.; Squires, R. L.Polyhedron 1988,7,1609.
Phosphine-SubstitutedManganese Carbonyl Complexes Table 1. Energetics HF
MP2
Total Energies (au) [Mn(Co)sl+ (1) [Mn(CO)s(PH3)1+ H-
-1816.91034 -2045.49798 -0.40042 -112.09330
co
Relative Energies (kcal mol-')" [Mn(CO),# (I) H0.0 (0.0) Mn(CO)s(CHO) (m) -181.5 (-175.1) Mn(C0)sH (VI) CO -141.2 (-139.5) Mn(CO)5(CHO)(M, TS) -154.7 [Mn(CO)s(PH3)lf (II) + H0.0 (0.0) cis-Mn(C0)4(PH,)(CHO) (IV) -176.1 (-169.6) -175.0 (-168.4) trans-Mn(C0)4(PH3)(CHO) (V) cis-Mn(CO)4(PH3)H(VII) CO -134.7 (-133.2) trans-Mn(C0)4(PH3)H (VIII) f -130.7 (-129.6)
+
+
+
co
-1818.09157 -2046.64128 -0.40638 -1 12.30484
0.0 (0.0) -228.1 (-221.7) -233.2 (-231.5) 0.0 (0.0) -217.2 -202.2 -221.7 -201.0
(-210.7) (-195.6) (-220.2) (-199.9)
cis-Mn(C0)4(PH3)(CHO)(X,TS) - 136.4 rrans-Mn(C0)4(PH3)(CHO) -138.8 (XI, TS) Thermochemical Datab (kcal mol-') m f
unsubstituted complexes cis-substituted complexes trans-substituted complexes
-221.7 -210.7 -195.5
m
d
-9.8 -9.5 -4.3
L2 Values with HF ZPE corrections in parentheses. Values at the MP2 level with ZPE corrections.
by phosphines, since A H d for the unsubstituted and cissubstituted complexes differ by an insignificant amount, -9.8 and -9.5 kcal mol-l, respectively. It is noted that it is the cis-monosubstituted form that is found experimen tall^.^ Lane and Squires15 recently estimated from published thermochemical data that the decomposition of [Fe(C0)4(CHO)I- to [Fe(C0)4Hl- and CO was moderately exothermic, -21 kcal mol-l. This compares favorably with the values of m d in this study. These small exothermic values for suggest that the vie+,6 that large metal hydrogen bond strengths render CO insertion into neutral metal hydride complexes difficult and that neutral formyl complexes are thermodynamically unstable is suspect. On the other hand, the values for the trans-monosubstituted complexes do differ substantially from both d the cis-monosubstituted and unsubstituted forms. m for the trans-substituted complexes is -4.3 kcal mol-l, which is significantly less exothermic than the previous values of -9.8 and -9.5 kcal molp1. This thermodynamic difference is due to the trans geometry being less favorable in the hydride complex than in the formyl complex because of the presence of two strong donor ligands along one coordinate axis in the trans hydride, an arrangement which leads to electron density being forced back into the phosphine ligand. The optimized geometries at the MP2 level for species I-VI11 and at the HF level for the transition states MXI are given in Figure 1. The effects of substituting CO with PH3 are clearly evident. For example, the MnC(0) bonds trans to PH3 in [Mn(CO)dPH3)1+are shorter than in [Mn(C0)6]+, and correspondingly, the C - 0 bonds in the phosphine-substituted species are longer, in accord with the presence of the phosphine increasing back-bonding to the other carbonyls, particularly in the trans position. A similar effect is observed on those bond lengths in Mn(C0)sH compared to [Mn(CO)6]+(Figure 1). Again comparing the cis and trans hydrides VI1 and VIII,the Mn-C(0) bonds trans to PH3 and H are shorter than
Organometallics, Vol. 14,No. 1, 1995 153 those which are cis, and the corresponding C-0 bonds are longer, suggesting that the relative thermodynamic stability of VI1 is due to enhanced back-bonding to the carbonyls trans to substituents of less back-bonding ability. In the case of the formyls 111-V,the calculated Mn-C(0) and C-0 bond lengths are shorter and longer, respectively, than those in [Mn(CO)6]+,and moreover, the Mn-C(0) bond lengths in I11 are shorter than those in [Mn(C0)5(PH3)lf(11),indicating greater back-bonding to carbonyls in the neutral formyl species I11 than in the cationic phosphine complex 11. A comparison of the cis and trans species IV and V is also of interest (see Figure 1). In trans-Mn(C0)4(PH3)(CHO)(V),the MnC(0) and C-0 bond lengths trans to CO are 1.762 and 1.187 A,respectively, whereas in the cis complex IV the Mn-C(0) trans to PH3 is 1.695 A and to CHO is 1.720 A. Thus, carbonyls trans to PH3 and CHO show enhanced back-bonding leading to a slightly greater thermodynamic stability for cis-Mn(C0)4(PH3)(CHO) (rv> than the trans isomer V. The calculated Mn-CHO bond lengths may be discussed in terms of the relative contribution of the carbenoid resonance form B (Figure 2). In general, unsubstituted anionic formyls such as [Cr(C0)5(CHO)I- and [Fe(C0)4(CHO)]-l7 are stabilized by significant contributions from the carbenoid resonance form D,' as indicated by their unusually low formyl CO stretching frequencies, -1570 cm-l as opposed to the value of -1600 cm-l for neutral formyls, which thus involve a smaller contribution from the carbenoid form B. Comparing Mn(C0)5(CHO)(111)with the correspondingcis and trans PH3-substituted formyls IV and V, it is immediately obvious that phosphine substitution leads t o an increase in electron density on the metal and hence increased importance from (B). Thus, in Mn(C0)5(CHO),r(Mn-CHO) = 2.024 A,while in cis- and trans-Mn(C0)4(PH3)(CHO) the values of r(Mn-CHO) are 1.998 and 1.933 A,respectively. Similarly, without phosphine substitution, r(C(H)-0) = 1.239 A, while in the cis- and trans-Mn(CO)dPHs)(CHO), r(C(H)-0) = 1.243 (IV)and 1.245 A (W. Kinetic Factors. The similarity of the above calculated enthalpies ( m d ) suggests that the difference in stabilities of the unsubstituted formyl Mn(C0)5(CHO) and the corresponding cis and trans monophosphinesubstituted formyls is not thermodynamic in origin and may be kinetic. Accordingly we searched for transition states for decomposition of the formyl species 111-V to their corresponding hydrides VI-VIII. The respective transition states which we located using the basis sets discussed above are shown in M-XI. The transition states X and XI are similar, and in both cases, the geometry of the formyl group has changed from that of the parent formyl, with the formyl hydrogen now being less associated with the carbon and more with the metal and with the C-H bond length being significantlylonger and the Mn-H bond significantly shorter while the MnCH bond angle is very small and the formyl MnCO bond angle approaches 180". These transition states correspond t o a concerted mechanism for hydride formation and are in accord with experimental evidence for the decomposition of both monophosphine- and diphosphine-substituted metal formyls to their corresponding hydrides. For example, the decomposition (16)Casey, C . P.; Neumann, S. M. J.Am. Chem. Soc. 1976,98,5395. (17) Collman, J.P.; Winter, S. R.J.Am. Chen. SOC.1973,95,4089.
Brougham et al.
154 Organometallics, Vol.14,No.1, 1995
# ,
H3
P
E..
0
c-0
0
0
-
2411 A
Figure 1. Optimized geometries at the MP2 level of the complexes of this study. Bond lengths are in angstroms and bond angles in degrees. Decomposition transition states are at the HF level.
of the neutral stable formyl Mn(C0)2(P(OPh)3)3(CHO) to the corresponding hydride shows a clear kinetic isotope effect, k H / k D = 3.24 consistent with considerable C-H bond breaking in the transition state.8 In the case of the formation of trans-[RuH(CO)(dppb)21(dppb = 1,4bis(diPhenYlPhosPhino)butane,PPhZ(CH2)4PPhZ) from trans-[Ru(CHo)(Co)(dPPb)zl,Barr& and Cole-l+"nton18 invoked a three-centered transition state to explain an isotope effect of k H / k D = 2.3. Their transition state is very similar to that calculated above. In
addition, our own experimental studies described below also indicate a different mechanism for hydride formation as between unsubstituted and phosphine-substituted formyl complexes. In the case of the unsubstituted formyl, Mn(C0)5(CHO), we were not able to locate a transition state of the above type. Instead, a transition state was located (18)Barratt, D.S.;Cole-Hamilton, D.J.J.Chem. Soc., Dalton T~uns. 1987,2683.
Phosphine-SubstitutedManganese Carbonyl Complexes Neutral:
+
M-C
M=C H'
/O-
Organometallics, Vol. 14,No. 1, 1995 155 Table 2. Low-Temperature (-65 "C) 'H NMR Data (in ppm) from the Reactions of [Mn(CO)Xdppx)lClO,, (dppx = Bis(diphenylphosphinoa1kane) with NaB& in Acetone46 dDDX
H'
WH) Anionic:
-
M-C
H
Yo '
L
dPPm 14.66)
J(W(Hz) /O-
M-C
(C) Figure 2. Significant resonance forms.
thermal stability (fin)
H'
( D)
as shown in M in which one carbonyl group has almost fully dissociated from the metal, r(MnC(0)) = 8.610 A, while the geometry of the formyl group remains close to that of the parent formyl complex at the HF level. This result suggests that the unsubstituted formyl decomposes to the corresponding hydride by a mechanism different from that of the phosphine-substituted formyls. It appears that initial carbonyl dissociation occurs, possibly with subsequent migration of the CO of the formyl group and formation of the corresponding hydride. Unfortunately we were not able to locate any other transition states to support this argument, so it must remain speculative. Nevertheless, the phosphinesubstituted complexes, IV and V,clearly have different transition states from I11 involving a concerted mechanism and so we conclude that the difference in stability between the unsubstituted and phosphine-substituted metal formyls is kinetic in origin with thermodynamic factors only playing a small part. Comparison with Experiment. Hydride Addition to [Mn(C0)4(PPh2(CH2),PPh2)lC104 (n = 1, dppm; n = 2, dppe;n = 3, dppp). In view of the above theoretical results, it was of interest to compare them with detailed studies of the reaction of a hydride donor such as borohydride with the related series of diphosphine-substituted manganese carbonyl complexes, [Mn(C0)4(PPh2(CH2)nPPh2)1C104, n = 1,2,and 3,using lowtemperature lH NMR spectroscopy to identify intermediates as in analogous studies of hydride addition to [(q5-C5H5)Fe(C0)31+ and [(715-CgH,)Fe(C0)31+.19 Room-Temperature Studies. Addition of NaBH4 t o a solution of [Mn(CO)4(dppe)lC104in THF or acetone gave an immediate color change from yellow to a redl brown suspension and formation of fuc-[Mn(C0)3(dppe)Hl as indicated by replacement of the starter v(C0) infrared peaks with bands a t 2003, 1928, and 1915 cm-l in agreement with published values.20 Attempts to isolate and purify [Mn(CO)s(dppe)H]were unsuccessful but the lH NMR spectrum of the product contained a metal-hydride peak at 6 -8.0 ppm (t, J 46.5 Hz) identical with the published value.21 Similar results were obtained with [Mn(CO)4(PPh2(CH2),PPh2)1c104, n = 2 and 3, but again attempts to isolate the hydrides were unsuccessful. Low-Temperature Studies. The reaction of [Mn(COk(dppe)lC104 and NaBH4 in a 2:l molar ratio in acetone-ds was carried out at -80 "C by use of tech(19)Ahmed, H. A.; Brown, D. A.; Fitzpatrick, N. J.; Glass W. K. Inorg. Chim. Acta 1989,164, 5 . (20)Booth, R. L.; Hazeldine, R. N. J. Chem. SOC.A 1966,157. (21) Lapinte, C.; Cathelin, D.; Astruc, D. Organometallics 1988,7 , 1683.
15, RTa
dPPe 13.4(t),4 H 15.2(d), 1 H (t) 10.3 (d) 12.5 (t) 15, RT (d) 5, RT
dPPP 14.5(s), 12 H 13.7(d), 1 H (d) 8.9
(s) 15, RT (d) ]+only one metal formyl at 6 14.6ppm was observed (see Table 2). During the preparation of this paper, a paper by Orchin and co-workers22was published reporting a
156 Organometallics, Vol. 14,No. 1, 1995 room-temperature study of the same systems as above but in a different solvent mixture (CH3CN/CH30W HzO). This enabled them to isolate both the mer- and fac-metal formyls and record spectroscopic data before decomposition occurred together with an X-ray structure of the methoxymethyl derivativefm-(dppb)Mn(C0)3CHzOCH3. Low-temperature lH NMR studies were not reported. Unlike the acetone system of our study, they found the formyl complexes decomposed directly to the hydride complexes. The lH data for the complexes of their study are similar to ours, given that a different solvent was used. It appears therefore that the reaction is solvent dependent, indicating that the decomposition of both diphosphine and monophosphine formyls is under kinetic control.
Experimental Section Solvents were freshly dried by standard methods. All reactions and workup were carried out under high-purity nitrogen. Tertiary diphosphines (dppm, dppe, and dppp) were (22) Mandal, S. K.; Krause, J. A.; Orchin, M. J . Organomet. Chem. 1994, 467, 113.
Brougham et al. obtained commercially and used without further purification. Infrared spectra were measured using a 0.1 mm CaFz cell on a Perkin-Elmer 1720FT spectrometer linked to a 3700 data station. lH, 13C, and 31PNMR spectra were recorded on a JEOL GX270 spectrometer. The complexes [Mn(CO)4(dppx)]c104, x = m, e, and p, were prepared by published methods.23
Conclusion The increased stability of monosubstituted phosphine carbonyl formyl complexes is shown theoretically to be largely kinetic in origin and to arise from quite different transition states between the unsubstituted formyl which forms the corresponding hydride by a simple carbonyl dissociative mechanism and the monophosphine-substituted formyls which form the corresponding hydrides by a concerted mechanism. Experimental studies are in accord with the theoretical conclusions. Acknowledgment. We thank Ms. Geraldine Fitzpatrick for her expert help with the NMR studies and also the reviewers for helpful comments. OM940511S (23) Carriedo, G. A.; Riera, V.; Santamaria, J. J . Orgunomet. Chem. 1982,234, 17.
