You should to be soldering to the nickel. The intermetallic is Ni3Sn4.
"At relatively low temperatures, the tin-nickel layers form about as rapidly as the tin-copper layers do, but at higher temperatures their growth rate is distinctly lower. At 100*C a 2um layer is formed in 50 days, at 170*C within two days. A 10um layer is formed at 170*C in 38 days. The appearance of the intermetallic layers, as well as their composition, depends on the temperature at which they are formed." [K-Wassink]
Recognize that soldering to nickel is not for amateurs. Oxidized nickel is difficult to solder. So, be aware of the in-bound quality, age, storage conditions, and general condition of the connector leads.
I want to say there are a couple of threads in the SMTnet Archives on soldering to nickel. Try a search.
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