This invention relates to a diboron derivative of formula (I) or a diboron derivative of formula (II) or a diboron derivative of formula (III) where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are each independently selected from the group consisting of optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, a group of the formula --(R.sup.5 Q).sub.m R.sup.6 where Q is selected from O, S, NR.sup.7, optionally substituted arylene and optionally substituted cycloalkylene, m is an integer selected from 1 to 3, the or and each R.sup.5 is independently an optionally substituted C.sub.1 -C.sub.3 alkylene, R.sup.6 is C.sub.1 -C.sub.3 alkyl or hydrogen, and R.sup.7 is hydrogen or C.sub.1 -C.sub.12 alkyl; each X is independently selected from O, S(O).sub.n and NR.sup.7, where n is an integer from 0 to 3, R.sup.7 is hydrogen or C.sub.1 -C.sub.12 alkyl, or one or more of --NR.sup.1 R.sup.7, --NR.sup.2 R.sup.7, --NR.sup.3 R.sup.7 and --NR.sup.4 R.sup.7 represent an optionally substituted 5 or 6 membered heterocyclyl group,and A, A.sup.1 and A.sup.2 are divalent groups which may or may not be different.
