What is the major focus of the crystal field model? Why are the d orbitals split into two sets for an octahedral complex? What are the two sets of orbitals?

Define each of the following. 

a. Weak-field ligand 

b. Strong-field ligand 

c. Low-spin complex 

d. High-spin complex

Why is Co(NH₃)₆³⁺ diamagnetic whereas CoF₆^3- is paramagnetic? Some octahedral complex ions have the same d-orbital splitting diagrams whether they are highspin or low-spin. For which of the following is this true?  

a. V³⁺ 

b. Ni²⁺ 

c. Ru²⁺