[melaniejs (OP)]

Mr Hariharan asks:

Transition metals, being metals, are electropositive (electron hating) and ligands like ammonia , chlorine, oxygen are mostly electronegative (electron liking). How come ligands donate to the electropositive transition metals when both have contradictory properties?

What do you think?

[chiralSPO]

Elemental chlorine is much more electronegative than metals like calcium. To the extent that chlorine will violently remove electrons from calcium... but a significant amount of that heat released is not just from the electrons transfer, but also . because there is an electrostatic attractive between the positive calcium ions and negative chloride ions. When the ions come together, heat is released, and it takes a LOT of heat to separate them.

Transition metals are a little more electronegative than alkali or alkaline earth metals. Titanium, for instance, is a few spots over from calcium, but it's just enough that titanium won't actually form a discrete Ti⁴⁺ ion when reacted with chlorine, instead, they share those electrons covalently. TiCl₄ is not a salt, it is a small, nonpolar molecule, and as such is a liquid with a low boiling point (https://en.wikipedia.org/wiki/Titanium_tetrachloride).

Ligand-transition metal interactions are typically essentially covalent in nature, but highly polarized (many are susceptible to ionization). You can think of the Cl^– as having lone pairs that are so strongly attracted to a Ti⁴⁺ center that the Cl^– actually donates both electrons needed for the bond.

The same goes for other good ligands, like phosphines, amines, thioesters, carbenes, even carbon monoxide is an excellent ligand (the bonding for that one is much more complex!) These neutral compounds have lone pairs that are attracted to the (partially) positive metal center.

The ligand rarely "gives" the electron entirely to the metal (although that is known to happen in some cases, especially when light is involved, though this is transient) Instead you can think of it as either: the ligand is very polar and aligns the partially negative end as close as possible to the positive metal center, or you can think of it as there are orbitals on the metal and the ligand that mix, and if the metal's contribution has no electrons, and the ligand contribute two, then there will be a bond order of 1 between them.