[research]

is there any relationship BTW bond order and covalent character?
for example if we imagine a C=C in two differnet compound in one of them bond order is 1.5 and in the other bond order is 2 so now which one has more covalent character?
every help will be appreciated.

[demoninatutu]

As far as I can see, there isn't.

Fractional bond orders come from molecular orbital theory (valence bond theory has no such concept). They are most common in multi-centre bonds, which in organic chemistry is most common in conjugated bonds (valence bond theory describes them as interchanging double/single bonds). In most organic compounds, it's safe to assume that all bonds are covalent unless there's compelling reason to believe otherwise.  Dipoles are an intermediate case between fully covalent and fully ionic bonds. Carbonate, for example, has three C-O bonds with a bond order of 1 1/3 but all the bonds are covalent, though with dipoles.

Note that the concept of 'covalent character' is open to interpretation and seems to come most commonly from experimental and computational observations of electron density distribution. NaCl and Cl₂ for instance both have a bond order of one. NaCl, however, clearly dissociates in water and computational calculations place the electron density squarely on the chlorine atom.  Cl₂, however, does not dissociate so easily in water and calculations place the electron density around both atoms.

In the case of transition metals, things get really complicated and there are certainly many grey areas but a rule of thumb is that low oxidation states with water soluble ions are usually more ionic and high oxidation states with insoluble ions are more covalent.

So to answer your question, molecular orbital theory will give one C=C bond a bond order of 2 if the pi bond is fixed between those two atoms but, for example, a bond order of 1 1/2 if it is delocalised over three carbon atoms (e.g. resonance). You would then need to run a calculation to find the electron density distribution to find the extent of polarisation. Strong polarisation would indicate reduced covalent character.