Inorganic experiments / edited by J. Derek Woollins. (OCLC #496229687)

RDA’s General Guideline for English Language Capitalization (RDA A.10) is basically to follow the Chicago Manual of Style. CMOS 10.66 on Naming conventions for chemical elements says that “symbols all have an initial capital” and “names of elements are always lowercased”, as found in various titles in this title’s contents note:

  Two-stage synthesis of Ph₂P(O)(CH₂)₄P(O)Ph₂ 
  Preparation of an iron dinitrogen complex 
  Nickel-catalyzed cross-coupling of alkylmagnesium with haloarene

Nickel is capitalized in that last title, following RDA A.4.1, which says the capitalize the first word of a title.