Where is chiastic structure in Psalm 51?
There are several uses of chiasm in Psalm 51, but various people see the structure in different ways.
Let's start with an easier example:
#""^(1b)#Have mercy on me, O God,
because of your loyal love!
Because of your great compassion,
wipe away my rebellious acts!
So here we have a structure A, B, B', A'
Some commentators include verse 2 in this A', grouping it with 'wipe away my rebellious acts':
#""^2#Wash away my wrongdoing!
Cleanse my of my sin!
These two lines seem like simple parallelism to me rather than part of the previous chiasm.
In verses 12 and 14 we see joy and deliverance mentioned twice. So this could be a chiasm, though it seems slightly less obvious:
#""^12#Let me again experience the joy of your deliverance!
Sustain me by giving me the desire to obey!
#""^13#Then I will teach rebels your merciful ways,
and sinners will turn to you.
#""^14#Rescue me from the guilt of murder, O God,
the God who delivers me!
Then my tongue will shout for joy because of your deliverance.
This could be an A, B, C, B', A' chiasm, but I'm not totally convinced.
We see another chiasm in verses 16 and 17:
#""^16#Certainly you do not want a sacrifice, or else I would offer it;
you do not desire a burnt sacrifice.
#""^17#The sacrifices God desires are a humble spirit -
O God, a humble and repentant hear you will not reject.
This has an A, B, B', A' structure.
Notice here that the parallel lines are in opposition to one another.
Some commentators see a larger chiastic structure in the whole psalm, but there are various different opinions. In addition note that some commentators see verses 18 and 19 as a later addition - not part of the original psalm - so not necessarily part of the original intended structure of the psalm.