Abstract: A compound of formula (I): ##STR1## n which M is a transition metal ion; A is a counter-ion if required;r, s and t are independently 0 to 3 such that r+s+t is in the range of 1 to 3;R.sup.a, R.sup.b, R.sup.c are each independently hydrogen or CH.sub.2 OR' where R' is hydrogen or an organic group;B and E are independently oxygen, CH.sub.2, NR.sup.d in which R.sup.d is alkyl, hydrogen, alkylcarbonyl, or arylcarbonyl or SO.sub.n where n is 0 or an integer 1 or 2, with the proviso that B and E are not simultaneously CH.sub.2 and that when B is oxygen, NR.sup.d or SO.sub.n, then r cannot be 0, and when E is oxygen, NR.sup.d or SO.sub.n, then t cannot be 0;R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9 and R.sub.10 are independently hydrogen, alkyl or alkoxy.