Pentadentate ligand

A pentadentate ligand is a ligand that coordinates via five donor atoms.
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 ffm_s ffm_a fff fmf fm_am_a fm_sm_a fm_sm_s fm_am_s 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.
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 NNMN. For octahedral coordination there are four arrangements designated: fm ff fm_a and fm_s where the parenthesis ow indicate how the side chain participates in coordination.
Ligands with four donor atoms on four chains around a central donor atom, will organise around the metal atom equatorially.
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.

Examples

IUPAC name	name	abbreviation	type	pKa	Metals	ref
N¹--N²-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--N2-ethyl)-N1,N2-dimethylethane-1,2-diamine		Me7-tetren	NNNNN		Co, Ni, Cu, Cu and Zn	-