Carbon , due to very high sum of first four ionisation enthalphies , cannot lose all the four valence electrons `( 2p^2 2p^2)` to form `C^(4+)` ion. Carbon cannot gain four eelctrons to form `C^(4+)` ion as this anion would be highly unstalble as it would be vety difficult for a nucleus with only `6` proton to held on firmly `10` electrons , Carbon can only form covalent componds by sharing og elecrons .
On the othe hand in lead , due to inert paire effect `+2` oxidation state is mnore stable as compard to `+4` oxidation state . Down the frop `(narr)`, ionisation enrthalpy decrease. Hence, the sum of first two ioinisation enthalpies of lead is quite low and lead can easily lose two electrons to form `Pb^(2+)` ion . As a result , lead forms ionci compound .
