Abstract: Mthods and apparatus for the extraction of a ligand such as molecular oxygen from a first fluid environment and for release of a ligand such as molecular oxygen, as well as ligand carrier compounds therefore comprising linear, pentadentate polyalkylamines and transition metal ions. The carrier compounds have the general formula: ##STR1## where, each of R.sub.1 and R.sub.2 is independently an organic group including a sulfur, an oxygen or a nitrogen coordinated to M;each of m, n. o, and p is 1, 2 3, or 4;X is selected from the group consisting of 2,6-pyridyl, 2,6-piperidyl, 2,5pyrrolyl, 2,4-imidazolyl, substituted heterocyclic amines, --O--, --S-- >P--R.sub.3, and >N--R.sub.3 where R.sub.3 is hydrogen, lower alkyl, or aralkyl; andM is a transition metal ion.