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6-C6H3-i-Prz; OR = OCMe(CF3),, OCMe(CFJ) form fivecoordinate adducts upon addition of PMe3 or quinuclidme. PMe3 attacks the C/N/O face of the pseudotetrahedral complexes to give chiral TBP species in which the phosphine is bound in an axial position and the imido and alkylidene ligands lie in the equatorial plane. Two isomers containing syn and anti rotamers of the alkylidene ligand are observed. The syn rotamer forms first; the anti rotamer is the final product. PMe3 binds weakly when OR = 0-t-Bu and is lost readily in vacuo. Quinuclidine adds to either the C 0/0 face or N/O/O face to give an achiral syn isomer and to the C/N/O face to of syn and anti forms is observed with time. An X-ray structure of syn-Mo(CH-t-Bu)(NAr)[OCMe-(CF,),I2(PMe3) shows that the t-Bu group points toward the imido ligand and the phenyl ring of the imido ligand lies approximately in the equatorial plane in a relatively crowded coordination environment (a = 10.979 (4) A, b = 17.945 (7) A, c = 18.375 (8) A, 19 = 106.34 (3)O, 2 = 4, V = 3474 (4) AS, p = 1.490 g/cm3, R = 0.037, R, = 0.045). Pyridine adducts of Mo complexes containing the 2,6-dichlorophenoxide ligand also have been characterized. Three isomers of fivecoordinate molybdenum or tungsten complexes containing a cis-or trans-2-butenylidene ligand and quinuclidine are found at equilibrium, syn and anti rotamers of the chiral core previously described and a syn rotamer with an achiral core. An X-ray structure of anti-W(tram-CHCH=CHMe)(NAr)[OCMe(CF3)z]2(quin) showed the expected trigonal-bipyramidal core with alkylidene and imido ligands occupying equatorial sites and OCMe(CF3)2 ligands occupying one axial and one e uatorial site (a = 12.972 (9) A, b = 18.049 (7) A, c = 15.038 (9) A, p = 92.07 (3)O, 2 = 4, V = 3518 (6) A?, p = 1.673 g cm3, R1 = 0.038, R, = 0.040). The only significant difference between the structure in the equatorial plane. Syn and anti rotamers in five-coordinate adducts have been shown to interconvert after losing the base in several cases. The barrier to rotation of the alkylidene ligand has been measured in several four-coordinate species and shown to lie in the range AG*= = 15-18 kcal mol-'. These findings are discussed in relation to the proposed mechanism of olefin metathesis by pseudotetrahedral complexes of the type M(CHR')(NAr)(OR),.give an anti chiral T 6 P species analogous to that formed for the PMe3 adduct. An equilibrium mixture of this anti adduct an d the syn adduct described above is that the anti adduct is markedly less crowded

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