why do transition metals form complex compounds

Thinking about a typical non-transition metal (calcium). Why is that? If you tried to make CaCl, (containing a Ca
ion), the overall process is slightly exothermic. ion instead, you have to supply more ionisation energy, but you get out lots more lattice energy. There is much more attraction between chloride ions and Ca ions than there is if you only have a 1+ ion. The overall process is very exothermic. releases much more energy than making CaCl, then CaCl is more stable - and so forms instead. This time you have to remove yet another electron from calcium. The first two come from the 4s level. The third one comes from the 3p. That is much closer to the nucleus and therefore much more difficult to remove. There is a large jump in ionisation energy between the second and third electron removed.

Although there will be a gain in lattice enthalpy, it isn't anything like enough to compensate for the extra ionisation energy, and the overall process is very endothermic. Thinking about a typical transition metal (iron) Here are the changes in the electronic structure of iron to make the 2+ or the 3+ ion. The 4s orbital and the 3d orbitals have very similar energies. There isn't a huge jump in the amount of energy you need to remove the third electron compared with the first and second. There is an increase in ionisation energy as you take more electrons off an atom because you have the same number of protons attracting fewer electrons. However, there is much less increase when you take the third electron from iron than from calcium.

In the iron case, the extra ionisation energy is compensated more or less by the extra lattice enthalpy or hydration enthalpy evolved when the 3+ compound is made. The net effect of all this is that the overall enthalpy change isn't vastly different whether you make, say, FeCl. That means that it isn't too difficult to convert between the two compounds. The formation of complex ions What is a complex ion? A complex ion has a metal ion at its centre with a number of other molecules or ions surrounding it. These can be considered to be attached to the central ion by co-ordinate (dative covalent) bonds. (In some cases, the bonding is actually more complicated than that. ) The molecules or ions surrounding the central metal ion are called ligands.

Simple ligands include water, ammonia and chloride ions. What all these have got in common is active lone pairs of electrons in the outer energy level. These are used to form co-ordinate bonds with the metal ion. Some examples of complex ions formed by transition metals Other metals also form complex ions - it isn't something that only transition metals do. Transition metals do, however, form a very wide range of complex ions. There are several types of bonds possible. There are hydrogen  bonds, covalent bonds, ionic bonds, and metallic bonds.   Atoms form compounds by sharing electrons to crea te full-levels  which keeps them balanced and uniform.

The first level is 2  electrons, the second is 8, the third is 18, and it keeps growing  from there. The bonds explain different ways the electrons can be  used between the multiple atoms. Covalent bonds are when electrons  are 'shared'. Hydrogen bonding is a form of covalent bonding except  it pretty much just deals with how hydrogen only has 1 electron.  For example, take water for instance (H2O). Oxygen has 6 electrons  in its natural state and hydrogen each have one. If hydrogen holds  hands with oxygen then the hydrogen is happy because its lowest  level (2) is full. Another hydrogen atom comes along and share it's  electron, now each hydrogen atom has 2 electrons and the oxygen has  8.

But why does oxygen want 8 and now not 2? Because it is  officially at its happiest when its at 8.   Then there are ionic bonds when the electrons are just given/taken  away and theres no sharing at all. This creates positive and  negative atoms because now it has an unbalanced charge between  protons and electrons (called ions). Metallic bonding deals with  metals and how they conduct electricity. These metal atoms are  bound together and have extra electrons that just flow through the  entire block of material which is why metals conduct electricity  (the movement of electrons).   So to some up why atoms bond, its to form complete energy levels in  order to be balance and at peace with themselves.