Sigma- and pi-coordinated metals

CAS structuring conventions for coordination compounds do not distinguish sigma and pi bonds metal to ligand bonds are treated as ordinary covalent bonds. The project will consider possible changes in conventions for these structures so that their records more nearly reflect the ways in which chemists think of these structures. 

Almost 70 of the approximately 115 elements are transition metals. These are elements with electrons successively added to their d or f subshells and have both filled and unfilled orbitals suitable for bonding. They exhibit a variety of oxidation states and bond formations, such as sigma and pi bonds or coordination bonds as are found in typical chelate structures. These characteristics are the reason that transition metal-containing macromolecules exist in a wide variety of structures including linear, two-dimensional, three-dimensional, dendrite, and assemblies. Many of these polymers are often referred to as organometallic polymers. We have, somewhat arbitrarily, separated this class from coordination polymers, which have been covered in Volumes 1, 3, and 5. 

PCMODEL Version 8 became available in 2002. New features in version 8 include support for different and improved force fields along with the MMX, MM3, MMFF94, Amber, and Oplsaa force fields currently supported. The atom limit has been increased to 2500 atoms, and support for reading and writing PDB and SDF files has been added. Transition-metal complexes can be built with explicit sigma bonding, lone-pair coordination, and pi-system coordination. Parameters are available for all transition metals. 

G.J. Kubas, Metal-dihydrogen and sigma-bond coordination the consummate extension of the Dewar-Chatt-Duncanson model for metal-olefin pi bonding . Journal of Organometallic Chemistry, 635, 37 (2001). 

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