The Highlands politician, who has served as a councillor, MP and MSP, will step down aged 65.
A total of 589 youngsters have been on the waiting list for 52 weeks or over for specialist mental health care.
The SNP's former Westminster leader will contest the seat currently held by Ruth Davidson.
Richard Leonard has written to the First Minister demanding to know why she opted to let Mackay resign from her cabinet.
Derek Mackay quit as finance secretary after it emerged he sent hundreds of messages to a schoolboy.
The former finance secretary was forced to resign just hours ahead of delivering the budget on Thursday.
Ferguson Marine went bust in August last year and was then taken over by the Scottish Government.
MSPs dismissed the bill blaming lack of clarity over the total cost of such a scheme.
Sports teams, pride event organisers, national organisations and local support groups are among those to put their name to the paper.
The procedural hearing will take place at the High Court in Edinburgh later on Wednesday.
Annie Wells MSP has called on the PM to hold a summit on the issue 'as soon as possible'.
The ankle-worn tags test an offender’s sweat every 30 minutes.
The number of private sector businesses operating in the country has risen by more than a quarter since 2007.
The Disclosure (Scotland) Bill is currently being considered by a Holyrood committee.
MSPs at the Scottish Parliament will consider the proposal as part of a new Bill.
An inquiry is to be held into whether student teachers are sufficiently prepared.