Organometallic chemists have synthesized a remarkable variety of new structural types. In these structures ligands, which are organic or inorganic molecules of variable independent stability, bind to one or more transition metal atoms. An approach to an understanding of the electronic structure, geometrical preferences, and reactivity of these complexes may be made if the molecule is "decomposed" conceptually into a metal fragment, ML η , and a ligand. A library of the molecular orbitals of these fragments is becoming available. One then "reconstructs" the molecule by examining the interaction of the orbitals of the ligand, typically an organic molecule, with the orbitals of the ML η fragment.