Reactions of [RhRu3H(η5-C5Me5)(CO)9 BH2] with didentate phosphines and the synthesis and crystal structure of [RhRu3H2(η5-C5Me5)(CO)8(µ-dppf-P,P′)AuB][dppf = 1,1′-bis(diphenylphosphino)ferrocene]