Rate laws, rate constants, and activation volumes are reported for reaction of MnBr(CO)5 with 2,2′-bipyridyl and with 1, 10-phenanthroline in methanol. The activation volume of +22cm3 mol−1 is consistent with a predominantlyD mechanism (CO loss) in this medium, but in 50% aqueous methanol both the kinetic pattern and activation volumes in the region of −20 cm3 mol−1 for the MnBr(CO)5 + bipy reaction indicate a key role for bromide dissociation. The solvatochromic behaviour of the MnBr(bipy)(CO)3 product of these reactions is compared with that of other ternaryd6-diimine complexes, and the piezochromic behaviour of its MLCT spectrum outlined.