Reactivity of Rh4(CO)12 with halides or pseudohalides; synthesis of the pentanuclear carbonyl clusters [Rh5(CO)14X]n- (X = Cl, Br, I, SCN, n = 2; X = PPh3, n = 1). Crystal structures of [N(PPh3)2][Rh5(µCO)6(CO)8(PPh3)] and [N(PPh3)2]2[Rh5(µCO)6(CO)8(SCN)]