(a) Write the hybridization and shape of the following complexes:

(i) [CoF₆]^3–

(ii) [Ni (CN)₄]^2–

(Atomic number: Co = 27, Ni = 28)

(b) Out of NH₃ and CO, which ligand forms a more stable complex with a transition metal and why?

(a)
(i) The atomic number of Co is 27 and its valence shell electronic configuration is 3d⁷4s².

Co is in +3 oxidation state in the complex [CoF₆]^3-.

Hence, [CoF₆]^3- is sp³d² hybridized and it is octahedral in shape.

(ii) The atomic number of Ni is 28 and its valence shell electronic configuration is 3d⁸4s².

Ni is in +2 oxidation state in the complex [Ni(CN)₄]^2–.

Hence, [Ni(CN)₄]^2– is dsp² hybridized and it is square planar in shape.

(b) Out of NH₃ and CO, CO ligand forms a more stable complex with transition metal because the metal- carbon bonds in metal carbonyls have both  and  characters .A  bond is formed when carbonyl carbon donates a lone pair of electrons to the vacant orbital of the metal. A  bond formed by the donation pair of electrons from filled metal d orbital into the vacant anti- bonding orbital ( also known as back bonding  of the carbonyl group). And sigma bond strengthen the pi bond vice-versa. Thus, a synergic effect s created due to this metal- ligand bonding. These synergic effects strengthen the bond between CO and the metal.