This letter is concerned with the problem of output consensus for two classes of heterogeneous nonlinear multi-agent systems which are interconnected via diffusive couplings over directed graphs. Specifically, for agents that are input feed forward passive (IFP), a condition in terms of passivity indices is proposed for asymptotic output consensus. Moreover, it is shown that the proposed condition can be exploited to design the coupling gain that ensures asymptotic consensus via a semi definite program, and the existence of such a coupling gain can be guaranteed provided all the agents are IFP. For agents that are input feed forward output feedback passive, a condition in terms of passivity indices for practical output consensus is provided, in which the relationship between the coupling gain and the consensus error bound is revealed