Although the hydrotris(pyrazoly1) borato complex [Mo{HB(pz),}(CO),]-reacted with 3-or 4-substituted arenediazonium cations [R'N,]' yielding carbonyl-substitution (i.e. aryldiazenido) products [Mo{HB(pz),}(CO),(N,R')],reaction of the methyl-substituted analogue [ML*(CO),]-[L* = tris(3.5-dimethylpyrazolyl) hydroborate; M = M o or W] led, via oxidative formation of aryl radicals and [ML*(CO),]', to q2-aroyl complexes [ML*(C0),(q2-COR')] [R' = C,H,X-4 (X = NO,, CN, COMe, CF,, H, Me, OMe or NMe,) or C,H,X-3 (X = NO, or OMe)] in acetonitrile or to the halogenocarbyne complexes [ML*(CO),(=CX)] (X = CI, Br or I) in the presence of the halogenoalkanes CH,CI, or CHX, (X = Br or I). The complex [MoL*(CO),]-reacted similarly with diphenyliodonium or triphenylsulfonium cations, but in the latter case anion-cation redox is very slow in the dark but rapid upon irradiation with sunlight. Comparison of these results with those obtained for [ML"(CO),]analogues with different substituents in the pyrazolyl rings demonstrates that oxidation of the former by arenediazonium cations occurs in response to the steric rather than the electronic effect of the 3-methyl s u bstituents. However further steric crowding in either the hydrotris(pyrazoly1) borate ligand or the diazonium cation promotes a reversion to the carbonyl-substitution pathway. A mechanism to account for these observations is proposed. Attempts to extend the chlorocarbyne synthesis to systems other than [ML*(CO),]-met with only limited success. Spectroscopic data for the new complexes are reported and discussed. Two aryldiazenido complexes, [MoL*(CO),(N,R')] (R' = 2,6-Me,C,H, or 2,3-dimethyl-5-0~0-1phenyl-3-pyrazolin -4-yl) have been c haracterised by single-crystal X-ray diffraction studies and are found to differ in the manner in which the aryldiazenide ligand accommodates to steric crowding in the molybdenum co-ordination sphere.The chemical similarity between related complexes containing the q-cyclopentadienide, [q-C5H5]-, and q 3-poly(pyrazolyl)borate ligands, [RB(pz),]-(R = H or pz, pz = pyrazol-l-yl), is illustrated by the reactions of their anionic tricarbonylmolybdenum complexes 1 and 3 with arenediazonium cations ' 9 ' to yield red neutral aryldiazenido complexes 2 and 4 (Scheme 1). However in 1971 Trofimenko reported that the tricarbonylmolybdenum derivative of the hydrotris(3,5-dimethyl-pyrazoly1)borate anion, [HB(dmpz),]-(henceforth abbreviated as L*), [MoL*(CO),]-5 reacted with arenediazonium cations to yield products which were blue, rather than red, and which appeared to be seven-co-ordinate o-aryl derivatives [MoL*-(CO),(o-R')] 6. We now report the outcome of a study of this anomalous reaction. Some of the results described have been published in preliminary f ~r m . ~. ~ Results and DiscussionProduct Characterisation and Outline Mechanism.-In Trofimenko's report3 the reaction between the anion 5 and t Transition Metal Organometallic Complexes containing 3-Substituted Poly(pyrazoly1)borate Ligands. Part 1.