The list with [0, 1, 0] is not a list of classes but just a list of values for a single class. If you have only one class, pos_weight should contain only one argument. For example, if you have 300 positive samples and 200 negative ones, you should set pos_weight to 200 / 300. It will virtually “reduce” the set of positive samples from 300 to 200.
An alternative is to sample from negative subset with higher frequency.