[Mo-2(MeCN)(8)][BF4](4) reacts with alpha,omega-dicarboxylic acids [HO2C(CH2)(n)CO2H, n = 0-8] to form salts with empirical formula [Mo-2{O2C(CH2)(n)CO2}(MeCN)(x)][BF4](2); the X-ray crystal structure of the butanedioic salt [Mo-4{eta(4)-mu-O2C-(CH2)(2)CO2-mu-eta(2)}(2)(MeCN)(12)][BF4](4) . 3MeCN is reported together with the use of the complexes as catalysts for the polymerization of norbornene.