Here's the link to the concept. Though I would advise you not follow this rule because it is not fixed for all compounds and there are a lot of exceptions to it

http://books.google.co.in/books?id=RCm43YjyTP0C&pg=PA13&lpg=PA13&dq=effective+atomic+number+coordination+compounds&source=web&ots=14BvAhfMWw&sig=NqSYrVcPhnE-RIXfIWqAavD2r8U&hl=en