When bonded side-on to a single metal atom, an alkyne serves as a dihapto usually two-electron donor.  For early metal complexes, e.g., CpTi(CR), strong π-backbonding into one of the π* antibonding orbitals of the alkyne is indicated. This complex is described as a metallacyclopropene derivative of Ti(IV). For late transition metal complexes, e.g., Pt(PPh)(MeCPh), the π-backbonding is less prominent, and the complex is assigned oxidation state 0.
In some complexes, the alkyne is classified as a four-electron donor. In these cases, both pairs of pi-electrons donate to the metal.  This kind of bonding was first implicated in complexes of the type W(CO)(RC).