How Do I Keep The Morale Of My Team Up?

I'm trying my best to keep my teammates from flaming and not communicating - calling gj after good plays, congratulating, cheering people up after something bad happens and making calls when we need them. But it just seems to never be working.

It just makes me sad that people start flaming and create a downward spirale of our morale no matter what I do.

I recently had a super devastating game where I just couldn't motivate anyone. Shaco was pissed from the start, saw my fake niceness and called me out on that, even though I just was trying to keep a positive atmosphere and get us to win. Top and mid joined in and just had to answer to that goddamn Shako and even though I tried my hardest to prevent it, everyone was focusing more on the chat than the game. That lost my promo series.

But I don't want to just ignore everyone and give up on it either. I want to keep everyone together, I want to be shotcalling, motivate everyone to work as a team, I feel like I could be a great team leader if anyone just gave me the chance to. I know that sounds cocky and really naive but what else am I going to do?

If you somehow managed to defuse situations like the one I described above please tell me. I had more and more problems with that the higher my elo got and Plat 5 has just been hell. Please help me, I want to get out of there.