1. Is ethylene diamine a strong or weak field ligand ?
2. Why is the shape of [Ni(CN)₄]^2- and [Ni(CO₄)] different , although both CN^- and (CO) are strong field ligands ?

1,ethylene diamine a strong field ligand as it has two group which is attached to the central atom. it form a "C" clamp looking structureSee the structure of EDTA(ethylene diamine)
2 hybridisation of [Ni(CN)4]^2- -It has a +2 charge on nickel atom-[Ni(CN)4]2-has dsp2hybridization and square planar geometry.


and hybridisation of [Ni(CO)4] -It has o charge in the central atom-CO is a high spin co-ordinating complex, so it doesn't occupy the inner orbitals(d-orbitals) of Ni. It starts occupying from the s-orbital. Four CO donates 8 electrons to the Ni atom. Starting with s-orbital, 2 electrons go to one s-orbital and the remaining 6 electrons go into three p-orbitals. So the hybridisation is sp3.
Basically the difference is due to charge on the central atom nickel

ni z= 28 3d8 4s2
oxidation state 0 CO is neutral ligand and strong field ligand therefore e- pairing occurs and final e- config after pairing is
3d10 4s0 hence 4s and 4p orbitals will be used for bonding and no of ligand is 4 so
hybridisation sp3