which of the following has least bond order

a)hexacarbonyl vanadium(-1)

b)pentacarbonyl iron

c)pentacarbonyl chromium

d) hexacarbonyl manganese(+1)

how do we calculate bond order in carbonyl compounds

plzzzz reply soon...and also plz give me some tips to crack jee 2011as there is very less time for the exam

Dear student,

The bond order of a simple molecule can be determined by looking at the number of electrons in bonding and antibonding molecu;ar orbitals. Like electrons in atomic orbitals , electrons in molecular orbitals fill the lowest energy orbitals first, but no more than two electrons can go in any given MO.

The bond order can then be computed by
0.5*(electrons in bonding orbitals - electrons in antibonding orbitals). (The 0.5 is due to the fact it takes two electrons to make a single bond.)

For the simple molecule H₂⁺, there is only one electron that must be placed in a molecular orbital. It goes into the lowest energy orbital as shown.

There is 1 electron in a bonding MO and none in antibonding orbitals: the bond order is thus 0.5*(1-0) = 0.5. H₂⁺ has a bond order of 1/2: fractions are ok! This bond has about 1/2 the strength of a normal H₂ covalent bond.

For H₂, there are two electrons. Both can fit into the bonding molecular orbital: the bond order is 0.5*(2-0) or 1.