Question

explain why cyano or phosphine ligands are place high in the spectrochemical series but the hydroxide ligand or the halo ligands are quite low in the spectrochemical series?

Answer 1

Ligands that produce a small splitting are called weak-field ligands and lie at the left end of the series. Ligands that produce a large splitting are called strong-field ligands and lie at the right end of the series.

It is based on sigma and pi bonding interaction. Also size of ligands play a role. Smaller the size, more closer will be ligand to the central atom and hence greater splitting.

It is a big process which is nicely explained in below link:
http://www.chm.davidson.edu/vce/coordchem/Ligands.html