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The new complexes [M0,L.J2-(H,L = 2.3-or 3.4-dihydroxybenzoic acid, 2,3-H2dhb or 3,4-H2dhb, M = Mo, W or 0 s ; H, L = 2,6-dihydroxybenzoic acid, 2,6-H2dhb, M = Mo), [OsO,(py),(2,6-dhb)] (py = pyridine), [ReOl(PPh,),L] (H,L = 2,3-, 3,4-or 2,6-H2dhb) and [M(PPh3),(2,3-dhb)] (M = Pd or Pt) have been prepared. The crystal structure of [NMe,],[MoO2(2.3-dhb),]~l .5H20 has been determined and exhibits catecholato rather than salicylato type bonding. Infrared, Raman, 'H and 13C-{'H} NMR spectroscopic data for the complexes are also presented.

The trichloride compounds [Ta(OC 6 HPh 2 -2,6-R 2 -3,5)Cl 3 ] (1: R = H a, Ph b, Me c, Pr i d or Bu t e) have been obtained by treating [Ta 2 Cl 10 ] with the corresponding 3,5-disubstituted-2,6-diphenylphenols Ia-Ie. The solid-state structures of 1c and 1d show a square-pyramidal structure with an axial aryloxide ligand. The reaction of 1 with LiCH 2 SiMe 3 (3 equivalents) led to the isolation of the tris(alkyls) [Ta(OC 6 HPh 2 -2,6-R 2 -3,5) 2 (CH 2 SiMe 3 ) 3 ] (4a-4d) except in the case of the 3,5-di-tert-butyl derivative 1e which generated the alkylidene compound [Ta(OC 6 H 3 Ph 2 -2,6-Bu t -3,5) 2 (᎐ ᎐ CHSiMe 3 )(CH 2 SiMe 3 )] 6e. The alkylidenes 6a-6d can be produced by photolysis of the corresponding tris(alkyls) 4a-4d. The alkylidenes 6a-6d undergo intramolecular cyclometallation of the aryloxide ligand (addition of an aromatic C᎐H bond to the tantalum alkylidene) at a rate which is extremely dependent on the meta substituents on the phenoxide nucleus. Kinetic studies show that conversion of 6a-6d into monometallated 7a-7d is first order with the phenyl, methyl and isopropyl substituents slowing the ring closure down by factors of 20, 90 and 360 respectively. The tert-butyl substituent completely shuts down cyclometallation of the adjacent phenyl ring. It is argued that bulky substituents inhibit rotation of the ortho-phenyl ring into a conformation necessary for C᎐H bond activation. Structural analysis of the torsion angles between ortho-phenyl and phenoxy rings has been carried out. The use of 1 H NMR chemical shifts has been demonstrated to be a valuable tool to probe the average conformations adopted in solution.