iron triad

Nickel and cobalt have similar properties and as they are adjacent period 4 transition metals ((v. iron triad)).

In Mendeleev's 1871 periodic table, copper, silver, and gold are listed twice, once under group VIII (with the iron triad and platinum group metals), and once under group IB.

Oxo compounds for the vanadium through iron triads (groups 3-8) are well known, whereas terminal oxo compounds for metals in the cobalt through zinc triads (groups 9-12) are rare and invariably feature metals with coordination numbers lower than 6.