The major HM-C CH and M-eta(2)-C2H2 products are observed in the matrix infrared spectra from reactions of laser-ablated group 3 metal atoms with acetylene, while the vinylidene product is not detected. These results reveal that coordination of group 3 metal atoms to the acetylene pi-bond and H-migration from C to M readily occur during codeposition and photolysis afterward. The product absorption assigned to the La-vinylidene complex in a previous study is reassigned to one of the absorptions of La-eta(2)-C2H2 center dot center dot center dot C2H2. Two strong Sc-H stretching absorptions are assigned to the free HSc-CCH- anion, in accord with a previous study, but a lower frequency counterpart is reassigned to HSc-CCH- coordinated to acetylene based on the increasing relative intensity of the low-frequency component at higher acetylene concentration. The group 3 metals evidently form weaker pi-complexes than the group 4 metals. The addition of an electron to HM-CCH elongates the M-H and M-C bonds in the anionic species due to the lower ionic contributions to the bonding.