The metallicity of a YDSC can determine whether or not an IMBH can form. Metallicity is simply the abundance of elements heavier than hydrogen and helium. Mapelli (2016) show that for a YDSC with the same metallicity as the Sun, the very massive star generated from the runaway collision scenario loses so much mass through powerful stellar winds that the resulting black hole that is formed is no more than ~30 times the mass of the Sun. At low metallicity (i.e. 0.01 to 0.1 times the Sun's metallicity), the rate of mass loss becomes much lower and the resulting black hole can be up to a few hundred times the mass of the Sun, well within the mass range of IMBHs.
Mapelli (2016), "Massive black hole binaries from runaway collisions: the impact of metallicity", arXiv:1604.03559 [astro-ph.GA]