Chemistry of transition elements: compare question

An aqueous solution containing the complex ion $\ce{[Ni(H2O)6]^{2+}}$ is pale green. When concentrated hydrochloric acid is added, the solution turns yellow and a new complex ion forms. In a separate experiment, an aqueous solution containing $\ce{[Ni(CN)4]^{2-}}$ is colourless. The $\ce{[Ni(CN)4]^{2-}}$ ion has a square planar geometry, while $\ce{[Ni(H2O)6]^{2+}}$ is octahedral.

Compare the two complexes $\ce{[Ni(H2O)6]^{2+}}$ and $\ce{[Ni(CN)4]^{2-}}$ in terms of: (a) the coordination number and charge of each complex ion, (b) the type of ligand (monodentate, bidentate or polydentate) and the nature of the metal–ligand bond, (c) the geometry of each complex ion.