In coordination chemistry, which of the following is a Lewis acid?

In coordination chemistry, a Lewis acid is defined as a species that can accept a pair of electrons to form a covalent bond. A metal ion fits this definition because it typically has vacant orbitals and a positive charge, allowing it to accept electron pairs from ligands, which act as Lewis bases.

Metal ions, such as transition metals, have the ability to coordinate with multiple ligands by accepting electron pairs. This electron-accepting behavior is the essence of being a Lewis acid. When ligands coordinate to a metal ion, they donate electron pairs to it, thus facilitating the formation of coordination complexes.

The other options provided do not fulfill the criteria for being a Lewis acid. Monodentate ligands can only attach to a central atom at a single site and primarily function as Lewis bases, donating an electron pair rather than accepting one. A carbon atom is generally considered a neutral species and does not display the typical behavior of a Lewis acid. While bidentate ligands can coordinate at two sites, they also behave as Lewis bases, donating electron pairs rather than accepting them.

This understanding of Lewis acids and bases is fundamental to the study of coordination chemistry, helping clarify the roles of different species in forming coordination complexes.