Most of the molecular compounds of transition metals are metal complexes and organometallic compounds in which ligands are coordinated to metals.
These molecular compounds include not only mononuclear complexes with a metal center but also multinuclear complexes containing several metals, or cluster complexes having metal-metal bonds.
The number of new compounds with a variety of bonding and structure types is increasing very rapidly, and they represent a major field of study in today's inorganic chemistry (refer to Chapter 6).