Transition metals are those elements in the periodic table with incompletely filled d shells with electrons. Transition metals start in the left with the scandium group which has the 4s2-3d1 electronic configuration, and ends in the right with the zinc group which has the 4s2-3d10 electronic configuration.
Based on the previous definition of transition metals the zinc group are not considered transition metals due to the complete d shell that they have.
Some scientists include aluminium on top of the scandium group due to its metallic character and due to its similar electronic configuration and oxidation state.
Yttrium in the scandium group is sometimes included in the lanthanides group due to its resemblance to lanthanides in its electronic and oxidation states.
In addition, the zinc group resembles the magnesium group in its chemistry. Both of these groups have the +2 oxidation state and both have an extensive organometallic chemistry which is manifested as the organomagnesium and organozinc and organocadmium chemistry.
Transition metals are divided to early transition metals and late transition metals. They both form organometallic complexes which have a coordination number of 6 in most compounds. The geometry in these complexes is octahedral.
It is formed by the hybridization of the metal orbitals which include one s orbital and three p orbitals and two d orbitals to form a d2sp3 hybridization. These orbitals are equivalent energetically in the free ion but are split to two sets of orbitals in the field of the ligands attached to it.
These two sets are called in group theory T2g and Eg levels. T2g level inlcudes three d orbitals which are energetically equivalent, while the eg level includes 2 d orbitals which are also energetically equivalent.
The gap in energy between the t2g and eg levels determines whether the complex will be magnetic or diamegnetic in its properties. A large gap between the two levels will favor a diamagnetic complex while a small energy gap will favor a magnetic complex.
This difference in magnetism or diamagnetism depends on the type of the ligand that is attached to the metal. Water complexes for example are usually high spin and magnetic in most complexes. while other ligands such as CO and CN- contribute largely to diamagnetism of the complex.
This is so due to an effect that is called backbonding which ligands such as CO and CN- can make. Thus splitting the t2g and eg energy gap further. This makes the complex diamagnetic. This type of backbonding does not occur in sigma donors ligands such as water.
This in turn contributes to low energy gap between the t2g and eg levels. Thus making the complex high spin and magnetic.
d5 and d10 electronic configurations are of unusual satability in organometallic complexes. Electronic transitions between the t2g and eg energy levels confer color to the complex that is specific to that metal and to the energy gap itself between the t2g and eg levels.
High spin d5 electronic configuration does not have any electronic transitions between the t2g and eg levels. This is so due to the conservation of spin. Such transitions are spin forbidden in high spin d5 configuration.
In octahedral complexes the t2g level lies below the eg level, while in tetrahedral complexes the reverse is observed. Namely, eg level lies below the t2g level.
d8 electronic configuration is unique in organometallic complexes. This is so due to the square planar geometry that the complex adopts. d8 complexes do not adopt tetrahedral geometry in the palladium group.
Tetrahedral geometry can be either diamagnetic or magnetic, while square planar geometry is always diamagnetic. This is so due to the difference in molecular orbitals diagrams in both of these structures.