A mononuclear iron bivalent complex having formula: wherein each of R 1 to R 6 independently is hydrogen, or an alkyl, aryl, aralkyl, organoxy, monoorganoamino, diorganoamino, monoorganophosphino, diorganophosphino, monoorganosilyl, diorganosilyl, triorganosilyl or organothio group which may be substituted with X, provided that at least one pair of any one of R 1 to R 3 taken together with any one of R 4 to R 6 may represent a crosslinking substituent selected from -O-, -S-, -NH-, -NR-, -PR-, -NH-(CH 2) k -NH-, -NR-(CH 2) k -NR-, -PH-(CH 2) k -PH-, -PR-(CH 2) k -PR-, -C=C-, C 1 -C 10 alkylene, C 6 -C 30 arylene, C 7 -C 30 aralkylene, -(CH 2 O) k -, -(CH 2) k -O-(CH 2) k -, -O-(CH 2) k -O-, -R'-O-(CH 2) k -O-R'- wherein each R' independently is a C 1 -C 10 alkylene group, C 6 -C 30 arylene group or C 7 -C 30 aralkylene group, -(CH 2 S) k -, -(CH 2) k -S-(CH 2) k -, -S-(CH 2) k -S-, -R'-S-(CH 2) k -O-R'- wherein each R' independently is as defined above, -SiR 2 - and -(CH 2) k -SiR 2 -(CH 2) k -; wherein in each case R is selected independently from C 1 -C 10 alkyl groups, C 6 -C 20 aryl groups and C 7 -C 20 aralkyl groups, which may be substituted with X, and each k independently is an integer of 1 to 10; X is a halogen atom, organoxy, monoorganoamino, diorganoamino or organothio group, L 1 is a two-electron isonitrile ligand, with the proviso that when plural L 1 's are present, two L 1 's may bond together, L 2 is a two-electron ligand other than CO ligand and L 1, with the proviso that when plural L 2 's are present, two L 2 's may bond together, m 1 is an integer of 1 to 4, m 2 is an integer of 0 to 3, and m 1 +m 2 is 3 or 4. wherein each of R 1 to R 6 independently is hydrogen, or an alkyl, aryl, aralkyl, organoxy, monoorganoamino, diorganoamino, monoorganophosphino, diorganophosphino, monoorganosilyl, diorganosilyl, triorganosilyl or organothio group which may be substituted with X, provided that at least one pair of any one of R 1 to R 3 taken together with any one of R 4 to R 6 may represent a crosslinking substituent selected from -O-, -S-, -NH-, -NR-, -PR-, -NH-(CH 2) k -NH-, -NR-(CH 2) k -NR-, -PH-(CH 2) k -PH-, -PR-(CH 2) k -PR-, -C=C-, C 1 -C 10 alkylene, C 6 -C 30 arylene, C 7 -C 30 aralkylene, -(CH 2 O) k -, -(CH 2) k -O-(CH 2) k -, -O-(CH 2) k -O-, -R'-O-(CH 2) k -O-R'- wherein each R' independently is a C 1 -C 10 alkylene group, C 6 -C 30 arylene group or C 7 -C 30 aralkylene group, -(CH 2 S) k -, -(CH 2) k -S-(CH 2) k -, -S-(CH 2) k -S-, -R'-S-(CH 2) k -O-R'- wherein each R' independently is as defined above, -SiR 2 - and -(CH 2) k -SiR 2 -(CH 2) k -; wherein in each case R is selected independently from C 1 -C 10 alkyl groups, C 6 -C 20 aryl groups and C 7 -C 20 aralkyl groups, which may be substituted with X, and each k independently is an integer of 1 to 10; X is a halogen atom, organoxy, monoorganoamino, diorganoamino or organothio group, L 1 is a two-electron isonitrile ligand, with the proviso that when plural L 1 's are present, two L 1 's may bond together, L 2 is a two-electron ligand other than CO ligand and L 1, with the proviso that when plural L 2 's are present, two L 2 's may bond together, m 1 is an integer of 1 to 4, m 2 is an integer of 0 to 3, and m 1 +m 2 is 3 or 4.