** Yanbu National Petrochemical Co (Yansab) tumbles 7.1 pct to 52.25 riyals ** Proposed a cash dividend of 1.5 riyals per share for the second half of 2014 last Thursday, down from 2.0 riyals a year earlier ** Said would shut down its ethylene glycol plant in April 2015 for between 35 and 60 days of planned maintenance ** Financial impact from maintenance work estimated at around 450 million riyals ($120 million) ** Saudi Arabia's index falls 0.8 pct (Reporting by Olzhas Auyezov; Editing by Andrew Torchia)