Steric effects on the ortho-metalation reactions at binuclear pyrazolyl-bridged rhodium compounds. Molecular structure of Rh2(3,5-dmpz)2[.mu.-P(o-C6F4)Ph2]Br(CO)(.eta.2-PCBr)(H2O) [3,5-dmpz = 3,5-dimethylpyrazolyl; PCBr = P(o-BrC6F4)Ph2]