Alkynes rotation

Steric factors probably prohibit simultaneous rotation of the olefin and alkyne C2 units which would crowd all four metal-bound carbons into the same plane. Separate rotation of each unsaturated ligand was explored theoretically using the EHMO method. Rotation of the olefin destroys the one-to-one correspondence of metal-ligand tt interactions. Overlap of the filled dxz orbital with olefin n is turned off as the alkene rotates 90°, creating a large calculated barrier for olefin rotation (75 kcal/mol). Alkyne rotation quickly reveals an important point the absence of three-center bonds involving dir orbitals allows the alkyne to effectively define the linear combinations of dxy and dyz which serve as dn donor and dir acceptor orbitals for 7T and ttx, respectively. Thus there should be a small electronic barrier to alkyne rotation (the Huckel calculation with fixed metal... 

Dynamic NMR studies of W(CO)(HC=CH)(S2CNEt2)2 and related complexes (58) indicate that the barrier to alkyne rotation is around 11-12 kcal/mol (Table IV). Discordant metal-ligand 77 interactions anticipated as the alkyne rotates away from the ground state orientation are evident in EHMO calculations (Fig. 23). When the alkyne is orthogonal to the M—CO axis the dyz orbital is stabilized by CO it but destabilized by alkyne ir while dxy becomes the lowest lying dir orbital due to... 

Fig. 23. Electronic factors influencing dir orbital energies and alkyne rotational barriers.

Furthermore, no exchange of the two independent olefin proton sites in either isomer was observed, indicating that olefin rotation is not ocurring at 77°C. Thus, the experimental results are consistent with molecular orbital expectations the barrier to olefin rotation is large while the barrier to alkyne rotation is small. 

More subtle effects determine alkyne rotational preferences in symmetric CpM(RC=CR)L2 complexes with identical L and L ligands as described by Hoffmann and co-workers (145). The extensive molecular orbital discussion of W(CO)2(RC=CR)LX2 complexes in Section VI described the factors which remove the dicarbonyl complexes from the simple guidelines appropriate for analysis of monocarbonyl complexes. The experimental barrier of 9.1 kcal/mol measured for [CpMo(MeC=CMe)-(PMePh2)2]+ is lower than those of related L = CO, L = PR3 complexes (72), but it is certainly not negligible as might first be expected for L = L. Steric factors may account for most of the activation energy required to rotate the 2-butyne ligand in this complex. 

Barriers of 12.5-15.5 kcal/mol for neutral CpMo(CO)(MeC=CMe)-(SR) complexes are quite similar to rotational barriers in cationic complexes (74). Given the 7r acidity of CO and the tt basicity of SR-, these barriers are surprisingly small. Sulfur donor ligands tend to be electronically flexible, and the soft thiolate may facilitate alkyne rotation by simultaneous rotation of the thiolate substituent. 

For cyclopentadienyl tungsten(II) derivatives barriers of 18-19 kcal/mol characterize alkyne rotation in both the alkyl and acyl cases (67,69,161). Extended Huckel calculations indicate that steric factors play a role in these complexes in addition to the standard dir orbital electronic factors (147). Smaller barriers attend replacement of CO in CpW(CO)-(HC=CH)X complexes with either P(OMe)3 or PMe3. No alkyne rotation... 

Rotational barriers have been probed for a number of bisalkyne complexes (Table VII). Cationic [CpM(RC=CR)2(CO)]+ complexes exhibit relatively high barriers (16-21 kcal/mol). Both standard variable-temperature NMR techniques (94) and two-dimensional methods (162) have been used to elucidate isomer interconversion schemes with two unsymmetrical alkynes in the coordination sphere. The plane of symmetry present when two symmetrical alkynes bind to a CpMX fragment is not retained in all isomers with RC=CH ligands. The availability of distal and proximal alkyne termini locations relative to the adjacent cis ligand leads to two cis isomers (R and R near one another) and one trans isomer (Fig. 25). Rotation of only one alkyne ligand converts cis to trans and vice versa, but direct cis to cis conversion is not possible unless both alkynes rotate simultaneously. 

Dynamic NMR studies indicate that the barrier to alkyne rotation in dithiocarbamate bisalkyne complexes is near 15 kcal/mol (87). For unsym-metrical alkyne ligands, as in Mo(PhC=CH)2(S2CNMe2)2, several isomers are possible with like substituents either adjacent ( cis ) or opposite ( trans ). Analysis of the NMR properties follows the logic presented by Faller and Murray for CpMo(RC=CR)2Cl (94). The C2 molecular symmetry dictated by the chelates can produce two different trans isomers with the two alkyne protons of PhC=CH in each isomer equivalent by C2 rotation (Fig. 26). Only one unique cis isomer is possible, but the two... 

Pyrrole-ZV-carbodithioate bisalkyne complexes display two distinct flux-ional processes. Rotation around the C—N bond of the S2C—NC4H4 ligand equilibrates both halves of the pseudoaromatic NC4H4 ring (AG = 10.7 kcal/mol) (88). Alkyne rotation exchanges both ends of the alkyne ligands at somewhat higher temperatures (AG =13.7 kcal/mol for MeC CMe, 13.8 kcal/mol for HC=CH). 

Hoffmann found that the perpendicular orientation is preferred over the parallel orientation. The observation of alkyne rotation in the mixed-metal compound [CoNi(, -PhC2CO Pr)(CO)3Cp]37 prompted a separate extended Hiickel study, and 6 has been postulated as an intermediate in alkyne rotation, as shown in Eq. (6), and carbonyl and alkyne substitution reactions.38... 

Related examples are the ReMe(0)2( -RC=CR) derivatives obtained from (68) where alkyne rotation is only observed at ca. 100 °C and isomers exist at room temperature. Other alkyne complexes of high-oxidation state halides are essentially limited to ReCLi(C2Ph2) OPCh, and derivatives of the ReCp fragment mentioned in Sections 8.1.4 and 8.1.5. 

ALKENE AND ALKYNE ROTATIONS PARADIGMS FOR STEREOCHEMICAL NONRIGIDITY... 

Alkene and Alkyne Rotations Paradigms for Stereochemical Nonrigidity 4... 

X-ray crystal structures of many new palladium 77 -alkyne complexes have been determined Pd(0) complexes are mainly trigonal planar showing in-plane coordination of the alkyne. Pd(ll) derivatives exhibit out-of-plane 77 -alkynes, rotated to an angle that is much smaller than 90° so that a perpendicular arrangement is never reached a sum of factors, not only electronic, may play a role in deciding the actual angle adopted, as mentioned above. 

The alkene ligands in rra -[M(CO)4(methylacrylate)2] (M = Mo or W) are mutually perpendicular and the two diastereomers 3 and 4 are not interconvertible by alkene rotation. Rotational barriers are somewhat higher for W than Mo and are different for the two diastereomers 69.4 2.0 (3 M = W) and 81.5 2.0 kJ mol (4 M = W). This illustrates the dangers in accounting for differences in barriers in simple terms in this case steric effects must be essentially the same in 3 and 4 while electronic differences must be rather subtle. Alkyne rotation is observed in c/5-[W(CO)(Me2NCS2)2(alkyne)] (5) to occur... 

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