Bondage Is A Two-Way Street

The simple "one-way" notion of chemical bonding described back here is part of the "lone pair theory" developed by G.N. Lewis and N.V. Sidgwick. According to that theory, a neutral molecule such as ammonia donates electrons from its lone pair to a metallic Lewis acid.  But things like ethylene, not having any lone pairs, confounded the theory, since they too formed neutral metal complexes very similar to ammonia-metal complexes.

In 1935, Linus Pauling introduced the novel concept of backbonding to explain the shorter than expected Ni-C bonds observed in the electron diffraction structure of Ni(CO)₄.  Like many concepts in chemistry, backbonding is better illustrated than described:

In this simplified scheme above, CO gives electronic juice to the metal's empty and receptive d-orbital (left-hand side). At the same time, the metal gives back electrons to the CO (right-hand side) using a different full d-orbital: the two sketches overlap. They are synergistic.