We have calculated the capture rate coefficients for collisions of C (P-3(j)) and Ge (S-1(0)) with unsaturated hydrocarbons and compared them with the experiments of Husain and co-workers [Clary, Haider, Husain, and Kabir, Astrophys. J. 422, 416 (1994); Husain, Ioannou, and Kabir, Z. Phys. Chemie 203, 213 (1998)]. Our findings indicate (i) that all nine electronic surfaces for the C (P-3) reactions must contribute to the observed reaction-rate coefficients, (ii) a central multipole approach is inappropriate for calculating the capture rate coefficients involving the-larger hydrocarbons, (iii) the trends observed in the measured rate coefficients for the Ge and C reactions can simply be understood in terms of the physical size of the hydrocarbons, and (iv) the electrostatic interaction plays a significant role in the determination of capture rate coefficients for reactions involving C (P-3) and highly polar species as well as acetylene.