Coordination complex

A coordination complex contains a central atom, often a metal ion, bound to surrounding ligands in coordination chemistry. The metal oxidation state and the ligand donor types are separate pieces of its description.

A metal carbonyl is a coordination complex containing carbon monoxide ligands. The ligand normally binds through carbon, donating an electron pair while accepting metal electron density into an antibonding orbital through pi backbonding.

These complementary interactions explain a useful spectroscopic test. Stronger backbonding tends to weaken the internal carbon–oxygen bond and lower its stretching frequency relative to weakly backbonded CO. A vibrational spectrum therefore probes electronic bonding, although terminal versus bridging coordination and the other ligands also affect the observed bands. Simple electron counting treats a neutral terminal CO as a two-electron donor, but that convention is not a complete description of bond strength.

A metal nitrosyl complex contains a coordinated nitric-oxide group, conventionally written M–NO. The metal–nitrogen–oxygen arrangement may be approximately linear or bent, and changes in geometry correlate with different bonding and reactivity.

Assigning every nitrosyl a single fixed formal charge can obscure the delocalized electronic structure. A useful counting approach groups the metal and nitrosyl together and counts the relevant electrons of that fragment. Geometry, spectroscopy and electron count then constrain one another. The original 1974 treatment explicitly relates these structural alternatives to chemical reactions.^[ref]

A complex with an accessible coordination site or low electron count that permits coordination ligand capture. electron deficiency and geometric accessibility must both be assessed.

A complex with a cyclobutenyl ligand. Eta-three binding of its allylic portion can stabilize the ring formed by closure of an appropriately folded metal-dienyl chain.

A four-coordinate complex whose donor atoms lie approximately at the corners of a square. Many low-spin d-eight metal complexes adopt this geometry.

A complex with a face-bound cyclic pi ligand as the seat and several other ligands as the legs. In $\mathrm{CpFe}(\mathrm{CO}{)}_2\mathrm{X}$ the two carbonyls and X form three legs.

A complex in which a ligand binds the same metal through at least two distinct donor atoms, closing a coordination ring.

The idealized eclipsed ion $[\mathrm{M}{\mathrm{o}}_2\mathrm{C}{\mathrm{l}}_8{]}^{4-}$ has two formally Mo(II), d${}^4$ centres and a quadruple bond. Each metal has four chloride ligands arranged approximately in a square.