When I conduct the pop quiz at the beginning of my training sessions for mentoring participants, the above question is asked and often with some people responding that it is. In some ways, it's a trick question done to serve a specific point.
In informal mentoring, chemistry is important because it is what brings both partners together. We all have had the experience of meeting someone and feeling the chemistry between us. Without this, informal mentoring would not exist. However, in a formal, professional program, we can't know in advance, when pairing, who will have chemistry and who will not. Instead, we match on the basis of compatibility. What does that mean? It means that both partners come to the table willing to engage in a developmental relationship. In the course of that relationship, some people may develop chemistry but it's not the predictor of success that people think. Look at it this way: chemistry in formal mentoring is nice to have but not a "must have".