Reaction of diag- or lat- (i.e. trans or cis)(η 5 -C 5 H 4 R)Re(CO) 2 X 2 (R=Me, t-Bu, SiMe 3 ; X=Br, I) with isocyanides, phosphites and triphenyl phosphine proceeded rapidly at room temperature (r.t.) in the presence of Me 3 NO to give (η 5 -C 5 H 4 R)Re(CO)(L)X 2 (L=CNC 6 H 3 Me 2 , P(OMe) 3 , P(O i Pr) 3 , P(OPh) 3 , PPH 3 ) in good yields (typically>75%) with the diagonal isomer as the dominant (>90%) product. The diag-(η 5 -C 5 H 4 R)Re(CO)(L)X 2 isomer were readily converted into the lateral isomer in excellent yield (>70%) by directly heating solid diag-(η 5 -C 5 H 4 R)Re(CO)(L)X 2 under nitrogen below its melting point. Solution phase isomerisation in CHCl 3 or C 6 D 6 (r.t., visible light irradiation) also proceeded from the diag to the lat isomer. The solid-state reaction between diag-(η 5 -C 5 H 4 Me)Re(CO)[P(OPh) 3 ]Br 2 and excess NaI surprisingly gave diag-(η 5 -C 5 H 4 Me)Re(CO) 2 I 2 in quantitative yield, revealing both Re-P and Re-Br bond cleavage. The new complexes, diag- and lat-(η 5 -C 5 H 4 R)Re(CO)(L)X 2 , have been fully characterized by elemental analysis and IR and NMR spectroscopy.