Metal complexes of diamines
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Metal complexes of diamines refers to coordination complexs of diamine ligands. The most common complexes are those of ethylenediamine. Complexes of en and related diamines have been thoroughly studied for their fundamental properties.[1] In a practical sense, diamines are mainly used to make polyamides such as nylon 66, not coordination complexes. This class of compounds are closely related to metal ammine complexes.
Diamines have properties expected for two amines,[2] i.e. they are dibasic and binds well to hard Lewis acids, such as metal cations.
The coordination chemistry of diamines emphasizes 1,2- and 1,3-diamines, which form 5- and 6-chelate rings. Both enthalpic and entropic factors favor their formation 1,4- and longer diamines are floppy, forming polymers vs chelate rings.[3] For ethylenediamine complexes, the five-membered MN2C2 chelate ring is nonplanar, but exist in two rapidly interconverting conformations, referred to as δ and λ.
Of the ditertiary diamines, tetramethylethylenediamine (TMEDA) is most commonly encountered. Being bulky and lacking N-H bonds, it is popular as a ligand in main group chemistry.[4]
Complexes
Octahedral complexes
Ethylenediamine forms many homoleptic octahedral complexes of the formula [M(en)3]n+.[5] Representative octahedral complexes are M = V2+,[6] Cr3+, Mn3+, Fe3+, Ru2+, Co2+ and Co3+, Rh3+, Ir3+, Ni2+, Pt4+ and Zn2+.[7] These complexes are chiral, and many have been resolved. Particularly famous is [Co(en)3]3+.
More common than homoleptic [M(en)3]n+ complexes are "mixed ligand" derivatives, e.g., of the type [M(en)L4]n+ and [M(en)2L2]n+. One example is cis-dichlorobis(ethylenediamine)cobalt(III) cation.
Square planar complexes
Square planar complexes of the formula [M(en)2](n+) are also well known. Representative square planar complexes are M = Pd2+, Pt2+, Cu2+, and Au3+.