An unusual effect of ligand methyl-substitution in the reaction of tricarbonyl (Group 6A metal)[hydrotris(3,5-R2-1-pyrazolyl)borato]anions (R = H, Me) with arenediazonium cations: X-ray crystallographic characterisation of a MoIIη2-aroyl complex, [Mo(η2-COC6H4Me-p)(CO)2{HB(3,5-Me2C3HN2)3}] and of an η2-hexahydrobenzoyl analogue, [Mo(η2-COC6H11)(CO)2{HB(3,5-Me2C3HN2)3}]

An X-ray crystallographic study has shown that the intensely-coloured products which result from the anomalous reaction of arenediazonium cations with [Mo(CO)3{HB(3,5-Me2C3HN2)}3]– are η2-aroyl dicarbonyl derivatives of MoII, [Mo(η2-COAr)(CO)2{HB(3,5-Me2C3HN2)3}] rather than η1-aryl tricarbonyl complexes as originally proposed: a by-product formed when the reaction is carried out in the presence of cyclohexane has been identified by X-ray crystallographic methods as the η2-hexahydrobenzoyl complex [Mo(η2-COC6H11)(CO)2{HB(3,5-Me2C3HN2)3}] the formation of which has been interpreted in terms of a free-radical mechanism for the reaction.