Impact dynamic equations for planar multi-link robotic collisions are modified, which can model both cases of full-ranked and rank-deficient Jacobian matrices. Rank-deficient Jacobian matrices cause the impact equations to be indeterminate. It is shown that (i) the post impact angular velocities are determined and can be solved explicitly without extra equations, and (ii) a set of linear equations with unknown impulses can be obtained where the impulses can only be solved if extra equations of impulses are provided. Two robot collision problems with rank-deficient Jacobian matrices are presented to exemplify the method