Oxalate (C2O4^2-) is a bidentate ligand because it can form two coordinate bonds with the central metal ion through its two oxygen atoms. Chloride, nitrate, and sulfate are generally monodentate ligands.

In [Co(en)2Cl2]+, cobalt is bonded to two ethylenediamine (en) ligands and two chloride (Cl) ligands. Each 'en' is bidentate, contributing 2 donor atoms, so 2 en ligands give 4 donor atoms. Two Cl ligands contribute 2 donor atoms. Total coordination number is 4 + 2 = 6.

[Co(en)3]3+ shows optical isomerism because it has a chiral structure with no plane of symmetry. The complex can exist as non-superimposable mirror images. Other complexes either have a plane of symmetry or do not form enantiomers.

[Ni(CN)4]2- has a square planar geometry due to the strong field ligand CN-. The nickel ion (Ni2+) undergoes dsp2 hybridization involving one d, one s, and two p orbitals to form the square planar complex.

A homoleptic complex has only one type of ligand attached to the central metal ion. In [Ni(CO)4], nickel is surrounded by four carbon monoxide (CO) ligands, making it homoleptic. Other complexes have more than one type of ligand.

In [Fe(CN)6]4-, the overall charge of the complex is -4. Each CN ligand carries a charge of -1, and there are 6 CN ligands, contributing a total charge of -6. Therefore, the oxidation state of Fe must be +2 to balance the charge: x + (-6) = -4, so x = +2.

[NiCl4]2- has tetrahedral geometry because Ni2+ with Cl- (weak field ligand) does not undergo pairing of electrons and adopts sp3 hybridization. [Ni(CN)4]2- is square planar, while [CoF6]3- and [Cr(H2O)6]3+ are octahedral.

In [Co(NH3)5Cl]Cl2, cobalt has an oxidation state of +3. Ligands are named first (ammine for NH3, chlorido for Cl), followed by the metal name with its oxidation state in Roman numerals, and then the anion outside the bracket (chloride). Hence, the name is pentaamminechloridocobalt(III) chloride.

[Ni(CN)4]2- is diamagnetic because Ni2+ in the presence of strong field ligand CN- undergoes pairing of electrons in a square planar geometry (dsp2 hybridization), resulting in no unpaired electrons. Other complexes have unpaired electrons and are paramagnetic.

F- is a weak field ligand, so the crystal field splitting energy (Δo) for [CoF6]3- is small, leading to absorption in the green region (lower energy). NH3 is a stronger field ligand than F-, so [Co(NH3)6]3+ has a higher Δo and absorbs higher energy light. Hence, Δo of [CoF6]3- is lower than [Co(NH3)6]3+.