The synthesis, characterization and mesomorphic properties of three series of isocyanides and their trigonal bipyramidal iron(0) complexes [Fe(CO)₄(CNR)] (R = C₆H₄O(O)CC₆H₄C₆H₄OC_nH_{2n + 1}, C₆H₄C(O)OC₆H₄C₆H₄OC_nH_{2n + 1}, C₆H₄C₆H₄O(O)CC₆H₄OC_nH_{2n + 1}, n = 6, 8, 10, 12) are described. The free isocyanides exhibit nematic and/or smectic A phases for shorter chains (n = 6, 8) and only smectic A phases for longer chains, except the isocyanides CNC₆H₄O(O)CC₆H₄C₆H₄OC_nH_{2n + 1} which melt with extensive decomposition and do not produce mesophases. Their coordination to give the iron complexes [Fe(CO)₄(CNR)] produces a slight decrease of their transition temperatures. The iron isocyanide complexes prepared show only a smectic C phase. The relationship between the molecular structure of the complexes and their thermal behaviour is discussed.