FORMATION OF ETA(2)-AROYL, ETA(1)-HALOGENOCARBYNE OR STERICALLY CROWDED ARYLDIAZENIDE LIGANDS IN THE REACTIONS OF RING-SUBSTITUTED TRICARBONYL[HYDROTRIS(PYRAZOLYL)BORATO]-MOLYBDATE AND TRICARBONYL[HYDROTRIS(PYRAZOLYL)BORATO]-TUNGSTATE ANIONS WITH ARENEDIAZONIUM CATIONS AND RELATED OXIDANTS

Although the hydrotris(pyrazolyl)borato complex [Mo{HB(pz)(3)}(CO)(3)] (-) reacted with 3- or 4-substituted arenediazonium cations [R'N-2](+) yielding carbonyl-substitution (ie, aryldiazenido) products [Mo{HB(pz )(3)}(CO)(2)(N(2)R')], reaction of the methyl-substituted analogue [ML (CO)(3)](-) [L* = tris(3,5-dimethylpyrazolyl)hydroborate: M = Mo or W ] led, via oxidative formation of aryl radicals and [ML(CO)(3)](.), t o eta(2)-aroyl complexes [ML(CO)(2)(eta(2)-COR')] [R' = C(6)H(4)X-4 ( X = NO2, CN, COMe, CF3, H, Me, OMe or NMe(2)) or C(6)H(4)X-3 (X = NO2 or OMe)] in acetonitrile or to the halogenocarbyne complexes [ML(CO)( 2)(=CX)] (X = Cl, Br or I) in the presence of the halogenoalkanes CH2C l2 or CHX(3) (X = Br or I). The complex [MoL(CO)(3)](-) reacted simil arly with diphenyliodonium or triphenylsulfonium cations, but in the l atter case anion-cation redox is very slow in the dark but rapid upon irradiation with sunlight. Comparison of these results with those obta ined for [ML(CO)(3)](-) analogues with different substituents in the pyrazolyl rings demonstrates that oxidation of the former by arenediaz onium cations occurs in response to the steric rather than the electro nic effect of the 3-methyl substituents. However further steric crowdi ng in either the hydrotris(pyrazolyl)borate ligand or the diazonium ca tion promotes a reversion to the carbonyl-substitution pathway. A mech anism to account for these observations is proposed. Attempts to exten d the chlorocarbyne synthesis to systems other than [ML(CO)(3)](-) me t with only limited success. Spectroscopic data for the new complexes are reported and discussed. Two aryldiazenido complexes, [MoL(CO)(2)( N-2,R')] (R' = 2.6-Me(2)C(6)H(3) or 2,3-dimethyl-5-oxo-1-phenyl-3-pyra zolin-4-yl) have been characterised by single-crystal X-ray diffractio n studies and are found to differ in the manner in which the aryldiaze nide ligand accommodates to steric crowding in the molybdenum co-ordin ation sphere.