Mendeleev's main criterion for group assignment was the maximum oxidation state of an element: on that basis, the group 11 elements could not be classified in group IB, due to the existence of copper(II) and gold(III) compounds being known at that time.
mˈɛndɪlˌiːvz mˈeɪn kɹaɪtˈiəɹɪən fɔːɹ ɡɹˈuːp ɐsˈaɪnmənt wʌzðə mˈæksɪməm ˌɑːksɪdˈeɪʃən stˈeɪt əvən ˈɛlɪmənt: ˌɔn ðæt bˈeɪsɪs, ðə ɡɹˈuːp ɪlˈɛvən ˈɛlɪmənts kʊd nˌɑːt biː klˈæsᵻfˌaɪd ɪn ɡɹˈuːp ˌaɪbˈiː, dˈuː tə ðɪ ɛɡzˈɪstəns ʌv kˈɑːpɚ ɹˌoʊmən tˈuː ænd ɡˈoʊld ɹˌoʊmən θɹˈiː kˈɑːmpaʊndz bˌiːɪŋ nˈoʊn æt ðæt tˈaɪm.