An unusual effect of ligand methyl-substitution in the reaction of tricarbonyl (Group 6Ametal)[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}]