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Looks like MLSE has been working the sponsorship angle for the raptors:

http://www.thestar.com/sports/raptors/2015/11/13/toronto-raptors-biosteel-strike-naming-rights-deal.html

Welcome to the...

Looks Like it started Sept 2006 based on this press release:

http://www.bmo.com/bmo/files/news%20release/4/1/bmo_bmofieldtor_sep2006_en.htm

The real interesting thing is I think the BMO field deal is up this year (Its was a 10 year deal signed in 2005) so they may be shopping for a new field sponsor as well:
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It was up last year and there was a 1 year extension this year.

http://www.thestar.com/business/sportonomics/2015/02/27/bmo-signs-up-for-one-more-year-as-tfc-jersey-sponsor.html

I have a bit of inside information (I'm not usually this cool). This is from a couple inside folks within the banking / sports sponsorship areas.

So BMO as a company is moving towards basketball...