5852
Organometallic Conformational Equilibria, XI. cis-trans Isomerism and Stereochemical Nonrigidity in Cyclopentadienylmolybdenum Complexes1'* J. W. Faller and A. S. Anderson Contribution f r o m the Department of Chemistry, Yale University, New Haven, Connecticut 06511. Received March 16, 1970 Abstract: Kinetic and thermodynamic parameters for the cis-trans interconversion of compounds having the general formula T-C~H~MO(CO)~LR, where L = PPh3, P(n-but)s, PMezPh,P(OMe)3,P(OPh)3; R = H, D, Me, PhCHz, were C1, Br, I, have been evaluated using a combination of pmr techniques. The equilibrium constants, [cis]/[trans], generally found to increase in the order R = PhCHz < Me < H, D < I < Br < C1; L = P Me2Ph PPh3 P(but), < P(OMe)3 < P(OPh)3,with greater effects observed upon changing R. Although the trans + cis barrier, AF*25, is only slightly dependent on the nature of L, it is strongly dependent on the nature of R and tends to increase in the order H, D (12-14 kcal/mol) < Me, PhCH2, (19-23 kcal/mol) < C1, Br, I(22-26 kcal/mol). A model incorporat-
-
N
ing square pyramid-trigonal bipyramid-square pyramid interconversion has been proposed, and the possible conformational interchanges have been considered from a topological viewpoint. The lowest energy pathway appears to involve an intermediate approximating a trigonal bipyramid with the cyclopentadienyl ring occupying one apical position and either R or L occupying the other. ompounds having the general formula of a-C5H6MO(CO)~LRmay be formally regarded as sevencoordinate complexes of molybdenum with the cyclopentadienyl ring occupying three coordination positions. Sufficient crystallographic data are now available to assure that the idealized lowest energy configuration would involve the remaining ligands located approximately at the corners of a square, such that a structure similar to a square pyramid would result. This square-
pyramidal geometry suggests the possibility of the existence of two geometric isomers (see Figure 1). Although the majority of these compounds are present in solution predominantly as one isomer, in favorable cases appreciable concentrations of both isomers ha\ e been found. A n equilibrium between these isomers has now been observed where L = a phosphine o r phosphite and R = H, D, Me, PhCH2, C H 3 0 C H 2 , CH3SCH2, C1, Br, or I. Our studies of the rearrangement mechanism re( I ) Part X : J. W. Faller, M. E. Thomsen, and M. J . Mattina, sponsible for the stereochemical nonrigidity of r-allyl submitted for publication. complexes of molybdenum and tungstenI8 led us to (2) This research was supported in part by the Connecticut Research Commission and the Petroleum Research Fund, administered by the investigate the nature of the rearrangement process American Chemical Society. Grant No. GP-6938 of the National in these analogous T - C ~ H J ~ O ( C O ) ? Lcompounds. R In Science Foundation provided funds for the Varian HA-100 spectrometer this paper we provide further verification of the aswhich was used in portions of this work. (3) An irregular structure may often be interpreted as a distorted signments of cis and trans isomers, demonstrate that form of a number of diferent more regular structural arrangements. the rearrangement process which interconverts isomers The choice of a particular idealized structure is somewhat arbitrary, and although it might be argued that another structure is more approis intramolecular, and report thermodynamic parampriate, it is convenient here to consider these particular compounds as eters obtained from the temperature dependence of bcing derived from a 3 : 4 type of coordination geometry or from a the equilibrium constants and activation parameters squarc-base-trigonal-cap coordination geometry. Obviously, the steric interactions between the ligands, particularly since some have obtained from fitting of the temperature dependence vastly difierent steric requirements than carbonyls, will cause distorof the rates to the Arrhenius and Eyring equations. tions from the idealized coordination geometry. Nevertheless, it is Since rates have been obtained in many cases ober a apparent from the solid-state structures that the 3 : 4 structure is more appropriate than the 3 :3 : 1 or monocapped-octahedra1 one. If the temperature range i n excess of looo, these barriers to four ligands besides the r-cyclopentadienyl moiety are identical, pseudorearrangement are believed to be some of the most C,,. symmetry is observed, e.&., T - C ~ H ~ V ( C O or) ~T-CsHsNb(C0)i.E ~ Despitc distortions the following examples provide convenient models accurate yet measured for intramolecular processes in for these idealized "square-pyramidal" structures: a-CsHaMo(CO)aorganometallic molecules. C?Hj,fi T - C ~ H ~ M O ( C O ) ~ T C-IC , ~~ H ~ M O ( C O ) ~ C H Z C O T-CSH~MOZH,~
C
~-CSH~MO(CO)B-S~[T-CSHSF~(C~)ZIZ(CO).,C3Fi,9[~-CjHsMo(C0)3]~,~0 CI," [x-CjHsMo(CO)p]p-~-P(CH~)~-~-H,' ~-CsHsMo(C0)z(PPha)C0- Experimental Section CHJ," [A-C:H~W(CO)~]Z, l 3 x-CsHsW(C0)3Au(PPh3)," [T-Clo HaMOInfrared spectra of cyclohexane solutions of all compounds were (CO)aCH3]?,l5 T-C~OH~MO:(CO)G,'G X-CIIHI~MO?(CO)G,'G and r-CsH5MO(CO)~(CH+&HICH~). 17 I n each of these cases, an approximately measured where solubility permitted. A Perkin-Elmer 421 grating square projection is obtained if the atoms which are directly attached to instrument was used with calibration from polystyrene film. Carthe nietal are projected onto the plane o f the n-cyclopentadienyl ring. (4) J . B. Wilford, A. Whitla, and H. M. Powell, J . Organometal. (11) J . E. O'Connor and E. R. Corey, J . Amer. Chem. SOC.,89, 3930 C h ~ m .8, , 495 (1967). (1967). ( 5 ) R . J. Docdens and L. F. Dahl, J . Amer. Chrm. Soc., 87, 2576 (12) M. R. Churchill and J . P. Fennessey, Inorg. Chem., 7 , 953 (1965). (1968). (6) M. J. Bennett and R . Mason, Proc. Chem. Soc. London, 273 F. C. Wilson and D. P. Shoemaker, Naturwissenshaffen, 43, 57 (13) (1963). ( 195 6). (7) S. Chaiwasic and R. H. Fenn, Acta Crjslallogr., Sect. B, 24, 525 (14) J. B. Wilford and H . M. Powell, J . Chem. SOC.A , 8 (1969). (1968). (15) P. H. Byrd and M. R. Churchill, Inorg. Chem., 7, 349 (1968). ( 8 ) 1. I50 1986,1916 >50 1989, 1905 9 . 2(8. 5b) 1988, 1905 2.9(2.4b) 1984,1913 0.30(0.34)b 1975,1886a >50 1974,18865 24 1971,1892O 0.86b 1975, 1883 > 50 1969,1884 7.6 1976, 1889 2.8b 1967, 1894 0.59(0,63p 1962,1889 0.18(0.15p 1955,1874 0.17(0.19) 1947, 1871 0 . 0 @[ O . 06)b 1943,1868
