CO2+ ions can be easily oxidised to CO3+ ions because the crystal field stabilisation energy of Ci3+ ions with a d6 configuration is higher than d7 configuration.
In presence of ligand the d- orbitals of metal gets splits up into two set of orbitals that is t2g and eg. The half filled or fully filled t2g is more stable than other configuration. This is because it gives maximum amount of low energy of t29 orbital. Lower the energy more is the stabilisation.
 
 
Ni has outer-most electronic configuration of 4s2 3d8. The hybridisation for octahedral complex is sp2d3.For low spin octahedral complexes the hybradisation is d2sp3.There are two cases in which nickle forms octahedral complexes.
First case: when the ligand is weak then there is no pairing up of electrons in the d-orbital, hence there is no d-orbital for acquiring the d2sp3 hybradisation. Hence, low spin complexes are not formed.
Second case:when the ligand is strong there is a pairing up of electrons.but there is only one vacant d-orbital available for hybradisation. So, here also d2sp3 hybradisation is not formed.hence low spin octahedral complex is not formed.
Thus, one can concluded that nickel doesn't form low spin octahedral complexes.