The structure of the previously reported (py)₃ZnFe(CO)₄ (py = pyridine) has been determined, confirming the monomeric nature of this species. The complex has average Zn–N and Zn–Fe bond lengths of 2.0970(7) and 2.4017(3) Å, and features a coordination geometry about Fe which is intermediate between trigonal bipyramidal and face monocapped tetrahedral. The space group is P2₁/c, with a = 8.22080(10) Å, b = 16.1668(3) Å, c = 15.4669(3) Å, β = 102.5869(11)°, V = 2006.21(6) Å³, D_calc. = 1.558 g/cm³ at 150(1) K. A monomeric cadmium analogue, (pyridine)(neocuproin)CdFe(CO)₄, has also been synthesized, and found to possess a similar geometry, with average Cd–N and Cd–Fe bond lengths of 2.352(2) and 2.5380(5) Å. The space group is P[`1] P\overline{1} with a = 10.8900(2) Å, b = 11.3042(3) Å, c = 15.5488(4) Å, α = 85.1251(10)°, β = 84.3468(14)°, γ = 72.0377(15)°, V = 1808.93(7) Å³, D_calc. = 1.478 g/cm³ at 150(1) K.

The structures of the monomeric (py)₃ZnFe(CO)₄ and (py)(neocuproin)CdFe(CO)₄ complexes have been determined, and suggest the use of significantly greater s character in the main group elements’ bonds with iron as opposed to nitrogen.