Pentadentate ligand
A pentadentate ligand is a ligand that coordinates via five donor atoms.[1]
There are different possible ways for a ligand to arrange around an ion. For an octahedral coordination with six positions, the possible arrangements of a linear pentadentate ligand are designated by ffms ffma fff fmf fmama fmsma fmsms fmams where each letter f or s represents three consecutive donor atoms: f represents facial fac arrangement, m is meridianal mer a is "anti" and s is "syn" for positioning of the mer arrangement relative to other donors.[1]
For a chain branched at the donor atom the tertiary atom will have two chains of length one and one of length one attached. The pattern can use parenthesis: () to indicate a side chain, eg NNM(N)N. For octahedral coordination there are four arrangements designated: f(m)m f(f)f f(f)ma and f(f)ms where the parenthesis ow indicate how the side chain participates in coordination.[1]
Ligands with four donor atoms on four chains around a central donor atom, will organise around the metal atom equatorially.[1]
The metal may be five-coordinate with arrangements being square-pyramidal or trigonal-bipyramidal, or somewhere between the two.
2,2':6',2'':6'',2''':6''',2''''-Quinquepyridine is fairly rigid, and tries to be in a plane, so coordination will be pentagonal planar. For silver it is planar, but for rhenium it slightly twists to a helix.[1]
Examples[edit]
| IUPAC name | name | abbreviation | type | pKa | Metals | ref |
|---|---|---|---|---|---|---|
| N1-(2-aminoethyl)-N2-(2-((2-aminoethyl)amino)ethyl)ethane-1,2-diamine | tetraethylenepentamine | tepa | NNNNN | 2.65, 4.25, 7.87, 9.08 and 9.92 | Mn Fe Co Ni Cu Zn | |
| N1-(2-(dimethylamino)ethyl)-N2-(2-((2-(dimethylamino)ethyl)(methyl)amino)ethyl)-N1,N2-dimethylethane-1,2-diamine | Me7-tetren | NNNNN | Co(II), Ni(II), Cu(I), Cu(II) and Zn(II) |