Why groups like `-CHO,-NO_2,-B(OR)_3,-overset(oplus)PBr_3 and -overset(oplus)SR_2` acts as meta-directing groups ?

Explanation

Since the carbon atom of the `-CHO` group is bonded to the oxygen atom by a double bond, the nitrogen atom of the `-NO_2` group is linked with the oxygen atom by a double bond , the boron atom of the `-B(OR)_3` group contains a vacant p-orbital and the phosphorus and sulphur atoms of the groups `-overset(oplus)PBr_3 and -overset(oplus)SR_2` have vacant d-orbitals , all of these groups reduce the electron densities of ortho- and para-positions by their-R effect. Consequently, the electron density at the meta-position becomes relatively higher and the electrophile preferably enters the meta-position. Thus , these groups behave as meta-directing groups.