My understanding: Transition (d-block) metals, as found in intracellular proteins, are limited to about 10 elements including amongst others Fe, Ni, and Cu. Due to the reductive environments inside cells, they are usually/expected to be found with an oxidation state of +2 (?). However, the number of bonds that coordinate the metal, assuming that you know that the oxidation state is +2, is not completely certain (?).
Confusion about bond and coordination: As the d-orbital of e.g. Fe(II) is not completely filled it can be considered somewhat reactive/unpredictable, and we cannot for sure say what coordination it will have in an enzyme (?). Or, is there some sort of general assumption we can make about its coordination based on its oxidation number (i.e. +2)? As I was looking for literature on the matter I stumbled on the chemistry stack exchange answers regarding ‘bonds in coordination complexes’ which used Fe(II) as an example of how Pauling’s Principle could be applied to simplify how metals are coordinated.
A brief summary of this answers would include that coordinating residues can be thought to contribute ½ electron to the coordination of Fe(II); and thus, Fe(II) would typically have four coordinating interactions while Fe(III) would have six coordinating interactions, in order to cancel the charge.
Summary of question(s): If my statements above are correct, then it could be generally assumed that all +2 metals in enzymes, such as Fe(II) , are coordinated by four groups? Furthermore, the coordinating bonds are considered dative (i.e. not entirely ionic)? Very speculatively, could it also be assumed that Fe(II) would typically be in a tetrahedral complex (why/why not)?
*update based on comments: Octahedral coordination of Fe(II) was mentioned as classical example in heme enzymes, while it is also known that non-heme enzymes can e.g. coordinate Fe(II) with three residues and a water molecules in a tetrahedral conformation. It could therefore be reasoned that coordination of the metal (e.g. tetrahedra vs octahedral) is related to enzyme function and evolution over time, and not so much the oxidation state of the metal?