Four-coordinate Rh(I) complexes {{Rh(CO)2Cl}n(a-diimine)] (n = 1,2) with s,s-N,N' chelate or s-N, s-N' bridge bonded RN=C(R')C(R')=NR ligands. Influence of the branching at Ca of R on the RhI-a-diimine interaction